ARM_shader_framebuffer_fetch

GL_ARM_shader_framebuffer_fetch

Aske Simon Christensen
Sandeep Kakarlapudi

Jan-Harald Fredriksen (jan-harald.fredriksen 'at' arm.com)

Shipping.

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.

None

Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
and by the <pname> parameter of GetBooleanv, GetIntegerv, and GetFloatv:
    FETCH_PER_SAMPLE_ARM                                0x8F65

Accepted by the <pname> parameter of GetBooleanv, GetIntegerv, and
GetFloatv:
    FRAGMENT_SHADER_FRAMEBUFFER_FETCH_MRT_ARM           0x8F66

#define GL_ARM_shader_framebuffer_fetch 1

mediump vec4 gl_LastFragColorARM

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
framebuffer.

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
constant FRAGMENT_SHADER_FRAMEBUFFER_FETCH_MRT_ARM. If FALSE is returned,
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
case.

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
FETCH_PER_SAMPLE_ARM.

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."

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."

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
    averaging.

    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
    possible.

    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"

None

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

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?

    RESOLVED.

    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?

    RESOLVED.

    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
    EXT_shader_framebuffer_fetch.

(3) What should the precision of gl_LastFragDepthARM be?

    RESOLVED.

    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?

    RESOLVED.

    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?

    RESOLVED.

    If only some formats are supported, the application needs some
    way to determine which formats are supported and which are not.

    Alternatives:
      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
    chosen.

(6) What performance recommendations should the extension contain?

    RESOLVED.

    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?

    RESOLVED.

    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?

    RESOLVED.

    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
    ARB_sample_shading.

    The latter approach is taken by this extension.

(9) Should the value read back include only the samples covered by the
    current fragment?

    RESOLVED.

    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?

    RESOLVED.

    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
       sample.
     * This extension may not be compatible with multiple render targets on
       all hardware.

(11) What is meant by undefined results in this extension?

    RESOLVED.

    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 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
    attachments.

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)
    Adding FRAGMENT_SHADER_FRAMEBUFFER_FETCH_DRAW_BUFFERS_ARM.
    Clarifying ESSL language about how reads are done when multisampling is
    enabled.

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.