CODEWITHSUNDEEP
openglglsl
Strawhare
Strawhare

Reputation: 486

Access GLSL Uniforms after shaders have used them

I've noticed that my shaders are performing a calculation that I need in the CPU code. Is it possible for me to load that results of that calculation into a uniform array, and then access that uniform from the CPU once the GPU has finished working?

Upvotes: 0

Views: 481

Answers (2)

Nicol Bolas
Nicol Bolas

Reputation: 474406

You can write arbitrary amounts of data through either Image Load/Store or SSBOs. While the number of image variables is restricted in image load/store, those variables can refer to buffer textures or array textures. Either of which give you access to a more-or-less arbitrarily large amount of data to write to:

layout(rgba32f, writeOnly) imageBuffer buffer;

imageStore(buffer, valueOffset1, value1);
imageStore(buffer, valueOffset2, value2);
imageStore(buffer, valueOffset3, value3);
imageStore(buffer, valueOffset4, value4);

SSBOs make this even easier:

layout(std430) buffer Data
{
    float giantArray[];
};

giantArray[valueOffset1] = data1;
giantArray[valueOffset2] = data2;
giantArray[valueOffset3] = data3;
giantArray[valueOffset4] = data4;

However, note that any such writes will be unordered with regard to writes from other shader invocations. So overwriting such data will be... problematic. And you'll need an appropriate glMemoryBarrier call before you try to read from it.

But if all you're doing is a compute operation, you ought to be using dedicated compute shaders.

Upvotes: 1

eliaspr
eliaspr

Reputation: 322

As far as i know, there is no way of retreiving uniform data from your GPU. But you could execute the calculation and set the output color to something you can identify on your screen depending on the expected result of your calculation. For exmaple:

#version 330 core

layout(location = 0) out vec4 color;

void main() {
    if( Something you're trying to debug )
        color = vec4(1, 1, 1, 1);
    else
        color = vec4(0, 0, 0, 1);
}

That's the only way I know of, and I use it all the time.

Upvotes: 0

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