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    shading state and parameters have been replaced by shaders/programs 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.

转载于:https://www.cnblogs.com/kylegui/p/3843809.html