Last Achievement Unlocked: 3D Shapes in Geometry Shader

So, I was busy working on the last part of my GSoC project – to be able to see 3D Spheres in Geometry Shader. This part required more time and than 1-D and 2-D combined. While trying to extend from 2-D to 3-D, the following problems were observed –

  1. The plane always remained perpendicular to the camera vector:  The following line was present in my 2-D implementation of the shader:> gl_Position = gl_in[0].gl_Position + offset;

    So, this was responsible for giving an offset to the original vertex. Since offset is a constant vec4, we would always see the point move relative to the camera normal.
    The approach towards solving it: So it was clear that the issue is with this offset variable. We don’t want to keep it constant, rather this should be continuously transformed as we moved the camera. So we need to multiply the offset with an appropriate transformation matrix. After lots of experimentation by me, David and Elef, it was found by David that multiplying the offset variable with the MCDC Matrix would result in the desired behavior. Hence, the modified expression for the produced vertex should be:
    >  gl_Position = gl_in[0].gl_Position + (MCDCMatrix * offset);

    Hence, now we have a perfectly rotating 2-D polygon.  Another important limitation is described below.

  2. A limited number of GS Vertex per Shader Vertex: This was one major challenge which we were working on for the last couple of weeks. There is a strict limit to the number of vertices that we can produce per Shader Vertex. This limit is 256 in my Windows Machine and 128 for Elef and David. This is too low for rendering even a sphere. So I used the ‘repulsion100’ model available in dipy. But for completing the sphere, I had to use them repeatedly and this made the number of required vertices go up to 330. This is more than what we can afford. After trying out various possible solutions, I came up with the idea of using two/three different actors for constructing a sphere, thus we split the list of vertices into three parts – each part contributing to a third of the sphere. Thus we get a perfectly rotating sphere.

Thus we finally have a bunch of spheres in 3-D with perfectly rotating spheres.  Thus, it was pleasant to see the tiny beautiful spheres on the screen. We can even render 30K spheres on the screen without much lag.



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