Can you use glVertexAttribPointer to assign a smaller vector to a larger one?

Question

From section 5.8 of The OpenGL® ES Shading Language (v1.00, r17) [PDF] (emphasis mine):

The assignment operator stores the value of the rvalue-expression into the l-value and returns an r-value with the type and precision of the lvalue-expression. The lvalue-expression and rvalue-expression must have the same type. All desired type-conversions must be specified explicitly via a constructor.

So it sounds like doing something like this would not be legal:

vec3 my_vec3 = vec3(1, 2, 3);
vec4 my_vec4 = my_vec3;

And to make it legal the second line would have to be something like:

vec4 my_vec4 = vec4(my_vec3, 1); // add 4th component

I assumed that glVertexAttribPointer had similar requirements. That is, if you were assigning to a vec4 that the size parameter would have to be equal to 4.

Then I came across the GLES20TriangleRenderer sample for Android. Some relevant snippets:

attribute vec4 aPosition;

maPositionHandle = GLES20.glGetAttribLocation(mProgram, "aPosition");

GLES20.glVertexAttribPointer(maPositionHandle, 3, GLES20.GL_FLOAT, false,
        TRIANGLE_VERTICES_DATA_STRIDE_BYTES, mTriangleVertices);

So aPosition is a vec4, but the call to glVertexAttribPointer that's used to set it has a size of 3. Is this code correct, is GLES20TriangleRenderer relying on unspecified behavior, or is there something else I'm missing?

Answer 1

The size of the attribute data passed to the shader does not have to match the size of the attribute in that shader. You can pass 2 values (from glVertexAttribPointer) to an attribute defined as a vec4; the leftover two values are zero, except for the W component which is 1. And similarly, you can pass 4 values to a vec2 attribute; the extra values are discarded.

So you can mix and match vertex attributes with uploaded values all you want.