I am writing a particle engine for iOS using Monotouch and openTK. My approach is to project the coordinate of each particle, and then write a correctly scaled textured rectangle at this screen location.
it works fine, but I have trouble calculating the correct depth value so that the sprite will correctly overdraw and be overdrawn by 3D objects in the scene.
This is the code I am using today:
//d=distance to projection plane
float d=(float)(1.0/(Math.Tan(MathHelper.DegreesToRadians(fovy/2f))));
Vector3 screenPos=Vector3.Transform(ref objPos,ref viewMatrix, out screenPos);
float depth=1-d/-screenPos.Z;
Then I am drawing a trianglestrip at the screen coordinate where I put the depth value calculated above as the z coordinate.
The results are almost correct, but not quite. I guess I need to take the near and far clipping planes into account somehow (near is 1 and far is 10000 in my case), but I am not sure how. I tried various ways and algorithms without getting accurate results.
I'd appreciate some help on this one.
What you really want to do is take your source position and pass it through modelview and projection or whatever you've got set up instead if you're not using the fixed pipeline. Supposing you've used one of the standard calls to set up the stack, such as glFrustum, and otherwise left things at identity then you can get the relevant formula directly from the man page. So reading directly from that you'd transform as:
z_clip = -( (far + near) / (far - near) ) * z_eye - ( (2 * far * near) / (far - near) )
w_clip = -z
Then, finally:
z_device = z_clip / w_clip;
EDIT: as you're working in ES 2.0, you can actually avoid the issue entirely. Supply your geometry for rendering as GL_POINTS and perform a normal transform in your vertex shader but set gl_PointSize to be the size in pixels that you want that point to be.
In your fragment shader you can then read gl_PointCoord to get a texture coordinate for each fragment that's part of your point, allowing you to draw a point sprite if you don't want just a single colour.