How can you scale a point sprite by its distance from the camera?
GLSL fragment shader: gl_PointSize = size / gl_Position.w;
seems along the right tracks; for any given scene all sprites seem nicely scaled by distance. Is this correct?
How do you compute the proper scaling for my vertex attribute size
? I want each sprite to be scaled by the modelview matrix.
I had played with arbitrary values and it seems that size
is the radius in pixels at the camera, and is not in modelview scale.
I've also tried:
gl_Position = pMatrix * mvMatrix * vec4(vertex,1.0);
vec4 v2 = pMatrix * mvMatrix * vec4(vertex.x,vertex.y+0.5*size,vertex.z,1.0);
gl_PointSize = length(gl_Position.xyz-v2.xyz) * gl_Position.w;
But this makes the sprites be bigger in the distance, rather than smaller:
Answer
To answer my own question, here's what I got working:
The scaling in the GLSL vertex shader is:
gl_PointSize = (heightOfNearPlane * pointSize) / gl_Position.w;
Where you compute your heightOfNearPlane
using the viewport height and the field-of-view angle you constructed the perspective matrix with:
float fovy = 60; // degrees
int viewport[4];
glGetIntegerv(GL_VIEWPORT,viewport);
float heightOfNearPlane = (float)abs(viewport[3]-viewport[1]) /
(2*tan(0.5*fovy*PI/180.0));
Which you pass in as a uniform.
Thx to the coders on irc.freenode.net ##opengl that helped me sort this out!
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