blob: 579afbc7d00f5fc7dce71486e8dfafef8e404a45 [file] [log] [blame]
Name Strings
Jeff Bolz, NVIDIA Corporation (jbolz 'at'
Copyright (c) 2014-2015 The Khronos Group Inc. Copyright terms at
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
Last Modified Date: May 9, 2015
Revision: 5
ARB Extension #167
This extension is written against The OpenGL 4.4 (Compatibility Profile)
This extension relaxes the restrictions on rendering to a currently
bound texture and provides a mechanism to avoid read-after-write
New Procedures and Functions
void TextureBarrier(void);
New Tokens
Additions to Chapter 9 of the OpenGL 4.4 Specification (Per-Fragment
Operations and the Frame Buffer)
Modify Section 9.3.1 Rendering Feedback Loops, p. 289
(Replace the complicated set of conditions with the following)
"Specifically, the values of rendered fragments are undefined if any
shader stage fetches texels and the same texels are written via fragment
shader outputs, even if the reads and writes are not in the same Draw
call, unless any of the following exceptions apply:
- The reads and writes are from/to disjoint sets of texels (after
accounting for texture filtering rules).
- There is only a single read and write of each texel, and the read is in
the fragment shader invocation that writes the same texel (e.g. using
"texelFetch2D(sampler, ivec2(gl_FragCoord.xy), 0);").
- If a texel has been written, then in order to safely read the result
a texel fetch must be in a subsequent Draw separated by the command
void TextureBarrier(void);
TextureBarrier() will guarantee that writes have completed and caches
have been invalidated before subsequent Draws are executed."
Additions to the AGL/GLX/WGL Specifications
New State
New Implementation Dependent State
GLX Protocol
The following rendering command is sent to the server as
a glXRender request:
2 4 rendering command length
2 4348 rendering command opcode
(1) What algorithms can take advantage of TextureBarrier?
This can be used to accomplish a limited form of programmable blending
for applications where a single Draw call does not self-intersect, by
binding the same texture as both render target and texture and applying
blending operations in the fragment shader. Additionally, bounding-box
optimizations can be used to minimize the number of TextureBarrier
calls between Draws. For example:
foreach (object in scene) {
if (dirtybbox.intersects(object.bbox())) {
dirtybbox = bound(dirtybbox, object.bbox());
Another application is to render-to-texture algorithms that ping-pong
between two textures, using the result of one rendering pass as the input
to the next. Existing mechanisms require expensive FBO Binds, DrawBuffer
changes, or FBO attachment changes to safely swap the render target and
texture. With texture barriers, layered geometry shader rendering, and
texture arrays, an application can very cheaply ping-pong between two
layers of a single texture. i.e.
X = 0;
// Bind the array texture to a texture unit
// Attach the array texture to an FBO using FramebufferTexture3D
while (!done) {
// Stuff X in a constant, vertex attrib, etc.
Draw -
Texturing from layer X;
Writing gl_Layer = 1 - X in the geometry shader;
X = 1 - X;
However, be warned that this requires geometry shaders and hence adds
the overhead that all geometry must pass through an additional program
stage, so an application using large amounts of geometry could become
geometry-limited or more shader-limited.
Revision History
Rev. Date Author Changes
---- -------- -------- -----------------------------------------
1 jbolz Initial revision.
2 mjk Assign number.
3 srahman Add glx protocol specification.
4 04/22/14 pdaniell Modify for inclusion into OpenGL 4.5
5 05/09/15 Jon Leech Add copyright statement.