OpenGL.GL.NV.shader_buffer_load

OpenGL extension NV.shader_buffer_load

This module customises the behaviour of the OpenGL.raw.GL.NV.shader_buffer_load to provide a more Python-friendly API

Overview (from the spec)

At a very coarse level, GL has evolved in a way that allows applications to replace many of the original state machine variables with blocks of user-defined data. For example, the current vertex state has been augmented by vertex buffer objects, fixed-function shading state and parameters have been replaced by shaders/programs and constant buffers, etc.. Applications switch between coarse sets of state by binding objects to the context or to other container objects (e.g. vertex array objects) instead of manipulating state variables of the context. In terms of the number of GL commands required to draw an object, modern applications are orders of magnitude more efficient than legacy applications, but this explosion of objects bound to other objects has led to a new bottleneck - pointer chasing and CPU L2 cache misses in the driver, and general L2 cache pollution.

This extension provides a mechanism to read from a flat, 64-bit GPU address space from programs/shaders, to query GPU addresses of buffer objects at the API level, and to bind buffer objects to the context in such a way that they can be accessed via their GPU addresses in any shader stage.

The intent is that applications can avoid re-binding buffer objects or updating constants between each Draw call and instead simply use a VertexAttrib (or TexCoord, or InstanceID, or...) to "point" to the new object's state. In this way, one of the cheapest "state" updates (from the CPU's point of view) can be used to effect a significant state change in the shader similarly to how a pointer change may on the CPU. At the same time, this relieves the limits on how many buffer objects can be accessed at once by shaders, and allows these buffer object accesses to be exposed as C-style pointer dereferences in the shading language.

As a very simple example, imagine packing a group of similar objects' constants into a single buffer object and pointing your program at object <i> by setting "glVertexAttribI1iEXT(attrLoc, i);" and using a shader as such:

struct MyObjectType { mat4x4 modelView; vec4 materialPropertyX; // etc. }; uniform MyObjectType *allObjects; in int objectID; // bound to attrLoc

...

mat4x4 thisObjectsMatrix = allObjects[objectID].modelView; // do transform, shading, etc.

This is beneficial in much the same way that texture arrays allow choosing between similar, but independent, texture maps with a single coordinate identifying which slice of the texture to use. It also resembles instancing, where a lightweight change (incrementing the instance ID) can be used to generate a different and interesting result, but with additional flexibility over instancing because the values are app-controlled and not a single incrementing counter.

Dependent pointer fetches are allowed, so more complex scene graph structures can be built into buffer objects providing significant new flexibility in the use of shaders. Another simple example, showing something you can't do with existing functionality, is to do dependent fetches into many buffer objects:

GenBuffers(N, dataBuffers); GenBuffers(1, &pointerBuffer);

GLuint64EXT gpuAddrs (i = 0; i < N; ++i) { BindBuffer(target, dataBuffers[i ); BufferData(target, size myData[i , STATIC_DRAW);

// get the address of this buffer and make it resident. GetBufferParameterui64vNV(target, BUFFER_GPU_ADDRESS, gpuaddrs[i]); MakeBufferResidentNV(target, READ_ONLY); }

GLuint64EXT pointerBufferAddr; BindBuffer(target, pointerBuffer); BufferData(target, sizeof(GLuint64EXT)*N, gpuAddrs, STATIC_DRAW); GetBufferParameterui64vNV(target, BUFFER_GPU_ADDRESS, &pointerBufferAddr); MakeBufferResidentNV(target, READ_ONLY);

// now in the shader, we can use a double indirection vec4 **ptrToBuffers = pointerBufferAddr; vec4 *ptrToBufferI = ptrToBuffers[i];

This allows simultaneous access to more buffers than EXT_bindable_uniform (MAX_VERTEX_BINDABLE_UNIFORMS, etc.) and each can be larger than MAX_BINDABLE_UNIFORM_SIZE.

The official definition of this extension is available here: http://www.opengl.org/registry/specs/NV/shader_buffer_load.txt