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Name Strings
Aske Simon Christensen
Sandeep Kakarlapudi
Jan-Harald Fredriksen (jan-harald.fredriksen 'at'
Revision 12
Last Modified Date: November 25, 2013
OpenGL ES Extension #165
OpenGL ES 2.0 or higher is required.
ESSL 1.00 or higher is required.
This extension is written against the OpenGL ES Shading Language
specification, Language Version 1.00, Document Revision 17 and revision
OpenGL ES 2.0.25 of the API specification.
This extension enables fragment shaders to read existing framebuffer
data as input. This permits use-cases such as programmable blending,
and other operations that may not be possible to implement with
fixed-function blending.
This extension also adds the ability to indicate that a shader should
be run once per sample instead of once per pixel.
Reading framebuffer data as input in combination with multiple render
targets (MRT) may not be supported by all implementations. This
extension allows applications to query for this capability.
New Procedures and Functions
New Tokens
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
and by the <pname> parameter of GetBooleanv, GetIntegerv, and GetFloatv:
Accepted by the <pname> parameter of GetBooleanv, GetIntegerv, and
New Macro Definitions
#define GL_ARM_shader_framebuffer_fetch 1
New Built-in Variables
mediump vec4 gl_LastFragColorARM
Changes to the OpenGL ES 2.0.25 Specification, Chapter 3
Remove the last sentence of Paragraph 2 of Chapter 3.8.1, page 86 ("These
built-in varying variables include [...]" and add:
"These built-in varying variables include the fragment's position, eye z
coordinate, and front-facing flag, as well as the current color value in the
When reading the current color value from the framebuffer, the values
associated with the image attached to color attachment point 0 are returned.
Reading the current color value from the framebuffer may not be supported on
all hardware if more than one color attachment has an image attached. This
capability can be determined by calling GetBooleanv with the symbolic
shaders that read the current value from the framebuffer when more than one
color attachment point has an image attached produce undefined results.
Reading the current color value from the framebuffer is only supported for
fixed-point color components. Undefined results are produced if a shader
reads from gl_LastFragColorARM while either no image is attached to color
attachment 0 or the image attached to color attachment point 0 has a format
that is not unsigned normalized fixed-point. No error is generated in this
Add to Chapter 3.2 Multisampling:
"Per-sample fetch can be used to specify that reads of current values from
the framebuffer, colors and other associated data, including varying
interpolation, should be evaluated for each sample. Per-sample fetch
is controlled by by calling Enable or Disable with the symbolic constant
If SAMPLE_BUFFERS is not one, or the fragment shader does not statically
access current values from the framebuffer, per-sample fetch has no effect."
Additions to Chapter 3 of the OpenGL ES Shading Language Specification
Remove Paragraph 2 of section 3.8, page 17, Identifiers ("Identifiers
starting with "gl_" are reserved [...]") and add:
"Identifiers starting with "gl_" are reserved for use by OpenGL ES, and
may not be declared in a shader as either a variable or a function.
However, as noted in the specification, certain predeclared "gl_" names
are allowed to be redeclared in a shader for the specific purpose of
changing their precision qualifier."
Additions to Chapter 7 of the OpenGL ES Shading Language Specification
In section 7.2 (Fragment Shader Special Variables), after the
8th paragraph ("If the shader executes the discard keyword,") and before
the paragraph on about gl_FragCoord, add:
"The fragment shader has access to the read-only built-in
variable gl_LastFragColorARM. The value of this variable is the
color of the pixel to which the current fragment is destined, i.e.,
the color that will be used as the destination color during blending,
for draw buffer 0.
If the current render target is multisampled, and the destination
pixel thus contains more than one sample per fragment, the value of
gl_LastFragColorARM is an implementation-dependent combination of the
samples within the destination pixel that are covered by the current
fragment. The value will be between the minium and maximum value of the
samples in the pixel.
If the current GL state would cause the destination color to be
converted from sRGB to linear at input to blending, then the color read
from the framebuffer is converted from sRGB to linear before going into
gl_LastFragColorARM. If the destination pixel contains more than one
sample, this conversion is applied to each color sample prior to the
If no samples within the destination pixel are covered by the current
fragment, the value of gl_LastFragColorARM is undefined.
If more than one color attachment has an image attached, reads from
gl_LastFragColorARM, may produce undefined results. This is, however,
not an error. See section 3.8.1 "Shader Variables" of the OpenGL ES
2.0.25 Graphics System Specification for more details.
gl_LastFragColorARM is declared with a default precision qualifier.
This can be changed by redeclaring the variable with the desired
precision qualifier. Redeclarations must be at global scope and must
not otherwise alter the declared type of the variable.
Reads from gl_LastFragColorARM must wait for the processing of all
previous fragments destined for the current pixel to complete. For best
performance, it is therefore recommended that reads from this built-in
variable is done as late in the execution of the fragment shader as
Access to gl_LastFragColorARM is optional, and must be enabled by:
#extension GL_ARM_shader_framebuffer_fetch : <behavior>
Where <behavior> is as specified in section 3.4."
In section 7.2 (Fragment Shader Special Variables), at the end of
the list of built-in variables, add:
"mediump vec4 gl_LastFragColorARM"
New State
Add to table 6.6 (Multisampling):
Get Value Get Command Type Initial Value Description
--------- ----------- ---- ------------- ---------
FETCH_PER_SAMPLE_ARM IsEnabled B FALSE Per-sample fetch enabled
New Implementation Dependent State
Add to table 6.19 (Implementation Dependent Values (cont.)):
Get Value Type Get Command Minimum Value Description Section
--------- ---- ----------- ------------- -------------- -------
FRAGMENT_SHADER_FRAMEBUFFER_FETCH_MRT_ARM B GetBooleanv - Reading existing 3.8.1
framebuffer color data
from a fragment
shader when more than
one color attachment
point has an image attached
gives defined results.
(1) What should the built-in variables be called?
In the current implementation, it is called gl_FBColor, but
since we need to change that anyway for the public version (to
add the ARM suffix), we could change the name completely if we
come up with something better.
The current proposal is to use gl_LastFragColorARM as in
NV_shader_framebuffer_fetch with an added extension post-fix.
This could then be extended to include
gl_LastFragDataARM[gl_MaxDrawBuffers] as in EXT_framebuffer_fetch in a
future extension.
(2) What should the precision of gl_LastFragColorARM be?
Is it usually appropriate for the variable to be mediump (or
perhaps lowp), but that precludes the mechanism from being
used effectively with float32 render targets. float32 render targets
are not required in either OpenGL ES 2.0 or OpenGL ES 3.0.
gl_LastFragColor is currently defined as mediump by default, but the
precision can be redeclared in the shader in the same manner as in
(3) What should the precision of gl_LastFragDepthARM be?
No longer relevant for this extension as the depth buffer support
has been split out to a separate extension.
(4) Which framebuffer formats are supported?
All UNORM formats are supported. No other formats are supported.
Since gl_LastFragColor is a built-in variable, with a given
type, other types are not trivial to add.
(5) Should there be a query for the valid framebuffer formats?
If only some formats are supported, the application needs some
way to determine which formats are supported and which are not.
A) Specify the exact set of formats in the extension.
B) Add a query, for example something based on the internal format
queries in OpenGL ES 3.0.
Given the resolution of Issue 4, alternative A is effectively
(6) What performance recommendations should the extension contain?
There is currently a recommendation to place the framebuffer
read as late as possible in the shader.
(7) Should gl_LastFragStencil and gl_LastFragDepth be split into
separate extensions?
Yes. This is is now added by ARM_shader_framebuffer_fetch_depth_stencil.
(8) Should shaders that read the current fragment color, depth, or stencil
be run per-sample?
The EXT_framebuffer_fetch extension automatically runs the parts of the
shader that depend on gl_LastFragData per sample if this variable is
read. In some use-cases (e.g., tone-mapping), this is important to
avoid shader aliasing. This approach is, however, not possible to
implement on all hardware.
An alternative is to allow the application to control the shader
iteration rate. This could be done similarly to the API part of
The latter approach is taken by this extension.
(9) Should the value read back include only the samples covered by the
current fragment?
Yes, only the samples covered by the current fragment will be read back.
The alternative would be to ignore the coverage mask when returning the
value, but this seems less desirable.
If the shader is run per sample (see Issue 8) both options would
give the same result.
(10)How is this different from EXT_shader_framebuffer_fetch?
The core functionality is the same, that is, fragment shaders may read
existing framebuffer data as input.
The two main differences are:
* This extension places the mechanism for running the shader per sample
under application control. It is not possible to run only those parts
of the shader that depend on the current framebuffer color per
* This extension may not be compatible with multiple render targets on
all hardware.
(11) What is meant by undefined results in this extension?
Reads from gl_LastFragColorARM may return undefined results in some
cases as described in the text. This means that there is no guarantees
on the exact value returned in these cases. The values will typically
be a GPU specific 'default' value, or correspond to the API clear value.
It is guaranteed that these values will never originate from other GL
contexts or applications.
Revision History
Revision 12, 25/11/2013 (Jan-Harald Fredriksen)
Added and resolved issue 11.
Revision 11, 26/09/2013 (Jan-Harald Fredriksen)
Restricting redeclarations of built-in variables to global scope.
Revision 10, 15/07/2013 (Jan-Harald Fredriksen)
Minor clarification.
Revision 9, 10/07/2013 (Jan-Harald Fredriksen)
Renaming SAMPLE_SHADING_ARM to FETCH_PER_SAMPLE_ARM and restricting it
to shaders that use fetch to avoid confusion with possible per per-
sample shading extensions in the future.
Revision 8, 02/05/2013 (Jan-Harald Fredriksen)
Separated out depth and stencil functionality.
Changing MRT interactions to depend on the number of color attachments
that have images attached to them.
Removing error condition when reading from non-fixed-point color
Revision 7, 19/04/2013 (Jan-Harald Fredriksen)
Language clarifications.
Revision 6, 12/04/2013 (Jan-Harald Fredriksen)
Removing explicit references to specific GPUs.
Resolved issues 4 and 5.
Revision 5, 11/04/2013 (Jan-Harald Fredriksen)
Allowing the new built-in variables to be redeclared for purposes of
redefining their precision qualifier.
Revision 4, 10/04/2013 (Jan-Harald Fredriksen)
Clarifying ESSL language about how reads are done when multisampling is
Revision 3, 09/04/2013 (Jan-Harald Fredriksen)
Tentatively resolved issue 8 and 9. Added issue 10.
Revision 2, 08/10/2012 (Jan-Harald Fredriksen)
Added issue 9.
Revision 1, 22/07/2012 (Jan-Harald Fredriksen)
First draft based on ARM_framebuffer_read.