OpenGL EXT: shader_buffer_load
http://www.opengl.org/registry/specs/NV/shader_buffer_load.txt
Overview
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shading state and parameters have been replaced by shaders/programs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[N];
for (i = 0; i < N; ++i) {
BindBuffer(target, dataBuffers[i]);
BufferData(target, size[i], 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.
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