Do I need to take care to pack vertex attributes together?

Question

If I want to pass two nominally independent attribute arrays of floats to a draw call, can I happily have a GLSL in float variable for each of them, or do I need to ensure to pack them into an in vec2 or similar and use the various components to ensure not consuming unnecessary GL_MAX_VERTEX_ATTRIBS "slots"?

Or, in other words; GL_MAX_VERTEX_ATTRIBS specifies, according to the docs, "the maximum number of 4-component generic vertex attributes accessible to a vertex shader". Does an attribute that is less than 4 components always count as one attribute towards this limit?

Answer 1

Does an attribute that is less than 4 components always count as one attribute towards this limit?

Yes, that is exactly what it means.

Each vertex attribute is 4-component, if you write it as float in your shader that actually does not change anything. If you want to see this in action, try setting up a 1-component vertex attribute pointer and then declaring that attribute vec4 in your vertex shader -- GL will automatically assign the values 0.0, 0.0, 1.0 for y, z and w respectively.

If you are hitting the vertex attribute limit (minimum 16) because you're using a bunch of scalars, then you should consider packing your attributes into a vec4 instead for optimal utilization.

There is a minor exception to this rule I described above, for data types with more than 4-components (e.g. mat4). A vertex attribute declared mat4 has 16-components and consumes 4 sequential attribute locations.