Multiple UBOs without specifying binding point index explicitly

Question

I am looking into OpenGL Uniform Buffer Objects and Shader Storage Buffers. I think the question applies to both of them, but let's just focus on UBOs.

Tutorials seem to be saying that "block index" and "binding point index" are different things, and if I have more than one UBO in a shader, I need to specify the binding index

layout(binding = 0) uniform first_buffer  {...};
layout(binding = 1) uniform second_buffer {...};

I get the impression that the block index is determined by the linker and can be read from glGetUniformBlockIndex, but the binding point index has to be hard-coded with arbitrary distinct binding=N in the layout, and a corresponding glBindBufferBase(GL_UNIFORM_BUFFER, N, ubo_handle), which strikes me as fragile and a nuisance.

What is the difference between a block index and a binding point?

Can I omit the binding=N in the layout? What happens then with glBindBufferBase?

Is this all the same for SSBOs?

Think about the block index as the location index of a texture sampler uniform and the binding point is somehow like the texture unit. — Rabbid76
See Difference between glBindBuffer and glBindBufferBase respectively Is it safe to use the block index as the binding point for UniformBufferObject, ShaderStorageBufferObjects, etc? and see also GLSL Tutorial – Uniform Blocks — Rabbid76

Rabbid76 Rabbid76 · Accepted Answer · 2019-09-01T07:30:47

See OpenGL 4.6 API Core Profile Specification - 7.6 Uniform Variables

Named uniform blocks, as described in the OpenGL Shading Language Specification, store uniform values in the data store of a buffer object corresponding to the uniform block. Such blocks are assigned a uniform block index.

The block index is the "handle" for the active program resource of a shader program. If you have different shader programs with the "same" uniform block, then they may have different buffer indices.

A buffer is bound to the uniform block of a shader program the binding point (e.g. in shader by a Binding point Layout Qualifier). On the one side a uniform block (index) is associated to a binding point and on the other side a buffer is bound to the binding point (by glBindBufferBase). So one buffer can be bound to the uniform blocks of different programs.

While the buffer index is fixed and and can't be changed after the program is linked, the binding point is a dynamic value and can be changed by glUniformBlockBinding. When the program is linked, then the binding point is initialized by 0 or by the value which is set by the Binding point Layout Qualifier.

This principle is the same for a Shader Storage Buffer Object.

Multiple UBOs without specifying binding point index explicitly

1 Answers