Explicit vs Automatic attribute location binding for OpenGL shaders

Question

When setting up attribute locations for an OpenGL shader program, you are faced with two options:

glBindAttribLocation() before linking to explicitly define an attribute location.

or

glGetAttribLocation() after linking to obtain an automatically assigned attribute location.

What is the utility for using one over the other?

And which one, if any, is preferred in practice?

Answer 1

I know one good reason to prefer explicit location definition.

Consider that you hold your geometry data in Vertex Array Objects. For a given object, you create a VAO in such way that the indices correspond to, for example:

• index 0: positions,
• index 1: normals,
• index 2: texcoords

Now consider that you want to draw one object with two different shaders. One shader requires position and normal data as input, the other - positions and texture coords.

If you compile those shaders, you will notice that the first shader will expect the positions at attribute index 0 and normals at 1. The other would expect positions at 0 but texture coords at 1.

Quoting https://www.opengl.org/wiki/Vertex_Shader:

Automatic assignment

If neither of the prior two methods assign an input to an attribute index, then the index is automatically assigned by OpenGL when the program is linked. The index assigned is completely arbitrary and may be different for different programs that are linked, even if they use the exact same vertex shader code.

This means that you wouldn't be able to use your VAO with both shaders. Instead of having one VAO per, say, object, you'd need - in the worst case - a separate VAO per object per shader.

Forcing the shaders to use your own attribute numbering convention via glBindAttribLocation can solve this problem easily - all you need to do is to keep a consistent relation between attributes and their estabilished IDs, and force the shaders to use that convention when linking.

(That's not really a big issue if you don't use separate VAOs, but still might make your code clearer.)

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BTW:

When setting up attribute locations for an OpenGL shader program, you are faced with two options

There's a third option in OpenGL/GLSL 3.3: Specify the location directly in shader code. It looks like this:

layout(location=0) in vec4 position; 

But this is not present in GLSL ES shader language.