Problem:Light volume projection & Deferred Shading

OpenGL OpenGL: Advanced Coding
1

Hi guys,

The problem comes up when I try to project a point light volume, a sphere, to screen. The process is following:

1, I draw a sphere with a vertex shader, get the vertex position in normalized screen space.


uniform float fViewAspect;
uniform float fTanFOV;

uniform mat4 mat4ModelViewProjection;

varying vec3 vec3EyeScreenRay;

void main()
{
	gl_Position = mat4ModelViewProjection * gl_Vertex;
	
	vec2 vec2ScreenPosition = gl_Position.xy / gl_Position.w;
	vec3EyeScreenRay = vec3(vec2ScreenPosition.x * fViewAspect, vec2ScreenPosition.y, fTanFOV);
}

2, In fragment shader, I calculate the texture coordinates from the screen space position, which indicate the pixel may be in the light volume.


vec2 vec2TexCoord = 0.5 * (vec3EyeScreenRay.xy + vec2(1.0));

But, the result is weird. When I bind a render target of diffuse color for test,


gl_FragColor = texture2D(txColor, vec2TexCoord);

pixels are drawn in the sphere area and twisted. Looks like it is affected by the sphere’s polygon tessellation.

So, is my way to project light volume wrong?

2

You shouldn’t do the perspective divide operation in the vertex shader, interpolation of vec2TexCoord will not be correct for your purpose (there is perspective correction, but you don’t want that).

The following line:
vec2 vec2ScreenPosition = gl_Position.xy / gl_Position.w;

should be done in the fragment shader. This means you need to interpolate gl_Position.xyw of the (projected) vertex position in the fragment shader, and do the division by .w there.

3

Thanks, and I use gl_FragCoord to get correct texcoord.