如何查询SSBO结构的对齐方式/步幅? [英] How do I query the alignment/stride for an SSBO struct?

问题描述

我不确定哪种结构布局最适合我的应用程序:shared，packed，std140，std430.我并不是要对每种情况进行解释，因为这些信息很容易找到，很难弄清楚每种情况对供应商兼容性/性能的影响.如果shared是默认值，我怀疑这是一个很好的起点.

I'm not sure which structure layout is most suited for my application: shared, packed,std140, std430. I'm not asking for an explanation of each, that information is easy to find, it's just hard to figure out the impact each will have on vendor compatibility/performance. If shared is the default, I'm suspecting that's a good starting point.

据我所知，在使用shared或packed时，我必须查询对齐方式/偏移量，因为它是特定于实现的.

From what I can gather, I have to query the alignment/offsets when using shared or packed, because it's implementation specific.

用于查询它的API是什么?在绑定计算着色器时，是否有一些参数传递给glGetShaderiv，让我可以找出对齐方式?

What's the API for for querying it? Is there some parameter I pass to glGetShaderiv while the compute shader is bound, that lets me figure out the alignments?

推荐答案

使用GL_SHADER_STORAGE_BLOCK nofollow noreferrer> glGetProgramInterface 来获取 着色器存储缓冲区对象和最大名称长度.
缓冲区变量的最大名称长度可以从程序接口GL_BUFFER_VARIABLE中获取:

Use glGetProgramInterface with the parameter GL_SHADER_STORAGE_BLOCK to get the number of the Shader Storage Buffer Objects and the maximum name length.
The maximum name length of the buffer variables can be get from the program interface GL_BUFFER_VARIABLE:

GLuint prog_obj; // shader program object

GLint no_of, ssbo_max_len, var_max_len;
glGetProgramInterfaceiv(prog_obj, GL_SHADER_STORAGE_BLOCK, GL_ACTIVE_RESOURCES, &no_of);
glGetProgramInterfaceiv(prog_obj, GL_SHADER_STORAGE_BLOCK, GL_MAX_NAME_LENGTH, &ssbo_max_len);
glGetProgramInterfaceiv(prog_obj, GL_BUFFER_VARIABLE, GL_MAX_NAME_LENGTH, &var_max_len);

可以通过> 和 glGetProgramResourceIndex :

The name of the SSBO can be get by glGetProgramResourceName and a resource index by glGetProgramResourceIndex:

std::vector< GLchar >name( max_len );
for( int i_resource = 0; i_resource < no_of; i_resource++ ) {

    // get name of the shader storage block
    GLsizei strLength;
    glGetProgramResourceName(
        prog_obj, GL_SHADER_STORAGE_BLOCK, i_resource, ssbo_max_len, &strLength, name.data());

    // get resource index of the shader storage block
    GLint resInx = glGetProgramResourceIndex(prog_obj, GL_SHADER_STORAGE_BLOCK, name.data());

    // [...]
}

可以通过 glGetProgramResource 来检索着色器存储块的数据. .另请参见程序自省.

从程序接口，GL_SHADER_STORAGE_BLOCK和着色器存储块资源resInx获取缓冲区变量的数量及其索引:

Get the number of of buffer variables and its indices from program interface and GL_SHADER_STORAGE_BLOCK and the shader storage block resource resInx:

for( int i_resource = 0; i_resource < no_of; i_resource++ ) {

    // [...]

    GLint resInx = ...

    // get number of the buffer variables in the shader storage block
    GLenum prop = GL_NUM_ACTIVE_VARIABLES;
    GLint num_var;
    glGetProgramResourceiv(
        prog_obj, GL_SHADER_STORAGE_BLOCK, resInx, 1, &prop,
        1, nullptr, &num_var);

    // get resource indices of the buffer variables
    std::vector<GLint> vars(num_var);
    prop = GL_ACTIVE_VARIABLES;
    glGetProgramResourceiv(
        prog_obj, GL_SHADER_STORAGE_BLOCK, resInx,
        1, &prop, (GLsizei)vars.size(), nullptr, vars.data());

    // [...]
}

从程序接口GL_BUFFER_VARIABLE和资源索引vars[]获取相对于缓冲区的基数及其名称的缓冲区变量的偏移量(以基本机器单位为单位):

Get the offsets of the buffer variables, in basic machine units, relative to the base of buffer and its names from the program interface GL_BUFFER_VARIABLE and the resource indices vars[]:

for( int i_resource = 0; i_resource < no_of; i_resource++ ) {

    // [...]

    std::vector<GLint> offsets(num_var);
    std::vector<std::string> var_names(num_var);
    for (GLint i = 0; i < num_var; i++) {

        // get offset of buffer variable relative to SSBO
        GLenum prop = GL_OFFSET;
        glGetProgramResourceiv(
            prog_obj, GL_BUFFER_VARIABLE, vars[i],
            1, &prop, (GLsizei)offsets.size(), nullptr, &offsets[i]);

        // get name of buffer variable
        std::vector<GLchar>var_name(var_max_len);
        GLsizei strLength;
        glGetProgramResourceName(
            prog_obj, GL_BUFFER_VARIABLE, vars[i], 
            var_max_len, &strLength, var_name.data());
        var_names[i] = var_name.data();
    }

    // [...]
}

另请参见 ARB_shader_storage_buffer_object

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