NV_texture_barrier
GL_NV_texture_barrier
Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com)
Shipping (August 2009, Release 190)
Last Modified Date: August 10, 2009
NVIDIA Revision: 1
Unassigned.
This extension is written against the OpenGL 3.0 specification.
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);
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
None.
| New Implementation Dependent State |
None.
(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 FramebufferTextureARB
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.
Rev. Date Author Changes
---- -------- -------- -----------------------------------------
1 jbolz Initial revision.
1 jbolz Initial revision.
 |  | Last update: November 14, 2006.
Cette page doit être lue avec un navigateur récent respectant le standard XHTML 1.1.
|
