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