skia / external / github.com / KhronosGroup / OpenGL-Registry / bc93187978b46e0baae0b8556071d6b292b0bb97 / . / extensions / KHR / KHR_blend_equation_advanced.txt

Name | |

KHR_blend_equation_advanced | |

Name Strings | |

GL_KHR_blend_equation_advanced | |

GL_KHR_blend_equation_advanced_coherent | |

Contact | |

Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com) | |

Notice | |

Copyright (c) 2012-2015 The Khronos Group Inc. Copyright terms at | |

http://www.khronos.org/registry/speccopyright.html | |

Specification Update Policy | |

Khronos-approved extension specifications are updated in response to | |

issues and bugs prioritized by the Khronos OpenGL and OpenGL ES Working Groups. For | |

extensions which have been promoted to a core Specification, fixes will | |

first appear in the latest version of that core Specification, and will | |

eventually be backported to the extension document. This policy is | |

described in more detail at | |

https://www.khronos.org/registry/OpenGL/docs/update_policy.php | |

Contributors | |

OpenGL ES Working Group in Khronos | |

Jeff Bolz, NVIDIA Corporation | |

Mathias Heyer, NVIDIA Corporation | |

Mark Kilgard, NVIDIA Corporation | |

Daniel Koch, NVIDIA Corporation | |

Rik Cabanier, Adobe | |

Slawek Grajewski, Intel | |

Status | |

Complete. | |

Ratified by the Khronos Board of Promoters on 2014/03/14. | |

Version | |

Last Modified Date: February 14, 2018 | |

Revision: 17 | |

Number | |

ARB Extension #174 | |

OpenGL ES Extension #168 | |

Dependencies | |

This extension is written against the OpenGL 4.1 Specification | |

(Compatibility Profile). | |

This extension is written against the OpenGL Shading Language | |

Specification, Version 4.10 (Revision 6). | |

OpenGL 2.0 is required (for Desktop). | |

OpenGL ES 2.0 is required (for mobile). | |

EXT_blend_minmax is required (for mobile). | |

This extension interacts with OpenGL 4.0. | |

This extension interacts with OpenGL 4.1 (Core Profile). | |

This extension interacts with OpenGL 4.3 or later. | |

This extension interacts with OpenGL ES 2.0. | |

This extension interacts with OpenGL ES 3.0. | |

This extension interacts with NV_path_rendering. | |

Overview | |

This extension adds a number of "advanced" blending equations that can be | |

used to perform new color blending operations, many of which are more | |

complex than the standard blend modes provided by unextended OpenGL. This | |

extension provides two different extension string entries: | |

- KHR_blend_equation_advanced: Provides the new blending equations, but | |

guarantees defined results only if each sample is touched no more than | |

once in any single rendering pass. The command BlendBarrierKHR() is | |

provided to indicate a boundary between passes. | |

- KHR_blend_equation_advanced_coherent: Provides the new blending | |

equations, and guarantees that blending is done coherently and in API | |

primitive order. An enable is provided to allow implementations to opt | |

out of fully coherent blending and instead behave as though only | |

KHR_blend_equation_advanced were supported. | |

Some implementations may support KHR_blend_equation_advanced without | |

supporting KHR_blend_equation_advanced_coherent. | |

In unextended OpenGL, the set of blending equations is limited, and can be | |

expressed very simply. The MIN and MAX blend equations simply compute | |

component-wise minimums or maximums of source and destination color | |

components. The FUNC_ADD, FUNC_SUBTRACT, and FUNC_REVERSE_SUBTRACT | |

multiply the source and destination colors by source and destination | |

factors and either add the two products together or subtract one from the | |

other. This limited set of operations supports many common blending | |

operations but precludes the use of more sophisticated transparency and | |

blending operations commonly available in many dedicated imaging APIs. | |

This extension provides a number of new "advanced" blending equations. | |

Unlike traditional blending operations using the FUNC_ADD equation, these | |

blending equations do not use source and destination factors specified by | |

BlendFunc. Instead, each blend equation specifies a complete equation | |

based on the source and destination colors. These new blend equations are | |

used for both RGB and alpha components; they may not be used to perform | |

separate RGB and alpha blending (via functions like | |

BlendEquationSeparate). | |

These blending operations are performed using premultiplied source and | |

destination colors, where RGB colors produced by the fragment shader and | |

stored in the framebuffer are considered to be multiplied by alpha | |

(coverage). Many of these advanced blending equations are formulated | |

where the result of blending source and destination colors with partial | |

coverage have three separate contributions: from the portions covered by | |

both the source and the destination, from the portion covered only by the | |

source, and from the portion covered only by the destination. Such | |

equations are defined assuming that the source and destination coverage | |

have no spatial correlation within the pixel. | |

In addition to the coherency issues on implementations not supporting | |

KHR_blend_equation_advanced_coherent, this extension has several | |

limitations worth noting. First, the new blend equations are not | |

supported while rendering to more than one color buffer at once; an | |

INVALID_OPERATION will be generated if an application attempts to render | |

any primitives in this unsupported configuration. Additionally, blending | |

precision may be limited to 16-bit floating-point, which could result in a | |

loss of precision and dynamic range for framebuffer formats with 32-bit | |

floating-point components, and in a loss of precision for formats with 12- | |

and 16-bit signed or unsigned normalized integer components. | |

New Procedures and Functions | |

void BlendBarrierKHR(void); | |

New Tokens | |

Accepted by the <cap> parameter of Disable, Enable, and IsEnabled, and by | |

the <pname> parameter of GetIntegerv, GetBooleanv, GetFloatv, GetDoublev | |

and GetInteger64v: | |

BLEND_ADVANCED_COHERENT_KHR 0x9285 | |

Note: The BLEND_ADVANCED_COHERENT_KHR enable is provided if and only if | |

the KHR_blend_equation_advanced_coherent extension is supported. On | |

implementations supporting only KHR_blend_equation_advanced, this enable | |

is considered not to exist. | |

Accepted by the <mode> parameter of BlendEquation and BlendEquationi: | |

MULTIPLY_KHR 0x9294 | |

SCREEN_KHR 0x9295 | |

OVERLAY_KHR 0x9296 | |

DARKEN_KHR 0x9297 | |

LIGHTEN_KHR 0x9298 | |

COLORDODGE_KHR 0x9299 | |

COLORBURN_KHR 0x929A | |

HARDLIGHT_KHR 0x929B | |

SOFTLIGHT_KHR 0x929C | |

DIFFERENCE_KHR 0x929E | |

EXCLUSION_KHR 0x92A0 | |

HSL_HUE_KHR 0x92AD | |

HSL_SATURATION_KHR 0x92AE | |

HSL_COLOR_KHR 0x92AF | |

HSL_LUMINOSITY_KHR 0x92B0 | |

NOTE: These enums are not accepted by the <modeRGB> or <modeAlpha> | |

parameters of BlendEquationSeparate or BlendEquationSeparatei. | |

Additions to Chapter 2 of the OpenGL 4.1 Specification (OpenGL Operation) | |

None. | |

Additions to Chapter 3 of the OpenGL 4.1 Specification (Rasterization) | |

None. | |

Additions to Chapter 4 of the OpenGL 4.1 Specification (Per-Fragment | |

Operations and the Frame Buffer) | |

Modify Section 4.1.8, Blending (p. 359). | |

(modify the first paragraph, p. 361, allowing for new values in the | |

<mode> parameter) ... <modeRGB> and <modeAlpha> must be one of | |

FUNC_ADD, FUNC_SUBTRACT, FUNC_REVERSE_SUBTRACT, MIN, or MAX as listed | |

in Table 4.1. <mode> must be one of the mode values in Table 4.1, | |

or one of the blend equations listed in tables X.1 and X.2. ... | |

(modify the third paragraph, p. 361, specifying minimum precision and | |

dynamic range for blend operations) ... Blending computations are treated | |

as if carried out in floating-point. For the equations in table 4.1, | |

blending computations will be performed with a precision and dynamic range | |

no lower than that used to represent destination components. For the | |

equations in table X.1 and X.2, blending computations will be performed | |

with a precision and dynamic range no lower than the smaller of that used | |

to represent destination components or that used to represent 16-bit | |

floating-point values as described in section 2.1.1. | |

(add unnumbered subsection prior to "Dual Source Blending and Multiple | |

Draw Buffers", p. 363) | |

Advanced Blend Equations | |

The advanced blend equations are those listed in tables X.1 and X.2. When | |

using one of these equations, blending is performed according to the | |

following equations: | |

R = f(Rs',Rd')*p0(As,Ad) + Y*Rs'*p1(As,Ad) + Z*Rd'*p2(As,Ad) | |

G = f(Gs',Gd')*p0(As,Ad) + Y*Gs'*p1(As,Ad) + Z*Gd'*p2(As,Ad) | |

B = f(Bs',Bd')*p0(As,Ad) + Y*Bs'*p1(As,Ad) + Z*Bd'*p2(As,Ad) | |

A = X*p0(As,Ad) + Y*p1(As,Ad) + Z*p2(As,Ad) | |

where the function f and terms X, Y, and Z are specified in the table. | |

The R, G, and B components of the source color used for blending are | |

considered to have been premultiplied by the A component prior to | |

blending. The base source color (Rs',Gs',Bs') is obtained by dividing | |

through by the A component: | |

(Rs', Gs', Bs') = | |

(0, 0, 0), if As == 0 | |

(Rs/As, Gs/As, Bs/As), otherwise | |

The destination color components are always considered to have been | |

premultiplied by the destination A component and the base destination | |

color (Rd', Gd', Bd') is obtained by dividing through by the A component: | |

(Rd', Gd', Bd') = | |

(0, 0, 0), if Ad == 0 | |

(Rd/Ad, Gd/Ad, Bd/Ad), otherwise | |

When blending using advanced blend equations, we expect that the R, G, and | |

B components of premultiplied source and destination color inputs be | |

stored as the product of non-premultiplied R, G, and B components and the | |

A component of the color. If any R, G, or B component of a premultiplied | |

input color is non-zero and the A component is zero, the color is | |

considered ill-formed, and the corresponding component of the blend result | |

will be undefined. | |

The weighting functions p0, p1, and p2 are defined as follows: | |

p0(As,Ad) = As*Ad | |

p1(As,Ad) = As*(1-Ad) | |

p2(As,Ad) = Ad*(1-As) | |

In these functions, the A components of the source and destination colors | |

are taken to indicate the portion of the pixel covered by the fragment | |

(source) and the fragments previously accumulated in the pixel | |

(destination). The functions p0, p1, and p2 approximate the relative | |

portion of the pixel covered by the intersection of the source and | |

destination, covered only by the source, and covered only by the | |

destination, respectively. The equations defined here assume that there | |

is no correlation between the source and destination coverage. | |

Mode Blend Coefficients | |

-------------------- ----------------------------------- | |

MULTIPLY_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = Cs*Cd | |

SCREEN_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = Cs+Cd-Cs*Cd | |

OVERLAY_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = 2*Cs*Cd, if Cd <= 0.5 | |

1-2*(1-Cs)*(1-Cd), otherwise | |

DARKEN_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = min(Cs,Cd) | |

LIGHTEN_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = max(Cs,Cd) | |

COLORDODGE_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = | |

0, if Cd <= 0 | |

min(1,Cd/(1-Cs)), if Cd > 0 and Cs < 1 | |

1, if Cd > 0 and Cs >= 1 | |

COLORBURN_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = | |

1, if Cd >= 1 | |

1 - min(1,(1-Cd)/Cs), if Cd < 1 and Cs > 0 | |

0, if Cd < 1 and Cs <= 0 | |

HARDLIGHT_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = 2*Cs*Cd, if Cs <= 0.5 | |

1-2*(1-Cs)*(1-Cd), otherwise | |

SOFTLIGHT_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = | |

Cd-(1-2*Cs)*Cd*(1-Cd), | |

if Cs <= 0.5 | |

Cd+(2*Cs-1)*Cd*((16*Cd-12)*Cd+3), | |

if Cs > 0.5 and Cd <= 0.25 | |

Cd+(2*Cs-1)*(sqrt(Cd)-Cd), | |

if Cs > 0.5 and Cd > 0.25 | |

DIFFERENCE_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = abs(Cd-Cs) | |

EXCLUSION_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = Cs+Cd-2*Cs*Cd | |

Table X.1, Advanced Blend Equations | |

When using one of the HSL blend equations in table X.2 as the blend | |

equation, the RGB color components produced by the function f() are | |

effectively obtained by converting both the non-premultiplied source and | |

destination colors to the HSL (hue, saturation, luminosity) color space, | |

generating a new HSL color by selecting H, S, and L components from the | |

source or destination according to the blend equation, and then converting | |

the result back to RGB. The HSL blend equations are only well defined | |

when the values of the input color components are in the range [0..1]. | |

In the equations below, a blended RGB color is produced according to the | |

following pseudocode: | |

float minv3(vec3 c) { | |

return min(min(c.r, c.g), c.b); | |

} | |

float maxv3(vec3 c) { | |

return max(max(c.r, c.g), c.b); | |

} | |

float lumv3(vec3 c) { | |

return dot(c, vec3(0.30, 0.59, 0.11)); | |

} | |

float satv3(vec3 c) { | |

return maxv3(c) - minv3(c); | |

} | |

// If any color components are outside [0,1], adjust the color to | |

// get the components in range. | |

vec3 ClipColor(vec3 color) { | |

float lum = lumv3(color); | |

float mincol = minv3(color); | |

float maxcol = maxv3(color); | |

if (mincol < 0.0) { | |

color = lum + ((color-lum)*lum) / (lum-mincol); | |

} | |

if (maxcol > 1.0) { | |

color = lum + ((color-lum)*(1-lum)) / (maxcol-lum); | |

} | |

return color; | |

} | |

// Take the base RGB color <cbase> and override its luminosity | |

// with that of the RGB color <clum>. | |

vec3 SetLum(vec3 cbase, vec3 clum) { | |

float lbase = lumv3(cbase); | |

float llum = lumv3(clum); | |

float ldiff = llum - lbase; | |

vec3 color = cbase + vec3(ldiff); | |

return ClipColor(color); | |

} | |

// Take the base RGB color <cbase> and override its saturation with | |

// that of the RGB color <csat>. The override the luminosity of the | |

// result with that of the RGB color <clum>. | |

vec3 SetLumSat(vec3 cbase, vec3 csat, vec3 clum) | |

{ | |

float minbase = minv3(cbase); | |

float sbase = satv3(cbase); | |

float ssat = satv3(csat); | |

vec3 color; | |

if (sbase > 0) { | |

// Equivalent (modulo rounding errors) to setting the | |

// smallest (R,G,B) component to 0, the largest to <ssat>, | |

// and interpolating the "middle" component based on its | |

// original value relative to the smallest/largest. | |

color = (cbase - minbase) * ssat / sbase; | |

} else { | |

color = vec3(0.0); | |

} | |

return SetLum(color, clum); | |

} | |

Mode Result | |

-------------------- ---------------------------------------- | |

HSL_HUE_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = SetLumSat(Cs,Cd,Cd); | |

HSL_SATURATION_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = SetLumSat(Cd,Cs,Cd); | |

HSL_COLOR_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = SetLum(Cs,Cd); | |

HSL_LUMINOSITY_KHR (X,Y,Z) = (1,1,1) | |

f(Cs,Cd) = SetLum(Cd,Cs); | |

Table X.2, Hue-Saturation-Luminosity Advanced Blend Equations | |

Advanced blending equations are supported only when rendering to a single | |

color buffer using fragment color zero. If any non-NONE draw buffer uses | |

a blend equation found in table X.1 or X.2, the error INVALID_OPERATION is | |

generated by [[Compatibility Profile: Begin or any operation that | |

implicitly calls Begin (such as DrawElements)]] [[Core Profile and OpenGL | |

ES: DrawArrays and the other drawing commands defined in section 2.8.3]] | |

if: | |

* the draw buffer for color output zero selects multiple color buffers | |

(e.g., FRONT_AND_BACK in the default framebuffer); or | |

* the draw buffer for any other color output is not NONE. | |

[[ The following paragraph applies to KHR_blend_equation_advanced only. ]] | |

When using advanced blending equations, applications should split their | |

rendering into a collection of blending passes, none of which touch an | |

individual sample in the framebuffer more than once. The results of | |

blending are undefined if the sample being blended has been touched | |

previously in the same pass. The command | |

void BlendBarrierKHR(void); | |

specifies a boundary between passes when using advanced blend equations. | |

Any command that causes the value of a sample to be modified using the | |

framebuffer is considered to touch the sample, including clears, blended | |

or unblended primitives, and BlitFramebuffer copies. | |

[[ The following paragraph applies to KHR_blend_equation_advanced_coherent | |

only. ]] | |

When using advanced blending equations, blending is typically done | |

coherently and in primitive order. When an individual sample is covered | |

by multiple primitives, blending for that sample is performed sequentially | |

in the order in which the primitives were submitted. This coherent | |

blending is enabled by default, but can be enabled or disabled by calling | |

Enable or Disable with the symbolic constant BLEND_ADVANCED_COHERENT_KHR. | |

If coherent blending is disabled, applications should split their | |

rendering into a collection of blending passes, none of which touch an | |

individual sample in the framebuffer more than once. When coherent | |

blending is disabled, the results of blending are undefined if the sample | |

being blended has been touched previously in the same pass. The command | |

void BlendBarrierKHR(void); | |

specifies a boundary between passes when using advanced blend equations. | |

Any command that causes the value of a sample to be modified using the | |

framebuffer is considered to touch the sample, including clears, blended | |

or unblended primitives, and BlitFramebuffer copies. | |

Advanced blending equations require the use of a fragment shader with a | |

matching "blend_support" layout qualifier. If the current blend equation | |

is found in table X.1 or X.2, and the active fragment shader does not | |

include the layout qualifier matching the blend equation or | |

"blend_support_all_equations", the error INVALID_OPERATION is generated by | |

[[Compatibility Profile: Begin or any operation that implicitly calls | |

Begin (such as DrawElements)]] [[Core Profile and OpenGL ES: DrawArrays | |

and the other drawing commands defined in section 2.8.3]] The set of | |

layout qualifiers supported in fragment shaders is specified in sectino | |

4.3.8.2 of the OpenGL Shading Language Specification. | |

Additions to Chapter 5 of the OpenGL 4.1 Specification (Special Functions) | |

None. | |

Additions to Chapter 6 of the OpenGL 4.1 Specification (State and | |

State Requests) | |

None. | |

Additions to Appendix A of the OpenGL 4.1 Specification (Invariance) | |

None. | |

Additions to the AGL/GLX/WGL/EGL Specifications | |

None. | |

Additions to the OpenGL Shading Language Specification, Version 4.10 | |

Including the following line in a shader can be used to control the | |

language features described in this extension: | |

#extension GL_KHR_blend_equation_advanced : <behavior> | |

where <behavior> is as specified in section 3.3. | |

A new preprocessor #define is added to the OpenGL Shading Language: | |

#define GL_KHR_blend_equation_advanced 1 | |

Modify Section 4.3.8.2, Output Layout Qualifiers, p. 47 | |

(add to the end of the section, p. 50) | |

Fragment shaders additionally support the following layout qualifiers, | |

specifying a set of advanced blend equations supported when the fragment | |

shader is used. These layout qualifiers are only permitted on the | |

interface qualifier out, and use the identifiers specified in the "Layout | |

Qualifier" column of Table X.3. | |

If a layout qualifier in Table X.3 is specified in the fragment shader, | |

the fragment shader may be used with the corresponding advanced blend | |

equation in the "Blend Equation(s) Supported" column. Additionally, the | |

special qualifier "blend_support_all_equations" indicates that the shader | |

may be used with any advanced blending equation supported by the OpenGL | |

Specification. It is not an error to specify more than one of these | |

identifiers in any fragment shader. Specifying more than one qualifier or | |

"blend_support_all_equations" means that the fragment shader may be used | |

with multiple advanced blend equations. Additionally, it is not an error | |

to specify any single any of these layout qualifiers more than once. | |

Layout Qualifier Blend Equation(s) Supported | |

------------------------ ------------------------------ | |

blend_support_multiply MULTIPLY_KHR | |

blend_support_screen SCREEN_KHR | |

blend_support_overlay OVERLAY_KHR | |

blend_support_darken DARKEN_KHR | |

blend_support_lighten LIGHTEN_KHR | |

blend_support_colordodge COLORDODGE_KHR | |

blend_support_colorburn COLORBURN_KHR | |

blend_support_hardlight HARDLIGHT_KHR | |

blend_support_softlight SOFTLIGHT_KHR | |

blend_support_difference DIFFERENCE_KHR | |

blend_support_exclusion EXCLUSION_KHR | |

blend_support_hsl_hue HSL_HUE_KHR | |

blend_support_hsl_saturation HSL_SATURATION_KHR | |

blend_support_hsl_color HSL_COLOR_KHR | |

blend_support_hsl_luminosity HSL_LUMINOSITY_KHR | |

blend_support_all_equations /all blend equations/ | |

Table X.3, Fragment Shader Output Layout Qualifiers for Blend Support | |

A draw-time error will be generated in the OpenGL API if an application | |

attempts to render using an advanced blending equation without having a | |

matching layout qualifier specified in the active fragment shader. | |

GLX Protocol | |

!!! TBD | |

Dependencies on OpenGL 4.0 | |

If OpenGL 4.0 is not supported, references to the BlendEquationi API should | |

be removed. | |

Dependencies on OpenGL 4.1 (Core Profile) | |

This extension throws an INVALID_OPERATION when Begin is called if advanced | |

blend equations are used in conjunction with multiple draw buffers. For | |

the core profile of OpenGL 4.1 (and other versions of OpenGL), there is no | |

Begin command; instead, the error is thrown by other rendering commands | |

such as DrawArrays. The language in this specification documenting the | |

error has separate versions for the core and compatibility profiles. | |

Dependencies on OpenGL 4.3 or later (any Profile) | |

References to Chapter 4 are replaced with references to Chapter 17 (Writing | |

Fragments and Samples to the Framebuffer). | |

References to section 4.1.8 are replaced with references to section 17.3.8. | |

References to Table 4.1 are replace with references to Table 17.1. | |

References to section 2.1.1 are replaced with references to section 2.3.3. | |

Dependencies on OpenGL ES 2.0 | |

If unextended OpenGL ES 2.0 is supported, references to BlendEquationi, | |

BlendEquationSeparatei, GetInteger64v, and GetDoublev should be ignored. | |

Ignore any references to multiple draw buffers if EXT_draw_buffers or | |

NV_draw_buffers is not supported. | |

Dependencies on EXT_blend_minmax | |

Requires EXT_blend_minmax on OpenGL ES 2.0 implementations and references | |

to MIN and MAX should be replace by references to MIN_EXT and MAX_EXT as | |

introduced by that extension. | |

Dependencies on OpenGL ES 3.0 | |

If unextended OpenGL ES 3.0 is supported, references to BlendEquationi, | |

BlendEquationSeparatei, and GetDoublev should be ignored. | |

Dependencies on NV_path_rendering | |

When NV_path_rendering is supported, covering geometry generated by the | |

commands CoverFillPathNV, CoverFillPathInstancedNV, CoverStrokePathNV, and | |

CoverStrokePathInstancedNV will automatically be blended coherently | |

relative to previous geometry when using the blend equations in this | |

extension. This guarantee is provided even on implementations supporting | |

only NV_blend_equation_advanced. | |

Insert the following language after the discussion of the | |

BlendBarrierKHR() command for both extensions: | |

[[ For KHR_blend_equation_advanced only: ]] | |

The commands CoverFillPathNV, CoverFillPathInstancedNV, | |

CoverStrokePathNV, and CoverStrokePathInstancedNV are considered to | |

start a new blending pass, as though BlendBarrierKHR were called prior | |

to the cover operation. If a cover primitive is followed by subsequent | |

non-cover primitives using advanced blend equations and touching the | |

same samples, applications must call BlendBarrierKHR after the cover | |

primitives to ensure defined blending results. | |

[[ For KHR_blend_equation_advanced_coherent, the language immediately | |

above should be used, but the first sentence should be prefixed with | |

"When coherent blending is disabled, ...". ]] | |

Errors | |

If any non-NONE draw buffer uses a blend equation found in table X.1 or | |

X.2, the error INVALID_OPERATION is generated by Begin or any operation | |

that implicitly calls Begin (such as DrawElements) if: | |

* the draw buffer for color output zero selects multiple color buffers | |

(e.g., FRONT_AND_BACK in the default framebuffer); or | |

* the draw buffer for any other color output is not NONE. | |

New State | |

Initial | |

Get Value Type Get Command Value Description Sec Attribute | |

-------------------- ---- ------------ ------------ ------------------------ ----- ------------ | |

BLEND_ADVANCED_ B IsEnabled TRUE are advanced blending 4.1.8 color-buffer | |

COHERENT_KHR equations guaranteed to | |

be evaluated coherently? | |

Note: The BLEND_ADVANCED_COHERENT_KHR enable is provided if and only if | |

the KHR_blend_equation_advanced_coherent extension is supported. On | |

implementations supporting only KHR_blend_equation_advanced, this enable | |

is considered not to exist. | |

New Implementation Dependent State | |

None. | |

Issues | |

Note: These issues apply specifically to the definition of the | |

KHR_blend_equation_advanced specification, which was derived from the | |

extension NV_blend_equation_advanced. The issues from the original | |

NV_blend_equation_advanced specification have been removed, but can be | |

found (as of August 2013) in the OpenGL Registry at: | |

http://www.opengl.org/registry/specs/NV/blend_equation_advanced.txt | |

(0) How does this extension differ from the NV_blend_equation_advanced | |

extension for OpenGL and OpenGL ES? | |

RESOLVED: A number of features have been removed from | |

NV_blend_equation_advanced, including: | |

* The BlendParameterivNV API has been removed, and with it, the | |

BLEND_PREMULTIPLIED_SRC_NV and BLEND_OVERLAP_NV parameters. The spec | |

has been refactored to assume premultipled source colors and | |

uncorrelated source and destination coverage. | |

* A number of less commonly used blend modes have been removed, | |

including: | |

- certain "X/Y/Z" blending modes supported by few, if any, standards | |

(INVERT, INVERT_RGB_NV, LINEARDODGE_NV, LINEARBURN_NV, | |

VIVIDLIGHT_NV, LINEARLIGHT_NV, PINLIGHT_NV, HARDMIX_NV) | |

- various versions of additive and subtractive modes (PLUS_NV, | |

PLUS_CLAMPED_NV, PLUS_CLAMPED_ALPHA_NV, PLUS_DARKER_NV, MINUS_NV, | |

MINUS_CLAMPED_NV) | |

- other uncommon miscellaneous modes (CONTRAST_NV, INVERT_OVG_NV, | |

RED, GREEN, BLUE) | |

Additionally, this extension adds blending support layout qualifiers to | |

the fragment shader (qualifying "out"). Each fragment shader can | |

specify a set of advanced blend equations that can be used when it is | |

active. For example: | |

layout(blend_support_hardlight, blend_support_softlight) out; | |

specifies that the HARDLIGHT_KHR and SOFTLIGHT_KHR equations are allowed | |

when using the shader. A draw-time error will be generated if an | |

advanced blend equation is enabled in the API and a matching layout | |

qualifier is not specified in the active fragment shader. | |

(1) What should we do about the BLEND_PREMULTIPLIED_SRC_NV blend parameter | |

from NV_blend_equation_advanced? | |

RESOLVED: Remove this parameter for simplicity. All equations in this | |

extension assume that the source and destination colors are both | |

premultiplied. | |

(2) What should we do about the BLEND_OVERLAP_NV blend parameter from | |

NV_blend_equation_advanced? | |

RESOLVED: Remove this parameter for simplicitly. All equations in this | |

extension assume an UNCORRELATED_NV overlap mode. Blending using the | |

UNCORRELATED_NV overlap mode is usually mathematically simpler than | |

blending using the DISJOINT_NV or CONJOINT_NV modes. | |

(3) What set of "complex" blending equations should we support in this | |

extension? | |

RESOLVED: During standardization of this extensions, the set of | |

equations provided in this extension has been reduced to a smaller | |

subset for simplicitly. The remaining equations are typically found in | |

a wide collection of compositing standards. In particular, the | |

standarization process removed several classes of blend equations from | |

the NV_blend_equation_advanced, as described in "differences" issue (0) | |

above. | |

(4) Should we support the "Porter-Duff" blend equations (e.g., | |

SRC_OVER_NV) from NV_blend_equation_advanced? | |

RESOLVED: All of these blend equations should be supportable in | |

unextended OpenGL ES 3.0, and have been removed for simplicity. The | |

primary rationale for this decision is to reduce the number of internal | |

paths required by the driver; some implementations may have separate | |

paths for traditional OpenGL blending and for the new advanced blending | |

equations. Redirecting "simple" advanced blending equations to | |

traditional fixed-function blending hardware may involved more driver | |

implementation work and may have different performance characteristics | |

than other "complex" blending equations. | |

This approach does mean that an application wanting to use both | |

Porter-Duff blend equations and advanced blending equations provided by | |

this extension will need GL_to program blending somewhat differently | |

when using the Porter-Duff equations: | |

if (isPorterDuff(equation)) { | |

glBlendEquation(GL_FUNC_ADD); // enable sf*S+df*D blending | |

glBlendFunc(srcFactor, dstFactor); // and program blend factors | |

} else { | |

glBlendEquation(equation); // advanced eqs. don't use BlendFunc | |

} | |

(5) Should we impose any requirements on fragment shaders when used in | |

conjunction with advanced blend equations? | |

RESOLVED: Yes. This extension adds fragment shader layout qualifiers | |

allowing individual shaders to specify that they will be used with one | |

or multiple advanced blending equations. When using an advanced | |

blending equation from this extension, it is necessary to use a fragment | |

shader with a matching layout qualifier. A draw-time error will be | |

generated if the current fragment shader doesn't include a layout | |

qualifier matching the current advanced blending mode. | |

The rationale for this decision is that some implementations of this | |

extension may require special fragment shader code when using advanced | |

blending equations, and may perhaps perform the entire blending | |

operation in the fragment shader. Knowing the set of blending equations | |

that a fragment shader will be used with at compile time may reduce the | |

extent of run-time fragment shader re-compilation when the shader is | |

used. | |

Note that NV_blend_equation_advanced doesn't include layout qualifiers | |

or the draw-time error specified here. | |

(6) How do we handle coherency when a fragment is hit multiple times? | |

RESOLVED: In this extension, blending equations will be done coherently | |

and in primitive order by default, as is the case with traditional | |

blending in OpenGL and OpenGL ES. | |

The NVIDIA extension provides two separate extension string entries: | |

* NV_blend_equation_advanced | |

* NV_blend_equation_advanced_coherent | |

Exposing the former without the latter signals that the implementation | |

can support blending with these equations, but is unable to ensure that | |

fragments are blended in order when the same (x,y) is touched multiple | |

times. To ensure coherent results and proper ordering using the | |

non-coherent version of the extension, an application must separate its | |

rendering into "passes" that touch each (x,y) at most once, and call | |

BlendBarrierNV between passes. There are important use cases (e.g., | |

many path rendering algorithms) where this limitation isn't too | |

restrictive, and NVIDIA chose to expose the non-coherent version to | |

allow the functionality to be used on a larger set of GPUs. | |

This extension is functionally comparable to an implementation of the | |

NVIDIA extension exposing both strings, where coherent behavior is | |

enabled by default. NV_blend_equation_advanced_coherent and this | |

extension both provide the ability to opt out of this automatic | |

coherence by disabling BLEND_ADVANCED_COHERENT_KHR and using | |

BlendBarrierKHR manually. This could theoretically result in higher | |

performance -- see issue (32) of the NVIDIA extension for more | |

discussion. | |

(7) How should the blend equations COLORDODGE_KHR and COLORBURN_KHR be | |

expressed mathematically? | |

RESOLVED: NVIDIA changed the definition of these equations after the | |

NV_blend_equation_advanced spec was originally published, as discussed | |

below. These changes add new special cases to the COLORDODGE_KHR and | |

COLORBURN_KHR equations that are found in newer compositing standard | |

specifications and in a number of implementations of old and new | |

standards. They believe that the omission of the special case in other | |

older specifications is a bug. They have no plans to add new blend | |

equation tokens to support "equivalent" modes without the new special | |

case. We are adopting the same approach in this extension. | |

Note, however, that older versions of this extension and older NVIDIA | |

drivers implementing it will lack these special cases. A driver update | |

may be required to get the new behavior. | |

There is some disagreement in different published specifications about | |

how these two blend equations should be handled. At the time the NVIDIA | |

extension was initially developed, all specifications they found that | |

specified blending equations mathematically (see issue 28 of | |

NV_blend_equation_advanced) were written the same way. Since then, they | |

discovered that newer working drafts of the W3C Compositing and Blending | |

Level 1 specification (for CSS and SVG) express "color-burn" as follows | |

(translated to our nomenclature): | |

if (Cd == 1) // their Cb (backdrop) is our Cd (destination) | |

f(Cs,Cd) = 1 // their B() is our f() | |

else if (Cs == 0) | |

f(Cs,Cd) = 0 | |

else | |

f(Cs,Cd) = 1 - min(1, (1-Cd)/Cs) | |

http://www.w3.org/TR/2013/WD-compositing-1-20131010/ | |

#blendingcolorburn | |

Earlier versions of the same W3C specification, an older SVG compositing | |

draft specification, the Adobe PDF specification (and the ISO 32000-1 | |

standard), and the KHR_advanced_blending extension to OpenVG all specify | |

the following equation without the initial special case: | |

if (Cs == 0) | |

f(Cs,Cd) = 0 | |

else | |

f(Cs,Cd) = 1 - min(1, (1-Cd)/Cs) | |

http://www.w3.org/TR/2012/WD-compositing-20120816/ | |

#blendingcolorburn | |

http://www.w3.org/TR/2011/WD-SVGCompositing-20110315/ | |

http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/ | |

pdfs/pdf_reference_1-7.pdf | |

http://www.khronos.org/registry/vg/extensions/KHR/ | |

advanced_blending.txt | |

For the Adobe PDF specification, the corrected blend equations are | |

published in an Adobe supplement to ISO 32000-1 and are expected to be | |

accepted in a future version of the standard: | |

http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/ | |

devnet/pdf/pdfs/adobe_supplement_iso32000_1.pdf | |

The author's understanding is that multiple shipping implementations of | |

these blending modes implement the special case for "Cd==1" above, | |

including various Adobe products and the open-source Ghostscript | |

project. | |

We believe that the extra special case in this specification is | |

consistent with the physical model of color burning. Burning is | |

described in | |

http://en.wikipedia.org/wiki/Dodging_and_burning | |

as making a print with normal exposure, and then adding additional | |

exposure to darken the overall image. In the general equation: | |

1 - min(1, (1-Cd)/Cs) | |

Cs operates as a sort of fudge factor where a value of 1.0 implies no | |

additional exposure time and 0.0 implies arbitrarily long additional | |

exposure time, where the initial amount of exposure (1-Cd) is multiplied | |

by 1/Cs and then clamped to maximum exposure by the min() operation. | |

The Cd==1 special case here implies that we get zero exposure in the | |

initial print, since 1-Cd==0. No amount of extra exposure time will | |

generate any additional exposure. This would imply that the final | |

result should have zero exposure and thus a final f() value of 1. This | |

matches the initial special case. Without that special case, we would | |

hit the second special case if Cs==0 (infinite exposure time), which | |

would yield an incorrect final value of 0 (full exposure). | |

A similar issue applies to COLORDODGE_KHR, where some specifications | |

include a special case for Cb==0 while others do not. We have added a | |

special case there as well. | |

(8) The NV_blend_equation_advanced extension has two variants: | |

NV_blend_equation_advanced and NV_blend_equation_advanced_coherent. | |

Some implementations of that extension are not capable of performing | |

fully coherent blending when samples are touched more than once | |

without a barrier, and may expose only the former. Should we follow | |

this pattern here or support only the "coherent" variant? | |

RESOLVED: Yes. The working group originally decided to support only | |

the "coherent" variant (revision 4) for simplicity but later decided to | |

support both extension string entries as some implementations on both | |

OpenGL and OpenGL ES are unable to support the "coherent" variant. | |

Applications not wanting to manage coherency manually should look for | |

the KHR_blend_equation_advanced_coherent extension and ignore | |

KHR_blend_equation_advanced. | |

(9) We don't permit the use of advanced blend equations with multiple draw | |

buffers. Should we produce compile-, link-, or draw-time errors if we | |

encounter a shader that includes both (a) one or more layout | |

qualifiers indicating that the shader wants to use advanced blending | |

and (b) a color output with a location other than zero? | |

RESOLVED: No. | |

In the current extension, there is a draw-time error generated if you | |

try to use one of the new blend equations with multiple color targets | |

(glDrawBuffers with a count > 1). With this restriction, any "extra" | |

fragment shader color outputs could never be successfully blended into | |

the framebuffer with one of these equations. | |

When only one draw buffer is enabled when using a shader with multiple | |

outputs, "extra" outputs will simply be dropped and have no effect on | |

the framebuffer. You can already do this in unextended OpenGL and | |

OpenGL ES without generating an error. We didn't feel that the value of | |

such a warning/error justifies the draw-time overhead needed to detect | |

and report such a condition. | |

Since this extension requires that you declare the intent to use | |

advanced blending using layout qualifers, it is possible to identify a | |

shader that may want to use "extra" color outputs with advanced blending | |

at compile time, with no draw-time overhead. We decided not to treat | |

this condition as an error for several reasons: | |

- Advanced blending layout qualifiers don't require that blending | |

actually be enabled. Multiple draw buffers with multiple outputs | |

work just fine in that case. | |

- If we treated this condition as an error and a future extension | |

relaxed the DrawBuffers restriction, it would be necessary to also | |

add a GLSL language feature to disable the now-undesirable error. | |

(10) What happens when converting a premultiplied color with an alpha of | |

zero to a non-premultiplied color? | |

RESOLVED: We specify that a premultiplied color of (0,0,0,0) should | |

produce non-premultiplied (R,G,B) values of (0,0,0). A premultiplied | |

color with an alpha of zero and a non-zero R, G, or B component is | |

considered to be ill-formed and will produce undefined blending results. | |

For a non-premultiplied color (R',G',B',A'), the corresponding | |

premultiplied color (R,G,B,A) should satisfy the equation: | |

(R,G,B,A) = (R'*A', G'*A', B'*A', A') | |

If the alpha of a non-premultiplied color is zero, the corresponding | |

premultiplied color (R,G,B,A) should be (0,0,0,0). | |

We specify that ill-formed premultiplied colors produce undefined | |

blending results to enable certain performance optimizations. In many | |

of these blending equations, the alpha component used as a denominator | |

to compute the non-premultiplied color ends up being multiplied by the | |

same alpha component in the coverage, resulting in cancellation. For | |

example, implementations may want to substitute a premultiplied | |

destination color into the last term of the basic blend equation: | |

R = f(Rs',Rd')*p0(As,Ad) + Y*Rs'*p1(As,Ad) + Z*Rd'*p2(As,Ad) | |

= ... + Z*Rd'*(Ad*(1-As)) | |

= ... + Z*(Rd'*Ad)*(1-As) | |

= ... + Z* Rd * (1-As) | |

This substitution would be invalid for ill-formed premultiplied | |

destination colors. We choose to specify undefined results for invalid | |

input colors rather than requiring implementations to skip such | |

optimizations or include logic to check for zero alpha values for each | |

input. | |

(11) For "HSL" blend equations, the blend equation involves a clipping | |

step where colors may be "clipped" if the blend would produce | |

components are outside the range [0,1]. Are there inputs where this | |

blend could produce ill-defined or nonsensical results? | |

RESOLVED: Yes, the results of HSL blend equations are undefined if the | |

input colors have components outside the range [0,1]. Even if the input | |

colors are in-range, the basic color adjustment done in these blends | |

could produce result components outside the range [0,1]. To compensate, | |

the ClipColor() function in the specification interpolates the result | |

color and a greyscale value that matches the luminance of the result. | |

The math for the clipping operation assumes the luminance of the result | |

color is in the range [0,1]. If that isn't the case, the clipping | |

operation could result in a divide by zero (when all result components | |

have the same out-of-bounds value) or perform an otherwise nonsensical | |

computation. | |

Revision History | |

Revision 17, February 14, 2018 | |

Fix ClipColor() equation where in the "if (maxcol > 1.0)" body the | |

"(color-lum)*lum" term should have been "(color-lum)*(1-lum)". Also | |

add new issue 11 for the case where the inputs to SetLum() are outside | |

the range [0..1] and could cause a divide-by-zero in ClipColor(). | |

Revision 16, April 16, 2016 (from a September 30, 2014 edit that wasn't | |

published) | |

Fix incorrectly specified color clamping in the HSL blend modes. | |

Revision 15, May 9, 2015 | |

Renumber as OpenGL ARB extension instead of vendor extension, by | |

symmetry with other KHR Khronos-approved extensions. Add copyright | |

notice. | |

Revision 14, March 14, 2014 | |

Cast as KHR extension. | |

Revisions 12 and 13, March 5, 2014 | |

For non-coherent blending, clarify that all writes to a sample are | |

considered to "touch" that sample and require a BlendBarrierKHR call | |

before blending overlapping geometry. Clears, non-blended geometry, and | |

copies by BlitFramebuffer or TexSubImage are all considered to "touch" a | |

sample (bug 11738). Specify that non-premultiplied values corresponding | |

to ill-conditioned premultiplied colors such as (1,1,1,0) are undefined | |

(bug 11739). Add issue (10) related to the ill-conditioned | |

premultiplied color issue. | |

Revision 11, January 30, 2014 | |

Cast as OES extension. | |

Revision 10, January 22, 2014 | |

Add issue (9), where we decided not to add compile- or link-time errors | |

when using both advanced blending and multiple color outputs (bug | |

11468). | |

Revision 9, January 2, 2014 | |

Fix typo in issue (0). | |

Revision 8, November 6, 2013 | |

Restore support for non-coherent-only implementations that was removed | |

in revision 4. Fix the language about non-coherent blending to specify | |

that results are undefined only if an individual *sample* is touched | |

more than once (instead of *pixel*). Minor language tweaks to use | |

"equations" consistently, instead of sometimes using "modes". | |

Revision 7, October 21, 2013 | |

Add a reference to the Adobe supplement to ISO 32000-1, which includes | |

the corrected equations for COLORDODGE_NV and COLORBURN_NV. Move | |

"NVIDIA Implementation Details" down a bit in the spec. | |

Revision 6, October 16, 2013 | |

Add new special cases for COLORDODGE_KHR and COLORBURN_KHR, as described | |

in issue (7). Mark issue (7) as resolved. | |

Revision 5, October 15, 2013 | |

Remove Porter-Duff blend equations from the specification (issue 4). | |

Add a Draw-time error if an advanced blending equation is used without | |

specifying a matching layout qualifier in the fragment shader (issue 5). | |

Add issues for the spec issues discussed during standardization in | |

Khronos. Remove OpenGL ES 2.0 and 3.0 interactions dealing with | |

handling tokens present in OpenGL but not the core OpenGL ES | |

specification, since the relevant equations (ZERO and XOR) have been | |

removed. | |

Revision 4, September 6, 2013 | |

Removed support for non-coherent-only implementations. Implementations | |

that could support NV_blend_equation_advanced (app-managed coherency | |

only) but not NV_blend_equation_advanced_coherent will be unable to | |

support this extension. | |

Revision 3, August 19, 2013 | |

Fixed typos in the OpenGL ES 2.0 and 3.0 interactions section of | |

NV_blend_equation_advanced. | |

Revision 2, August 13, 2013 | |

Removed issues from the original NV_blend_equation_advanced | |

specification. Rename "NV" prefixes and suffixes to "XXX" since the | |

future status of this extension is unknown. Remove the | |

BlendParameterivNV function and the BLEND_PREMULTIPLIED_SRC_NV and | |

BLEND_OVERLAP_NV parameters. Source colors are assumed to be | |

premultiplied. The source and destination pixel coverage, derived from | |

their respective alpha components, is assumed to be uncorrelated. | |

Removed the miscellaneous blend modes (PLUS_NV, PLUS_CLAMPED_NV, | |

PLUS_CLAMPED_ALPHA_NV, PLUS_DARKER_NV, MINUS_NV, MINUS_CLAMPED_NV, | |

CONTRAST_NV, INVERT_OVG_NV, RED, GREEN, BLUE) from Table X.4 of | |

NV_blend_equation_advanced. Removed some of the less common "X/Y/Z" | |

blend modes (INVERT, INVERT_RGB_NV, LINEARDODGE_NV, LINEARBURN_NV, | |

VIVIDLIGHT_NV, LINEARLIGHT_NV, PINLIGHT_NV, HARDMIX_NV). Add layout | |

qualifiers to the OpenGL Shading Language to indicate the set of | |

advanced blend equations are supported with a particular fragment | |

shader; using blend equations not identified in the current fragment | |

shader result in undefined blending results. | |

Revision 1, August 13, 2013 | |

Forked the original NV_blend_equation_advanced specification. |