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Geometry shader backface culling

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I'm trying to perform backface culling in a geometry shader. So I'm sending a point as a vec3 from the cpu to the gpu through a vbo and then I generate a cube relative to that point in the shader, and I only wish to generate the sides that are acually visible, so I must test if each side is visible to the camera. What I have right now kind of works, but the faces cull too early.


I'm using the dot product technique where I take the difference between the camera position and the position of one vertex on the side I wish to test against and dot it with the sides normal.

vec3 point = gl_in[0] + vec3(-0.5f,-0.5f,-0.5f);
vec3 diff = point - CameraPosition[0];
float k = dot(diff, vec3(-1.0f, 0.0f, 0.0f));
if (k <= 0.0f) {

That's how I do it in my shader, the gl_in[0].gl_Position lies in the center of the cube. The cube is not rotated (but the world is when the camera rotates) so I assumed that the normal for the left side would just be (-1,0,0).


I hope I've given enough information to give you an idea what might cause the problem.



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i'd suggest to transform the points first by the view*model matrix, then make dot product of the point (normalized) with the view direction, probably (0,0,-1)

to your solution:

float k = dot(diff, vec3(-1.0f, 0.0f, 0.0f));

gives you: k = |diff| * cos(angle_between_vectors) so this is actually a projection and not the angle you want, to erase the impact from the length of diff normalize diff first. 

Edited by DDoS

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