Shader position vec4 or vec3

performance opengl vector glsl shader
Skides picture Skides · Sep 21, 2013 · Viewed 8.8k times · Source

I have read some tutorials about GLSL. In certain position attribute is a vec4 in some vec3. I know that the matrix operations need a vec4, but is it worth to send an additional element? Isn't it better to send vec3 and later cast in the shader vec4(position, 1.0)? Less data in memory - it will be faster? Or we should pack an extra element to avoid casting?

Any tips what should be better?

layout(location = 0) in vec4 position;
MVP*position;

or 

layout(location = 0) in vec3 position;
MVP*vec4(position,1.0);

Answer

Andon M. Coleman picture Andon M. Coleman · Sep 22, 2013

For vertex attributes, this will not matter. The 4th component is automatically expanded to 1.0 when it is absent.

That is to say, if you pass a 3-dimensional vertex attribute pointer to a 4-dimensional vector, GL will fill-in W with 1.0 for you. I always go with this route, it avoids having to explicitly write vec4 (...) when doing matrix multiplication on the position and it also avoids wasting memory storing the 4th component.

This works for 2D coordinates too, by the way. A 2D coordinate passed to a vec4 attribute becomes vec4 (x, y, 0.0, 1.0). The general rule is this: all missing components are replaced with 0.0 except for W, which is replaced with 1.0.

However, to people who are unaware of the behavior of GLSL in this circumstance, it can be confusing. I suppose this is why most tutorials never touch on this topic.

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