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In a particle simulation, I am rendering particles as "spheres" using GL_POINTS instead of a mesh for efficiency. With glEnable(GL_PROGRAM_POINT_SIZE) I can use gl_PointSize from the vertex shader to set the size of my resulting 2D point in screen space pixels.

I have the projection matrix, view matrix and the particle position in my vertex shader.

My current approach has been

vec4 pos = proj * view * vec4(position, 1.0);
gl_PointSize = POINT_RADIUS / pos.w;

, which works, but POINT_RADIUS is not in world space, but rather the pixel size on my near plane (of the perspective projection).

How would I calculate the point size in screen space from a given radius in world space? I can only think of kinda hacky solutions. Is there some mathematically clean or obvious approach to this?

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    $\begingroup$ You have computed the NDC size, to finish your computation you have to now compute its screen space size. Searching for "compute screen space size of object glsl" get lots of hits and good examples $\endgroup$
    – pmw1234
    Commented Dec 6, 2020 at 1:07

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