Can you modify a uniform from within the shader? If so. how?

Question

So I wanted to store all my meshes in one large VBO. The problem is, how do you do have just one draw call, but let every mesh have its own model to world matrix?

My idea was to submit an array of matrices to a uniform before drawing. In the VBO I would make the color of every first vertex of a mesh negative (So I'd be using the signing bit to check whether a vertex was the first of a mesh).

Okay, so I can detect when a new mesh has started and I have an array of matrices ready and probably a uniform called 'index'. But how do I increase this index by one every time I encounter a new mesh?

Can you modify a uniform from within the shader? If so, how?

Answer 1

Can you modify a uniform from within the shader?

If you could, it wouldn't be uniform anymore, would it?

Furthermore, what you're wanting to do cannot be done even with Image Load/Store or SSBOs, both of which allow shaders to write data. It won't work because vertex shader invocations are not required to be executed sequentially. Many happen at the same time, and there's no way for any shader invocation to know that it will happen "after" the "first vertex" in a mesh.

The simplest way to deal with this is the obvious solution. Render each mesh individually, but set the uniforms for each mesh before each draw call. Without changing buffers between draws, of course. Uniform changes, while not exactly cheap, aren't the most expensive state changes that exist.

There are more complicated drawing methods that could allow you more performance. But that form is adequate for most needs. You've already done the hard part: you removed the need for any state change (textures, buffers, vertex formats, etc) except uniform state.