Aligning structures to std140, CPU side

Question

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 u

Answer 1

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

• 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

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

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

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.