CODEWITHSUNDEEP
c++openglglslshader
luksen
luksen

Reputation: 67

How does OpenGL differentiate binding points in VAO from ones defined with glBindBufferBase?

I am writing a particle simulation which uses OpenGL >= 4.3 and came upon a "problem" (or rather the lack of one), which confuses me.
For the compute shader part, I use various GL_SHADER_STORAGE_BUFFERs which are bound to binding points via glBindBufferBase().
One of these GL_SHADER_STORAGE_BUFFERs is also used in the vertex shader to supply normals needed for rendering. The binding in both the compute and vertex shader GLSL (these are called shaders 1 below) looks like this:

OpenGL part:

glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 1, normals_ssbo);

GLSL part:

...
layout(std430, binding = 1) buffer normals_ssbo
{
    vec4 normals[];
};
...

The interesting part is that in a seperate shader program with a different vertex shader (below called shader 2), the binding point 1 is (re-)used like this:
GLSL:

layout(location = 1) in vec4 Normal;

but in this case, the normals come from a different buffer object and the binding is done using a VAO, like this:
OpenGL:

glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, 0);

As you can see, the binding point and the layout of the data (both are vec4) are the same, but the actual buffer objects differ.

Now to my questions:
Why does the VAO of shader 2, which is created and used after setting up shaders 1 (which use glBindBufferBase for binding), seamingly overwrite (?) the binding point, but shaders 1 still remember the SSBO binding and work fine without calling glBindBufferBase again before using them?
How does OpenGL know which of those two buffer objects the binding point (which in both cases is 1) should use? Are binding points created via VAO and glBindBufferBase simply completely seperate things? If that's the case, why does something like this NOT work:

layout(std430, binding = 1) buffer normals_ssbo
{
    vec4 normals[];
};
layout(location = 1) in vec4 Normal;

Upvotes: 2

Views: 499

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 474376

Are binding points created via VAO and glBindBufferBase simply completely seperate things?

Yes, they are. That's why they're set by two different functions.

If that's the case, why does something like this NOT work:

Two possibilities present themselves. You implemented it incorrectly on the rendering side, or your driver has a bug. Which is which cannot be determined without seeing your actual code.

Upvotes: 2

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