Create view matrices in GLSL shader

Question

I have many positions and directions stored in 1D textures on the GPU. I want to use those as rendersources in a GLSL geometry shader. To do this, I need to create corresponding view matrices from those textures.

My first thought is to take a detour to the CPU, read the textures to memory and create a bunch of view matrices from there, with something like glm::lookat(). Then send the matrices as uniform variables to the shader.

My question is, wether it is possible to skip this detour and instead create the view matrices directly in the GLSL geometry shader? Also, is this feasible performance wise?

Answer 1

Nobody says (or nobody should say) that your view matrix has to come from the CPU through a uniform. You can just generate the view matrix from the vectors in your texture right inside the shader. Maybe the implementation of the good old gluLookAt is of help to you there.

If this approach is a good idea performance-wise, is another question, but if this texture is quite large or changes frequently, this aproach might be better than reading it back to the CPU.

But maybe you can pre-generate the matrices into another texture/buffer using a simple GPGPU-like shader that does nothing more than generate a matrix for each position/vector in the textures and store this in another texture (using FBOs) or buffer (using transform feedback). This way you don't need to make a roundtrip to the CPU and you don't need to generate the matrices anew for each vertex/primitive/whatever. On the other hand this will increase the required memory as a 4x4 matrix is a bit more heavy than a position and a direction.

Answer 2

Sure. Read the texture, and build the matrices from the values...

vec4 x = texture(YourSampler, WhateverCoords1);
vec4 y = texture(YourSampler, WhateverCoords2);
vec4 z = texture(YourSampler, WhateverCoords3);
vec4 w = texture(YourSampler, WhateverCoords4);
mat4 matrix = mat4(x,y,z,w);

Any problem with this ? Or did I miss something ?

Answer 3

The view matrix is a uniform, and uniforms don't change in the middle of a render batch, nor can they be written to from a shader (directly). Insofar I don't see how generating it could be possible, at least not directly.

Also note that the geometry shader runs after vertices have been transformed with the modelview matrix, so it does not make all too much sense (at least during the same pass) to re-generate that matrix or part of it.

You could of course probably still do some hack with transform feedback, writing some values to a buffer, and either copy/bind this as uniform buffer later or just read the values from within a shader and multiply as a matrix. That would at least avoid a roundtrip to the CPU -- the question is whether such an approach makes sense and whether you really want to do such an obscure thing. It is hard to tell what's best without knowing exactly what you want to achieve, but quite probably just transforming things in the vertex shader (read those textures, build a matrix, multiply) will work better and easier.