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Name
EXT_shader_pixel_local_storage2
Name Strings
GL_EXT_shader_pixel_local_storage2
Contributors
Jan-Harald Fredriksen, ARM
Sandeep Kakarlapudi, ARM
James Glanville, Imagination Technologies
Tobias Hector, Imagination Technologies
Contact
Tobias Hector (tobias.hector 'at' imgtec.com)
Status
Complete
Version
Revision 0.13
Last Modified Date: October 28, 2015
Number
OpenGL ES Extension #253
Dependencies
OpenGL ES 3.0 and GLSL ES 3.0 are required.
EXT_shader_pixel_local_storage is required.
This extension has interactions with EXT_color_buffer_float and
EXT_color_buffer_half_float
This extension is written against the OpenGL ES Shading Language
specification, Language Version 3.00, Document Revision 4 and revision
OpenGL ES 3.0.2 of the API specification.
Overview
This extension builds on EXT_shader_pixel_local_storage by lifting the
restriction that pixel local storage is not supported when rendering to
multiple draw buffers.
Moreover, pixel local storage values are no longer lost when writing to
user-defined fragment outputs, and, correspondingly, framebuffer pixel
values do not always become undefined when the shader writes to pixel local
storage.
This extension adds the following capabilities:
- support for pixel local storage in combination with multiple user-
defined fragment outputs
- support for clearing pixel local storage variables
- support for multi-word pixel local storage variables
New Procedures and Functions
void FramebufferPixelLocalStorageSizeEXT(uint target, sizei size);
sizei GetFramebufferPixelLocalStorageSizeEXT(uint target);
void ClearPixelLocalStorageuiEXT(sizei offset, sizei n, const uint *values);
New Tokens
Accepted by the <pname> parameters of GetBooleanv, GetIntegerv,
GetInteger64v, or GetFloatv:
MAX_SHADER_COMBINED_LOCAL_STORAGE_FAST_SIZE_EXT 0x9650
MAX_SHADER_COMBINED_LOCAL_STORAGE_SIZE_EXT 0x9651
Returned by CheckFramebufferStatus:
FRAMEBUFFER_INCOMPLETE_INSUFFICIENT_SHADER_COMBINED_LOCAL_STORAGE_EXT 0x9652
New Macro Definitions
#define GL_EXT_shader_pixel_local_storage2 1
New Built-in Variables
None
Changes to the OpenGL ES 3.0 Specification, Chapter 3
In Section 3.9.2, at the end of the last sub-section ("Shader Outputs"),
modify the three paragraphs added by GL_EXT_shader_pixel_local_storage,
such that it reads:
"Fragment data values may also be written to pixel local storage blocks.
These values are available for reading in subsequent shader invocations
covering the same pixel. Data values written to pixel local storage block
members are converted to the storage format specified in the shader.
If GL_EXT_shader_pixel_local_storage2 is supported a shader may write
to both user-defined fragment outputs and to pixel local storage blocks.
If GL_EXT_shader_pixel_local_storage2 is not supported and a shader
writes to any user-defined fragment output, the pixel local storage values
for that fragment are lost, and their values in subsequent shader
invocations are undefined. Similarly, if a shader writes to pixel local
storage blocks, the value of the framebuffer pixel covered by that fragment
becomes undefined."
Changes to the OpenGL ES 3.0 Specification, Chapter 4
In Section 4.1.7 ("Blending"), modify the paragraph added by GL_EXT_shader-
_pixel_local_storage to read:
"Blending only applies to user-defined fragment outputs. No blending is
performed for outputs to pixel local storage blocks. If the fragment
shader outputs only to pixel local storage blocks, proceed to the next
operation."
In Section 4.1.9 ("Dithering), modify the paragraph added by GL_EXT_shader-
_pixel_local_storage to read:
"Dithering only applies to user-defined fragment outputs. No dithering
is performed on outputs to pixel local storage blocks."
In Section 4.2.3 ("Clearing the Buffers") add the following:
"The command
void ClearPixelLocalStorageuiEXT(sizei offset, sizei n, const uint *values);
clears a consecutive region of the pixel local storage to a specified set
of values. <offset> is the start offset of the region in 32-bit words. <n>
is the number of 32-bit words to clear. <values> is an array of <n> 32-bit
words that specifies the values the pixel local storage should be cleared
to. The words in <n> are packed with the first component in the least
significant bits of the word. The most significant bit of each component is
packed in the most significant bit location of its location in the word. If
<values> is NULL, all pixel local storage in the supplied range has every
bit set to 0.
Clearing the pixel local storage will clear all values stored in the
specified range. This will have the effect of making the value of any user-
defined fragment output in this range undefined. Applications can determine
the fragment output or pixel local storage variables the range
corresponds to by the locations declared in the shader and by the storage
sizes described in section 4.3.8.2 ("Output Layout Qualifiers") of the
OpenGL ES Shading Language Specification. Further, an implementation is
required to allocate variables in storage the following order: pixel local
variables first, then fragment outputs.
When ClearPixelLocalStorageuiEXT is called, no per-fragment operations or
mask operations are applied.
An INVALID_VALUE error will be generated if the sum of <offset> and <n>
exceeds the total amount pixel local storage available to a shader. An
INVALID_OPERATION error will be generated if ClearPixelLocalStorageuiEXT is
called while pixel local storage is disabled."
Replace Section 4.4.3 ("Enabling pixel local storage") with the following:
"4.4.3 Enabling pixel local storage
Fragment shaders have access to pixel local storage blocks, but this access
must be enabled prior to use and disabled after use.
Pixel local storage for the current draw framebuffer is enabled by calling
Enable with SHADER_PIXEL_LOCAL_STORAGE_EXT.
The content of the pixel local storage for a pixel is initially undefined.
The contents of the pixel local storage persist until color data is flushed
to the framebuffer. After such an event, data in the pixel local storage
is lost and the contents are undefined. Events that cause a flush include:
* calling the GL commands Flush, Finish, and ClientWaitSync
* calling commands such as TexSubImage2D, CopyTexSubImage2D, and
BlitFramebuffer to update a texture that is also attached to the current
draw framebuffer while pixel local storage is enabled
* disabling pixel local storage by calling Disable with SHADER_PIXEL_-
LOCAL_STORAGE_EXT.
If pixel local storage is not enabled, an INVALID_OPERATION error will be
generated if any rendering command is issued while a program object that
accesses pixel local storage is bound.
While pixel local storage is enabled, an INVALID_OPERATION error will be
generated if any of the current draw framebuffer's attachment points are
modified, including changes to the underlying storage backing of objects
attached to these attachment points. An INVALID_OPERATION error will also
be generated on attempts to bind a different framebuffer object, to delete
the currently bound draw framebuffer, or change color buffer selection via
DrawBuffers while pixel local storage is enabled.
Pixel local storage is not supported in combination with multisample
rasterization. Attempting to enable pixel local storage while the value of
SAMPLE_BUFFERS is one will generate an INVALID_OPERATION error.
An INVALID_FRAMEBUFFER_OPERATION error will be generated when attempting to
enable pixel local storage while the current draw framebuffer is incomplete.
The maximum number of bytes of pixel local storage available to a shader is
specified by the value of the implementation-dependent constant MAX_-
SHADER_PIXEL_LOCAL_STORAGE_SIZE_EXT. A compile-time error will be generated
if an attempt is made to utilize more than the space available for pixel
local storage variables. An implementation may choose to subdivide the
amount of pixel local storage into a region for fast access and a region
for normal access. As many pixel local storage variables as possible will
be stored, in order of declaration, in the fast region before any variables
will be allocated in the normal region. The number of bytes available for
fast access is specified by the value of the implementation-dependent
constant MAX_SHADER_PIXEL_LOCAL_STORAGE_FAST_SIZE_EXT. This value will
always be less than or equal to the total amount of pixel local storage.
The maximum number of bytes of combined storage per pixel available to a
shader is specified by the value of the implementation-dependent constant
MAX_SHADER_COMBINED_LOCAL_STORAGE_SIZE_EXT. This value is the combined
storage of color attachments and pixel local storage. A compile-time error
will be generated if an attempt is made to utilize more than the combined
space for local storage. An INVALID_OPERATION error is generated by any
command that draws geometry if the combined storage required for the
current draw framebuffer and the pixel local storage of the current program
is greater than this value. As for the total amount of pixel local storage,
an implementation may choose to subdivide the amount of combined storage
into a region for fast access and a region for normal access. The number of
bytes available for fast access is specified by the value of the
implementation-dependent constant MAX_SHADER_COMBINED_LOCAL_STORAGE_FAST_-
SIZE_EXT. This value will always be less than or equal to the total amount
of combined storage."
Pixel local storage is disabled by calling Disable with SHADER_PIXEL_-
LOCAL_STORAGE_EXT.
In the initial state, SHADER_PIXEL_LOCAL_STORAGE_EXT is disabled.
If EXT_shader_pixel_local_storage2 is to be used, then the amount of pixel
local storage must be known to the framebuffer before pixel local storage
is enabled, similar to a framebuffer attachment.
The command
void FramebufferPixelLocalStorageSizeEXT(uint target, sizei size);
specifies the amount of storage required for pixel local variables whilst
SHADER_PIXEL_LOCAL_STORAGE_EXT is enabled. <target> specifies the
framebuffer that is modified by this function. <size> determines the size,
in bytes, required for pixel local storage. An INVALID_VALUE error is
generated if <size> is greater than MAX_SHADER_PIXEL_LOCAL_STORAGE_SIZE_-
EXT, or if it is not a multiple of 4. An INVALID_OPERATION error is
generated if an application tries to call this function whilst SHADER_-
PIXEL_LOCAL_STORAGE_EXT is enabled.
The amount of storage required for pixel local variables can be
determined by the storage sizes described in section 4.3.8.2 ("Output
Layout Qualifiers") of the OpenGL ES Shading Language Specification.
If the default framebuffer is bound to <target>, then calling this
function does not affect framebuffer completeness.
The command
sizei GetFramebufferPixelLocalStorageSizeEXT(uint target);
returns the size of pixel local storage previously set on the framebuffer
bound to <target> by FramebufferPixelLocalStorageSizeEXT."
Add the following to the list of framebuffer complete conditions in
sub-section "Whole Framebuffer Completeness" in section 4.4.4:
" - The amount of pixel local storage specified by
FramebufferPixelLocalStorageSizeEXT is zero, or the combined size of
pixel local storage and fragment outputs is less than MAX_SHADER_-
COMBINED_LOCAL_STORAGE_SIZE_EXT.
{ FRAMEBUFFER_INCOMPLETE_INSUFFICIENT_SHADER_COMBINED_LOCAL_STORAGE_EXT }"
Add the following to the list of functions that can affect framebuffer
completeness sub-section "Whole Framebuffer Completeness" in section 4.4.4:
" - Changing the size of shader pixel local storage with
FramebufferPixelLocalStorageSizeEXT."
Errors
Remove the following error added in EXT_shader_pixel_local_storage:
INVALID_OPERATION is generated if the application attempts to enable pixel
local storage while the current draw framebuffer is a user-defined frame-
buffer object and has an image attached to any color attachment other than
color attachment zero.
Add the following errors:
INVALID_VALUE is generated by ClearPixelLocalStorageuiEXT if the sum of
<offset> and <n> exceeds the total amount pixel local storage available to a
shader.
INVALID_OPERATION is generated if ClearPixelLocalStorageuiEXT is called while
pixel local storage is disabled.
INVALID_OPERATION is generated by any command that draws geometry if the
combined storage required for the current draw framebuffer and the pixel
local storage of the current program is is greater than the value of MAX_-
SHADER_COMBINED_LOCAL_STORAGE_SIZE_EXT.
New State
Add to Table 6.13 Framebuffer (state per framebuffer object)
State Type Get Command Initial Value Description Sec.
--------------- ---- ------------ ------------- ----------- -----
Z+ GetFramebuffer- 0 Amount of pixel 4.4.3
PixelLocal- local storage
StorageSizeEXT specified for
the framebuffer.
New Implementation Dependent State
Add to Table 6.32 Implementation Dependent Fragment Shader Limits
State Type Get Command Minimum Value Description Sec.
--------------- ---- ------------ ------------- ----------- -----
MAX_SHADER_COMBINED- Z+ GetIntegerv 16 Amount of fast 4.4.3
LOCAL_STORAGE_FAST- storage in units
_SIZE_EXT of bytes available
for per-pixel
storage.
MAX_SHADER_COMBINED- Z+ GetIntegerv 16 Amount of total 4.4.3
LOCAL_STORAGE_SIZE- storage in units
_EXT of bytes available
for per-pixel
storage.
Additions to Chapter 4 ("Variables and Types") of the OpenGL ES Shading Language Specification
Replace section "4.3.7 Pixel Local Variables" with the following:
"4.3.7 Pixel Local Variables
The __pixel_localEXT, __pixel_local_inEXT, and __pixel_local_outEXT,
qualifiers are used to declare variables whose values are persistent across
fragment shader invocations covering the same pixel, collectively referred
to as pixel local variables. Pixel local variables do not have any backing
store allocated through the OpenGL API and are not accessible to the API.
Variables declared with the __pixel_localEXT qualifier can be read and
written from the same fragment shader invocation. Variables declared with
the __pixel_local_inEXT qualifier can only be read. Variables declared
with the __pixel_local_outEXT qualifier can only be written.
Pixel local storage variable reads and writes within a single shader
invocation are processed in order.
It is legal for a shader to write to both user-defined fragment outputs and
pixel local storage variables.
Pixel local storage variables may not have initializers and their contents
are undefined until written to from a shader or initialized via the API. If
a pixel local storage variable is not written to in a fragment shader, the
value of that variable is undefined unless it was declared as part of a
__pixel_localEXT block.
Pixel local storage variables may be qualified with layout qualifiers
affecting how the values are stored in and retrieved from the underlying
storage, as described in section 4.3.8.4 "Pixel Local Block Layout
Qualifiers".
When reading from a pixel local storage variable, the in-storage value is
implicitly converted from the storage format specified by the layout
qualifier to the variable type. Similarly, when writing to a pixel local
storage variable, the value of the member is implicitly converted to the
storage format specified by the layout qualifier.
Pixel local storage variables may only be declared inside interface blocks
(section 4.3.7, "Interface Blocks"), which are then referred to as shader
pixel local storage blocks. It is a compile-time error to declare pixel
local storage variables at global scope (outside a block).
Pixel local storage blocks must be declared at global scope.
Pixel local storage variables declared inside pixel local storage
blocks will be laid out in local storage in monotonically increasing order
based on their location in the declaration. All pixel local variables
consume integer multiples of 4 bytes, as specified in section 4.3.8.4.
Pixel local storage blocks may be allocated from the same storage as any
user-defined color outputs, but do not alias. Writing to pixel local
variables has no effect on any user-defined outputs, and pixel local
variables are not affected by writes to any user-defined outputs.
A shader may only declare a single input and a single output pixel local
storage block. A pixel local storage block declared using the __pixel_-
localEXT qualifier is counted as both an input and an output block. Thus,
it is a compile-time error for a shader to declare more than one pixel
storage block, with the exception that it is legal to declare one pixel
local storage block using the __pixel_local_inEXT qualifier and one with
the __pixel_local_outEXT qualifier.
Modify the start of Section 4.3.7 (Interface Blocks) to read:
"Uniform and pixel local storage variable declarations can be grouped into
named interface blocks to provide coarser granularity backing than is
achievable with individual declarations. They can have an optional instance
name, used in the shader to reference their members. A uniform block is
backed by the application with a buffer object. A block of pixel local
storage variables is not backed by any object.
GLSL ES 3.0 does not support interface blocks for shader inputs or outputs.
An interface block is started by a uniform or pixel local keyword, followed
by a block name, followed by an open curly brace ( { ) as follows:
interface-block:
layout-qualifieropt interface-qualifier block-name { member-list } instance-nameopt;
interface-qualifier:
in
out
uniform
__pixel_localEXT
__pixel_local_inEXT
__pixel_local_outEXT
"
Modify the sentence:
"Repeating the uniform interface qualifier for a member's storage qualifier
is optional."
To read:
"Repeating the uniform, __pixel_localEXT, __pixel_local_inEXT, or
__pixel_local_outEXT interface qualifier for a member's storage qualifier
is optional."
Add a new paragraph after the one starting with:
"For uniform blocks, the application uses the block name to identify the
block."
That reads:
"For __pixel_localEXT, __pixel_local_inEXT, and __pixel_local_outEXT storage
blocks, the block name is not used."
In section 4.3.8.2, "Output Layout Qualifiers", add the following to the
list of layout qualifiers applicable to fragment outputs only:
layout-qualifier-id
float-output-format-qualifier
int-output-format-qualifier
uint-output-format-qualifier
generic-output-format-qualifier
float-pixel-local-format-qualifier
rgb565
r8
rg8
rgb8
rgba8
srgb8_a8
rgb5_a1
rgba4
rgb10_a2
[[ Only if EXT_color_buffer_float is supported ]]
r32f
rg32f
r11f_g11f_b10f
rgba32f
[[ Only if EXT_color_buffer_half_float or EXT_color_buffer_float is supported ]]
r16f
rg16f
rgba16f
[[ Only if EXT_color_buffer_half_float is supported ]]
rgb16f
int-pixel-local-format-qualifier
r8i
r16i
r32i
rg8i
rg16i
rg32i
rgba8i
rgba16i
rgba32i
uint-pixel-local-format-qualifier
r8ui
r16ui
r32ui
rg8ui
rg16ui
rg32ui
rgba8ui
rgb10_a2ui
rgba16ui
rgba32ui
generic-output-format-qualifier
implementation_defined
Add the following paragraphs to the end of this section:
"The output format qualifiers are optional and must be enabled by calling
#extension GL_EXT_shader_pixel_local_storage2: <behavior>
before use, where <behavior> is as specified in section 3.4.
Each output-format-qualifier specifies the format which each of the
user-defined fragment outputs is stored at the end of shader execution.
The values are packed to the specified format and stored as raw bits in
fragment output storage.
By default, the format for each output is set to "implementation_defined",
and the underlying implementation will make a decision about how to
store it. If any output is set to a format other than implementation_-
defined, all formats must be specified.
The output-format-qualifier format must match the base type and the
number of components of the variable declaration. It is a compile-time
error to declare a user-defined output variable where the format qualifier
does not match the variable type and the number of components.
If EXT_pixel_local_storage2 is enabled, the format of every user-defined
fragment output variable should be specified in order to use pixel local
storage blocks. Any outputs set as implementation_defined (the default)
behave as if consuming all available local storage, leaving no space for
pixel local variables, causing a compile time error if pixel local storage
blocks also exist in the shader.
The specified output-format-qualifier does not need to be constant
between shader invocations, but any resolves to the final framebuffer will
treat the data as being of the format specified on the active frame-
buffer's color attachment rather than the shader defined format. This
implies that color attachment values are undefined if the format of the
color attachment does not match the format specified in the shader. The
number of user-defined outputs and the size specified by their format
qualifier must remain consistent between shader invocations, or the values
will be undefined.
Each user-defined fragment output variable uses memory local to the
shading processor. Each format takes up a predetermined number of bytes
of storage, as specified in the table below.
|----------------|-------|
| Format | Bytes |
|----------------|-------|
| r8 | 4 |
| r8ui | 4 |
| r8i | 4 |
| r16ui | 4 |
| r16i | 4 |
| r32ui | 4 |
| r32i | 4 |
| rg8 | 4 |
| rg8ui | 4 |
| rg8i | 4 |
| rg16ui | 4 |
| rg16i | 4 |
| rg32ui | 8 |
| rg32i | 8 |
| rgb8 | 4 |
| rgb565 | 4 |
| rgba8 | 4 |
| srgb8_a8 | 4 |
| rgb5_a1 | 4 |
| rgba4 | 4 |
| rgb10_a2 | 4 |
| rgba8ui | 4 |
| rgba8i | 4 |
| rgb10_a2ui | 4 |
| rgba16ui | 8 |
| rgba16i | 8 |
| rgba32ui | 16 |
| rgba32i | 16 |
| r32f | 4 |
| rg32f | 8 |
| r11f_g11f_b10f | 4 |
| rgba32f | 16 |
| r16f | 4 |
| rg16f | 4 |
| rgb16f | 8 |
| rgba16f | 8 |
|----------------|-------|
The total amount of memory used by a pixel local storage block or a user-
defined fragment output can be determined by the size of each variable's
format as in the equation below. n is the number of pixel local storage
members or output variables in each case.
n
/_ i = sizeof(format-qualifier)
i=0
The total combined amount of memory used by fragment outputs and pixel
local storage can be worked out simply via the following:
total memory = sizeof(pixel local storage) + sizeof(fragment outputs)"
The total number of bytes of fragment output local storage available is
specified by the value of the implementation-dependent constant
gl_MaxShaderCombinedLocalStorageSizeEXT. A compile-time error will be
generated if the declared outputs use more storage than this value.
An implementation may choose to subdivide the amount of local storage into
a region for fast access and a region for normal access. As many local
variables as possible are allocated into the fast access region before any
are allocated into the normal access. Pixel local variables are
prioritized and are fully allocated into combined local storage before any
fragment output variables are allocated. The number of total bytes
available for fast access is specified by the value of the implementation-
dependent constant gl_MaxShaderCombinedLocalStorageFastSizeEXT. This value
will always be less than or equal to the total amount of combined local
storage."
In Section 4.3.8.4 ("Pixel Local Block Layout Qualifiers"), add the following
(add the following to the list of layout qualifiers for pixel local storage
variables)
float-pixel-local-format-qualifier
rg32f
rgba32f
rgba16f
int-pixel-local-format-qualifier
rgba16i
r32i
rg32i
rgba32i
uint-pixel-local-format-qualifier
rg32ui
rgba16ui
rgba32ui
(add to the end of the description of the pixel local block layout qualifiers)
"The size of each pixel local format qualifier is equivalent to that
specified for the output format qualifiers in section 4.3.8.2."
Additions to Chapter 6 of the OpenGL ES Shading Language Specification
In Section 6.4 (Jumps), change the sentence about the discard keyword that
currently reads:
'This keyword causes the fragment to be discarded and no updates to the
framebuffer will occur.'
to:
'This keyword causes the fragment to be discarded and no updates to the
framebuffer or any pixel local storage variables will occur.'
Additions to Chapter 7 of the OpenGL ES Shading Language Specification
In Section 7.3 (Built-In Constants), add new entries:
const mediump int gl_MaxShaderCombinedLocalStorageFastSizeEXT = 16
const mediump int gl_MaxShaderCombinedLocalStorageSizeEXT = 16
Examples
(1) G-buffer creation pass
#version 300 es
#extension GL_EXT_shader_pixel_local_storage2 : enable
varying vec2 coord;
varying vec3 normal;
uniform sampler2D albedo_metallicness_texture;
uniform sampler2D roughness_texture;
uniform float roughness;
uniform float albedo;
uniform vec3 metallicness;
__pixel_localEXT FragDataLocal {
layout (r32f) highp float depth;
layout (rgba8ui) highp vec4 normal_roughness;
layout (rgba8ui) highp vec4 albedo_metallicness;
} gbuf;
void main()
{
// Write to pls values
gbuf.depth = gl_FragCoord.z;
gbuf.normal_roughness = vec4(normal, texture2d(roughness_texture).r);
gbuf.albedo_metallicness = texture2d(albedo_metallicness_texture, coord);
}
(2) Light accumulation step (multiple lights can hit the same pixel)
#version 300 es
#extension GL_EXT_shader_pixel_local_storage2 : enable
uniform vec4 light_position;
__pixel_localEXT FragDataLocal {
layout (r32f) highp float depth;
layout (rgba8ui) highp vec4 normal_roughness;
layout (rgba8ui) highp vec4 albedo_metallicness;
} gbuf;
layout(location = 0, rgba10_a2) out highp vec4 accumulationBuffer;
void main()
{
// Accumulate to the accumulationBuffer, without invalidating the pls data
accumulationBuffer = do_lighting(gbuf.depth, gbuf.normal_roughness, gbuf.albedo_metallicness, light_position);
}
Issues
(1) Should there be a way to dumping the PLS contents to memory for
debugging purposes?
RESOLVED: Not in this extension.
A couple of ways this could be supported:
A) Reuse ReadPixels. Allow a new combination of <format> and <type>
parameters, say, "SHADER_PIXEL_LOCAL_STORAGE" and UNSIGNED_INT.
B) Add a new function, say glReadPixelLocalStorage. This would be
as the above, except that the <format> and <type> parameters
would be implicit.
Either approach would probably need a query function to determine
the amount of storage used, or a way to specify the amount of storage
to retrieve.
(2) How are local storage values initialized?
RESOLVED: Using ClearPixelLocalStorageuiEXT.
EXT_shader_pixel_local_storage stated:
"All pixel local storage variables are guaranteed to be zero if
all color components of the framebuffer are set to zero."
But with this extension, color data and pixel local storage are
distinct, so while this guarantee could be made, it may not be the
most efficient approach.
This is solved by adding an explicit API: ClearPixelLocalStorageuiEXT.
(3) Is the proposed resolution to (2) a compatibility break with EXT_-
shader_pixel_local_storage?
RESOLVED: No.
EXT_shader_pixel_local_storage guaranteed that the pixel local
storage was initialized to zero if all color components of the
framebuffer were set to zero. Given that this only applied to the
aliased pixel local storage in the original extension, and aliasing
is no longer present, the resolution is not a compatibility break,
as the spec itself is a compatibility break.
(4) Do we need to know the render target count at compile time?
RESOLVED.
Yes. Implementations will need to know the render target count and
the storage size of each render target in order to allocate and
partition the pixel local storage.
(5) Do we want to explicitly alias some pixel local storage variables onto
color rendertargets?
RESOLVED.
See Issue 7.
(6) Should the maximum PLS storage queries be per framebuffer?
RESOLVED: No - glGetInteger with a MAX_COMBINED_LOCAL_STORAGE_SIZE.
Dynamic, per framebuffer, queries will have all necessary information,
but applications can not access this information until the framebuffer
object has been created, which seems undesirable.
EXT_shader_pixel_local_storage specified static queries for the
amount of available pixel local storage. With this extension, the
semantics of that query changes since color and pixel local storage
no longer alias. Given that change, we need a query for the
total amount of storage available for pixel local storage and color
attachments.
(7) Do we want to keep the aliasing of pixel local storage variables onto
color rendertargets?
RESOLVED: No
The main benefit of aliasing was that it potentially allows more (fast)
local storage. Instead of sharing the local storage between color
render targets and pixel local storage variables, all local storage can
be allocated to pixel local storage initially. In this case, we expect
all pixel local storage to be resolved (and become undefined) when any
user-defined color output is written to (and vice versa).
However, given the existance (and support) of framebuffer fetch, it
seems unnecessary to continue using this - the same effect can be
achieved with side-by-side framebuffer fetch and pixel local storage,
without the drawback of requiring explicit resolves.
(8) Do we want the full range of framebuffer formats to work with this
extension? If so, how do we define the sizes of, say rgb565? Does it
take up 16bits or is it assumed to be padded to 32 bits?
RESOLVED: Add all formats, but storage for all formats is padded
to multiples of 32-bits.
(9) Do the usual per-fragment operations apply to color outputs in this
extension?
RESOLVED: Yes. All values written to user-defined color outputs pass
through per-fragment operations (including blending, alpha to coverage,
etc.) as normal.
Revision History
Revision 0.13, 28/10/2015 (Tobias Hector)
Added enumerant values
Revision 0.12, 08/10/2015 (Tobias Hector)
Added PLS interactions with the discard keyword.
Revision 0.11, 07/10/2015 (Tobias Hector)
Specified that PLS variables are undefined if they're not written to,
unless they were defined as both input and output (in which case they
are preserved).
Revision 0.10, 13/07/2015 (Tobias Hector)
Re-added error about multisampled rendering
Revision 0.9, 10/07/2015 (Tobias Hector)
Corrected ClearPixelLocalStorageuiEXT to accept a uint value, instead of int
Changed language about combined local storage to be clearer.
Revision 0.8, 03/07/2015 (Tobias Hector)
Added wording that disallows the default framebuffer becoming incomplete.
Added language allowing NULL to be passed into ClearPixelLocalStorageuiEXT
Added word alignment to size parameter of FramebufferPixelLocalStorageSizeEXT
Revision 0.7, 02/07/2015 (Tobias Hector)
Folded in updates from external review.
Added interactions with EXT_color_buffer_float and half_float
Added multi-word pixel local storage formats
Added framebuffer pixel local storage size specification
Added framebuffer completeness checks
Updated issues list to match updated spec.
Revision 0.6, 03/10/2014 (Jan-Harald Fredriksen)
Extended padding for all color outputs to 4 bytes.
Resolved and updated proposed resolutions for several issues.
Added Issue 10 and Issue 11.
Revision 0.5, 25/09/2014 (Jan-Harald Fredriksen)
Integrated a subset of IMG_fragment_output_format.
Revision 0.4, 20/06/2014 (Jan-Harald Fredriksen)
Minor wording changes.
Revision 0.3, 27/05/2014 (Jan-Harald Fredriksen)
Adding Issue 7.
Revision 0.2, 18/05/2014 (Jan-Harald Fredriksen)
Adding some issues and a new query.
Revision 0.1, 21/03/2014 (Jan-Harald Fredriksen)
Second internal draft.
Adding missing changes compared to EXT_shader_pixel_local_storage.
Added proposed resolution of Issue 1.
Added Issue 2, Issue 3, and Issue 4.
Revision 0, 19/11/2013 (Jan-Harald Fredriksen)
First internal draft.