# Writing the correct value in the depth buffer when using ray-casting

I am doing a ray-casting in a 3d texture until I hit a correct value. I am doing the ray-casting in a cube and the cube corners are already in world coordinates so I don't have to multiply the vertices with the modelviewmatrix to get the correct position.

world_coordinate_ = gl_Vertex;


vec3 direction = (world_coordinate_.xyz - cameraPosition_);
direction = normalize(direction);

for (float k = 0.0; k < steps; k += 1.0) {
....
pos += direction*delta_step;
float thisLum = texture3D(texture3_, pos).r;
if(thisLum > surface_)
...
}


Everything works as expected, what I now want is to sample the correct value to the depth buffer. The value that is now written to the depth buffer is the cube coordinate. But I want the value of pos in the 3d texture to be written.

So lets say the cube is placed 10 away from origin in -z and the size is 10*10*10. My solution that does not work correctly is this:

pos *= 10;
pos.z += 10;
pos.z *= -1;

vec4 depth_vec = gl_ProjectionMatrix * vec4(pos.xyz, 1.0);
float depth = ((depth_vec.z / depth_vec.w) + 1.0) * 0.5;
gl_FragDepth = depth;


But to you question: When you have sample in world space (your pos) you have to multiply it with View*Projection matrix to have correct output of depth. View matrix contains linear basis of the camera and converts the world position to the camera space and then you do projection.
Using this depth with regular rendering pipeline is easy (if you are not able to write the depth into the depth buffer and use HW acceleration). Store world position in Vertex shader (using varying in gl i think). Than in PS perform viewProjection multiplication on this worldPos. Normalize (pos /= pos.w). And compare pos.z with the value in a depth texture. Thats it ;)