opengl - Why do different variations of glVertexAttribPointer exist?

Question

There are

glVertexAttribPointer()
glVertexAttribIPointer()
glVertexAttribLPointer()

As far as I know, glVertexAttribPointer can be used instead of the other two.

If so, why do the I and L variations exist?

Answer 1

I read about this in OpenGL Insights

When using glVertexAttribPointer() everything gets cast to a float. While glVertexAttribIPointer() can only expose vertex arrays that store integers and glVertexAttribLPointer() is only for doubles.

As confirmed by a quote on this OpenGL.org page:

For glVertexAttribPointer, if normalized? is set to GL_TRUE?, it indicates that values stored in an integer format are to be mapped to the range [-1,1] (for signed values) or [0,1] (for unsigned values) when they are accessed and converted to floating point. Otherwise, values will be converted to floats directly without normalization.

For glVertexAttribIPointer, only the integer types GL_BYTE?, GL_UNSIGNED_BYTE?, GL_SHORT?, GL_UNSIGNED_SHORT?, GL_INT?, GL_UNSIGNED_INT? are accepted. Values are always left as integer values.

glVertexAttribLPointer specifies state for a generic vertex attribute array associated with a shader attribute variable declared with 64-bit double precision components. type? must be GL_DOUBLE?. index?, size?, and stride? behave as described for glVertexAttribPointer and glVertexAttribIPointer.