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Name | |

ARB_shading_language_packing | |

Name Strings | |

GL_ARB_shading_language_packing | |

Contact | |

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

Contributors | |

Barthold Lichtenbelt, NVIDIA | |

Chris Dodd, NVIDIA | |

Daniel Koch, Transgaming | |

Eric Werness, NVIDIA | |

Greg Roth, NVIDIA | |

Jeff Bolz, NVIDIA | |

Piers Daniell, NVIDIA | |

Notice | |

Copyright (c) 2011-2013 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 Working Group. 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 | |

Status | |

Complete. Approved by the ARB on 2011/06/20. | |

Approved by the Khronos Promoters on 2011/07/29. | |

Version | |

Last Modified Date: 06/13/2011 | |

Revision: 5 | |

Number | |

ARB Extension #116 | |

Dependencies | |

This extension is written against version 4.1 (revision 09) of the OpenGL | |

Shading Language Specification. | |

Overview | |

This extension provides the GLSL built-in functions to convert a 32-bit | |

unsigned integer holding a pair of 16-bit floating-point values to or from | |

a two-component floating-point vector (vec2). | |

This mechanism allows GLSL shaders to read and write 16-bit floating-point | |

encodings (via 32-bit unsigned integers) without introducing a full set of | |

16-bit floating-point data types. | |

This extension also adds the GLSL built-in packing functions included in | |

GLSL version 4.00 and the ARB_gpu_shader5 extension which pack and unpack | |

vectors of small fixed-point data types into a larger scalar. By putting | |

these packing functions in this separate extension it allows | |

implementations to provide these functions in hardware that supports them | |

independent of the other ARB_gpu_shader5 features. | |

In addition to the packing functions from ARB_gpu_shader5 this extension | |

also adds the missing [un]packSnorm2x16 for completeness. | |

New Procedures and Functions | |

None. | |

New Tokens | |

None. | |

Modifications to the OpenGL API Specification | |

None. | |

Modifications to The OpenGL Shading Language Specification, Version 4.10 | |

(Revision 6) | |

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

language features described in this extension: | |

#extension GL_ARB_shading_language_packing : <behavior> | |

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

New preprocessor #defines are added to the OpenGL Shading Language: | |

#define GL_ARB_shading_language_packing 1 | |

Modify Section 8.4, Floating-Point Pack and Unpack Functions | |

(add support for general packing functions) | |

Syntax: | |

uint packUnorm2x16(vec2 v); | |

uint packSnorm2x16(vec2 v); | |

uint packUnorm4x8(vec4 v); | |

uint packSnorm4x8(vec4 v); | |

vec2 unpackUnorm2x16(uint v); | |

vec2 unpackSnorm2x16(uint v); | |

vec4 unpackUnorm4x8(uint v); | |

vec4 unpackSnorm4x8(uint v); | |

The functions packUnorm2x16(), packSnorm2x16(), packUnorm4x8(), and | |

packSnorm4x8() first convert each component of a two- or four-component | |

vector of normalized floating-point values into 8- or 16-bit integer | |

values. Then, the results are packed into a 32-bit unsigned integer. | |

The first component of the vector will be written to the least | |

significant bits of the output; the last component will be written to | |

the most significant bits. | |

The functions unpackUnorm2x16(), unpackSnorm2x16(), unpackUnorm4x8(), | |

and unpackSnorm4x8() first unpacks a single 32-bit unsigned integer into | |

a pair of 16-bit unsigned integers, a pair of 16-bit signed integers, | |

four 8-bit unsigned integers, or four 8-bit signed integers. Then, each | |

component is converted to a normalized floating-point value to generate a | |

two- or four-component vector. The first component of the vector will be | |

extracted from the least significant bits of the input; the last | |

component will be extracted from the most significant bits. | |

The conversion between fixed- and normalized floating-point values will be | |

performed as below. | |

function conversion | |

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

packUnorm2x16 fixed_val = round(clamp(float_val, 0, +1) * 65535.0); | |

packSnorm2x16 fixed_val = round(clamp(float_val, -1, +1) * 32767.0); | |

packUnorm4x8 fixed_val = round(clamp(float_val, 0, +1) * 255.0); | |

packSnorm4x8 fixed_val = round(clamp(float_val, -1, +1) * 127.0); | |

unpackUnorm2x16 float_val = fixed_val / 65535.0; | |

unpackSnorm2x16 float_val = clamp(fixed_val / 32767.0, -1, +1); | |

unpackUnorm4x8 float_val = fixed_val / 255.0; | |

unpackSnorm4x8 float_val = clamp(fixed_val / 127.0, -1, +1); | |

(add support for 16-bit floating-point packing and unpacking functions) | |

Syntax: | |

uint packHalf2x16(vec2 v); | |

vec2 unpackHalf2x16(uint v); | |

The function packHalf2x16() returns an unsigned integer obtained by | |

converting the components of a two-component floating-point vector to the | |

16-bit floating-point representation found in the OpenGL Specification, | |

and then packing these two 16-bit integers into a 32-bit unsigned integer. | |

The first vector component specifies the 16 least-significant bits of the | |

result; the second component specifies the 16 most-significant bits. | |

The function unpackHalf2x16() returns a two-component floating-point | |

vector with components obtained by unpacking a 32-bit unsigned integer into a | |

pair of 16-bit values, interpreting those values as 16-bit floating-point | |

numbers according to the OpenGL Specification, and converting them to | |

32-bit floating-point values. The first component of the vector is | |

obtained from the 16 least-significant bits of v; the second | |

component is obtained from the 16 most-significant bits of v. | |

GLX Protocol | |

None. | |

Errors | |

None. | |

New State | |

None. | |

New Implementation Dependent State | |

None. | |

Issues | |

(1) What should this extension be called? | |

RESOLVED: Proposed name is "ARB_shading_language_pack2h" (i.e., pack | |

two half components). | |

(2) This functionality is also provided in the NV_gpu_shader5 extension. | |

Why provide a separate extension? | |

RESOLVED: This extension provides an additional pack/unpack function on | |

top of those added in GLSL 4.00 without requiring that implementors | |

provide the entirety of NV_gpu_shader5. | |

(3) Should we provide a different set of function names to disambiguate | |

from the similar NV_gpu_shader5 versions (which use the "f16vec2" type to | |

represent a pair of fp16 values)? If not, what happens if both this | |

extension and NV_gpu_shader5 are enabled? | |

RESOLVED: The functions in this extension will be called "packHalf2x16" | |

and "unpackHalf2x16", which matches the convention of the other | |

pack/unpack functions in GLSL 4.10 closely enough. For example, | |

packUnorm2x16() packs a pair of 16-bit unsigned normalized integers into | |

a 32-bit unsigned integer. | |

Had we used the same names as in NV_gpu_shader5, there would be a | |

conflict in the definitions -- the unpacked type here is "vec2", while | |

the unpacked type in NV_gpu_shader5 is "f16vec2" (a two-component vector | |

with explicit 16-bit floating-point types). This conflict poses a | |

problem if both extensions are enabled, and would also arise if future | |

GLSL versions chose to add explicit fp16 types. For the pack functions, | |

an implementation supporting both versions could resolve via function | |

overloading: | |

uint packFloat2x16(vec2 v); | |

uint packFloat2x16(f16vec2 v); | |

Unfortunately, function overloading is not possible for the return type | |

of the pack function. One could resolve this by providing only: | |

f16vec2 unpackFloat2x16(uint v); | |

If a shader included code like: | |

vec2 unpacked = unpackFloat2x16(uint_value); | |

the unpacked value would be returned as f16vec2 data. However, that | |

could be implicitly converted to vec2 if implicit conversions from fp16 | |

to fp32 were supported. Such behavior would be consistent with what we | |

did for ARB_gpu_shader_fp64, where fp32 values can be implicitly | |

converted to fp64. | |

(4) The packing functions introduced in ARB_gpu_shader5 may be available | |

on hardware that doesn't support the other features from ARB_gpu_shader5. | |

How can an implementation expose those packing functions? | |

RESOLVED: Added all the ARB_gpu_shader5 packing functions to revision | |

#5 of this spec; [un]packUnorm2x16, [un]packUnorm4x8 and | |

[un]packSnorm4x8. | |

(5) Should we also provide a [un]packSnorm2x16 packing function, which | |

was not included in ARB_gpu_shader5? | |

RESOLVED: Yes, these functions have been added for completeness. They | |

have been added to revision #5 of this extension and the extension has | |

been renamed from _pack2h to _packing to encompass all these packing | |

functions. | |

Revision History | |

Rev. Date Author Changes | |

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

5 06/13/11 pdaniell Rename the spec to ARB_shading_language_packing | |

and incorporate all the packing functions into | |

this spec, including the [un]packSnorm2x16 | |

functions, which have no extension spec. | |

4 12/17/10 johnk Put in the pack/unpack section instead of | |

common functions. Change a reference to double. | |

3 12/12/2010 pbrown Rename the pack/unpack functions to avoid | |

conflict with NV_gpu_shader5 names (bug | |

6858). Renamed last part of extension name | |

from "pack2f" to "pack2h" to match the | |

function name change. | |

2 11/08/2010 pbrown Fix minor errata in the first draft (bug | |

6999). | |

1 10/20/2010 pbrown Internal revision. |