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Chananya Freiman

WebGL: hardware skinning with a bone texture

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I have WebGL code running hardware skinning in a vertex shader for animations, using a uniform array of matrices for my bones.

The problem arises when I don't have enough vector slots, which happens when there are more than 62 bones (I don't control the models or the number of bones, and I've already seen a model with 173 bones, which is crazy).


I tried using a float texture to store all my bones in, and fetch them in the shader, but I can't seem to do that correctly.


There is no texelFetch in WebGL's version of GLSL, no 1D textures and obviously no texture buffers or uniform buffers.


What I tried was creating a X on 1 2D float texture, where X is the number of floats required for all the matrices, feeding it with all the matrices.


I send to the shader the size of each matrix and the size of each vector, relative to the size of the texture, so I can get to any matrix with a normal texture fetch.


I believe this should theory. But it doesn't.


This is the texutre initialization code:

var buffer = new Float32Array(...);
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, buffer.byteLength / 16, 1, 0, gl.RGBA, gl.FLOAT, buffer);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);

And the part of the vertex shader that constructs a matrix from the texture, given a matrix index:

uniform sampler2D u_bone_map;
uniform float u_matrix_fraction;
uniform float u_vector_fraction;
mat4 boneMatrix(float bone) {
  return mat4(texture2D(u_bone_map, vec2(u_matrix_fraction * bone, 0)),
              texture2D(u_bone_map, vec2(u_matrix_fraction * bone + u_vector_fraction, 0)),
              texture2D(u_bone_map, vec2(u_matrix_fraction * bone + u_vector_fraction * 2.0, 0)),
              texture2D(u_bone_map, vec2(u_matrix_fraction * bone + u_vector_fraction * 3.0, 0)));

u_matrix_fraction and u_vector_fraction are the relative sizes I wrote above.

E.g., if the texture is 512x1 each matrix is 16/512, and each vector is 4/512, so to get the ith matrix, the code would need to go to i*u_matrix_fraction and grab 4 texels, each the size of u_vector_fraction.


These matrices result in my meshes going crazy, so something is wrong, but I am just not sure what.


Got any ideas?


Thanks for any help.

Edited by Chananya Freiman

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marcClintDion    435

I'm intrigued... really, I'm surprised, when I first heard of WebGL I had assumed that texture access in the vertex shader would not be supported. 


Well after a quick two minutes of reading it seems that Chrome and Firefox are not holding firm to the OpenGL E.S. 2.0. spec.  I guess you can do it. 


Have you tried printing what you are putting into the texture out to a text file as well so you can see if the data makes sense?  That's usually how I debug.

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I don't have access to files, JavaScript sleep.png


In any case, the data is the same data I otherwise use in the matrix uniform array (where it works as expected), so it's correct.


There's probably something really stupid and obvious I am doing and not noticing.

Edited by Chananya Freiman

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I did not know of the existence of that inspector, I'll check it out.


Thanks. smile.png




Thanks a lot, that inspector really helped. Got the skinning working this time!

Edited by Chananya Freiman

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