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Name
NV_texture_barrier
Name Strings
GL_NV_texture_barrier
Contact
Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com)
Contributors
Mark Kilgard, NVIDIA
Shazia Rahman, NVIDIA
Status
Shipping (August 2009, Release 190)
Version
Last Modified Date: September 29, 2016
NVIDIA Revision: 4
Number
OpenGL Extension #381
OpenGL ES Extension #271
Dependencies
This extension is written against the OpenGL 3.0 specification.
Also written based on the wording of the OpenGL ES 3.2 specification.
Overview
This extension relaxes the restrictions on rendering to a currently
bound texture and provides a mechanism to avoid read-after-write
hazards.
New Procedures and Functions
void TextureBarrierNV(void);
New Tokens
None.
Additions to Chapter 2 of the OpenGL 3.0 Specification (OpenGL Operation)
None.
Additions to Chapter 3 of the OpenGL 3.0 Specification (Rasterization)
None.
Additions to Chapter 4 of the OpenGL 3.0 Specification (Per-Fragment
Operations and the Frame Buffer)
Modify Section 4.4.3, Rendering When an Image of a Bound Texture Object
is Also Attached to the Framebuffer, p. 288
(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 TextureBarrierNV(void);
TextureBarrierNV() will guarantee that writes have completed and caches
have been invalidated before subsequent Draws are executed.
Additions to Chapter 5 of the OpenGL 3.0 Specification (Special Functions)
None.
Additions to Chapter 6 of the OpenGL 3.0 Specification (State and
State Requests)
None.
Additions to the AGL/GLX/WGL Specifications
None
Additions to Chapter 9 of the OpenGL ES 3.2 Specification (Framebuffers
and Framebuffer Objects)
Modify section 9.3.1, Rendering Feedback Loops:
(Replace the complicated 2nd and 3rd paragraphs
"Specifically... ...only be executed conditionally." 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 TextureBarrierNV(void);
TextureBarrierNV() will guarantee that writes have completed and caches
have been invalidated before subsequent Draws are executed.
Errors
New State
None.
New Implementation Dependent State
None.
GLX Protocol
The following rendering command is sent to the server as
a glXRender request:
TextureBarrierNV
2 4 rendering command length
2 4348 rendering command opcode
Issues
(1) What algorithms can take advantage of TextureBarrierNV?
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 TextureBarrierNV
calls between Draws. For example:
dirtybbox.empty();
foreach (object in scene) {
if (dirtybbox.intersects(object.bbox())) {
TextureBarrierNV();
dirtybbox.empty();
}
object.draw();
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;
TextureBarrierNV();
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.
(2) Does this support OpenGL ES?
RESOLVED: Yes. ES specification language has been added, written
against the OpenGL 3.2 specification. The added language is
identical to the regular OpenGL language.
As this specification has no dependencies other than assuming
framebuffer objects, this extension could support any version of ES
from 2.0 up. However the texelFetch operation for fetching from a
texture is introduced by OpenGL ES 3.0's GLSL or the NV_gpu_shader4
extension.
Revision History
Rev. Date Author Changes
---- -------- -------- -----------------------------------------
1 jbolz Initial revision.
2 mjk Assign number.
3 srahman Add glx protocol specification.
4 9/29/16 mjk Add ES support