将结构与std140，CPU端对齐

问题描述

我想这是纯C ++问题和OpenGL问题之间的一种交叉.我有一个统一的缓冲区，并在其中分配了sizeof(ShaderData)个字节的空间.我在着色器的GPU端使用std140布局.

I suppose this is a kind-of cross between a pure C++ question and an OpenGL question. I have a uniform buffer and am allocating space in it of sizeof(ShaderData) bytes. I'm using std140 layout on the GPU side in the shader.

根据std140规则，我需要在结构的各个位置添加填充以确保矢量正确对齐.下面的结构是一个示例(针对我的观点):

According to std140 rules, I need to add padding in various places in my structure to make sure things like vectors are aligned correctly. The structure below is an example (for my light):

struct ShaderData {

    float Light_Intensity;
    float _pad1[3];    // align following vec3 on 4N boundary
    Math::Vec3f Light_Position;
    float _pad2;       // align following vec4 on 4N boundary
    Math::Colour4f Light_Ambient;
    Math::Colour4f Light_Diffuse;
    Math::Colour4f Light_Specular;
    float Light_AttenuationMin;
    float Light_AttenuationMax;

} MyShaderData;

这是人们通常在C ++中执行操作的方式吗?还是有一些特殊的关键字或实用注释来对齐结构CPU端的各个元素，使它们显得有些整洁?

Is this the way people generally do things in C++, or are there special keywords or pragmas for aligning individual elements of a structure CPU side that are a little tidier?

推荐答案

否，这样只会浪费空间.您必须根据std140规则找到优化的布局.

No, in this way you just waste space. You have to find the optimized layout according to std140 rules.

• 一个float需要4个字节，并且已对齐4个字节
• 一个vec3需要12个字节，并且已对齐16个字节
• 一个vec4需要16个字节，并且已对齐16个字节

• a float needs 4 bytes and it's 4 bytes aligned
• a vec3 needs 12 bytes and it's 16 bytes aligned
• a vec4 needs 16 bytes and it's 16 bytes aligned

这意味着您可以为您的结构找到更好的布局:

This means that you can find a better layout for your struct:

float Light_Intensity;          X
float _pad1[3];                  XXX
Math::Vec3f Light_Position;         XXX
float _pad2;                           X

您可以看到您浪费了4个字节，更糟糕的是，您只能执行以下操作:

As you can see you are wasting 4 bytes and what's worse is the fact that you can just do something like:

Math::Vec3f Light_Position      XXX
float Light_Intensity;             X

使其对齐并且无需浪费字节.之所以可行，是因为vec3将与16个字节的边界对齐，而float将仍与4个字节的边界对齐.

To have it aligned and without the need to waste a byte. This works because vec3 will be aligned to 16 bytes boundaries while the float will be still aligned to 4 bytes boundaries.

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