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Vincent_M

Easily Looking Up Uniform Locations

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I used to use glGetUniformLocation to get the handle to my uniform. This becomes an issue because if I wanted to write anything extensible, then I have to store an int for every single uniform location. I started to hold them in a map using the name they're referenced by as the key, but I'm sure a string comparison is slow when you have to upload the same data constantly per frame. Any thoughts on this?

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Don't know what "if I wanted to write anything extensible" exactly means here, but UBOs may be another option (in core since v3.1, ES v3.0; ARB_uniform_buffer_object as extension). Not as flexible as explicit locations, but they allow to reduce the amount of queries and stored indices (if the standard layout is used).

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That OpenGL 3.1 extension above looks promising, but I need it to be OpenGL ES 2.0-compatible for the time being since I'm supporting mobile as well. Once OpenGL ES 3.0 becomes widespread, though. I'll finally get current-gen-worthy APIs to play with.

 

What I meant by "write anything extensible", I meant, by setting a variable with a meaningful name to something automatically. For example, I probably won't associate the uniform location value (which could change if the shader is modified) of 1 to "u_pvmMat". If I had 20 uniform locations in a shader, I'd have to create 20 variables to store that info --each with a meaningful name. The current way I do this is by using an STL map using a string as a key. That's gotta be slow trying to iterate through a string-based list numerous times to upload uniforms to the shader for each object you want to draw though. Then again, the corresponding glUniform* call per upload is also costly on the calling thread.

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"The current way I do this is by using an STL map using a string as a key. "

 

Use something faster as your key? Or store the positions in an array, and now you can index into them using a symbolic name. Instead of using "u_pvmMat" as the key, use a constant called U_PVM_MAT. You're not losing flexibility because you've got hardcoded strings at the moment.

 

Define the constants in an enum somewhere with them.

 

For bonus points, autogenerate the source file containing the enum by scanning your shader sources...

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I imagine you'll typically have two kinds of uniform data:

- Those that the renderer actually understands, like ProjectionMatrix

- Material properties that could be whatever, the renderer doesn't know what they are, it just dumps them to the shader before rendering with the material

 

To treat both in a similar manner but gain faster access you could turn the names to integer hashes (use whatever string hashing algorithm you like). The hashes for the renderer-known uniforms naturally need to be calculated only once on startup, or even compile-time if you can manage that.

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