How Extract Frustum Planes from Clip Coordinates?

Question

I'm having some problems with my Frustum Culling and I want to debug it, so I'm trying to render it's planes, to see exatly the Frustum. But I'm using the Clip Coordinates to do the culling (like OpenGL pipeline do), but I dont know how to render a frustum, in another words, I dont know how to convert a frustum in Clip Coordinates to a "Frustum Mesh".

Actually to calculate the frustum I use this function:

glm::vec3 point = MVP * glm::vec4(-radius, -radius, -radius, 1.0f)) //leftTopPoint
return (point.x > -point.w) && //Inside left clipping pane
       (point.x < point.w) && //Inside right clipping pane
       (point.y > -point.w) && //Inside bottom clipping pane
       (point.y < point.w) && //Inside top clipping pane
       (point.z > -point.w) && //Inside near clipping pane
       (point.z < point.w); //Inside far clipping pane

Where MVP is a glm::mat4 of Projection * View * Model matrices.

This function is activated only when I press the left click of mouse, so I want to render the Frustum and see how it is.

Also I dont use a "lookAt" function to rotate the camera, I'm using Quaternions (dont know if that matters).

Sorry if it is a bit confusing, it's hard to explain. So I just need a tip or a orientation/example of how achive this.

Thanks for your attention.

Answer 1

I've done it by using a mix of Lighthouse3D tutorial, which I got by following the tip of @concept3d. My previous Frustum Culling routine was execute in about 12~16ms using Clip Space approach, but extracting planes from camera, I can execute it in 1~2ms....So, the peformance boost is awesome.

Here is my final code. Whenever my rotation/position changes, I update my frustum classe, like this:

localForward = glm::inverse(rotation) * -FORWARD; //Calculate the forward direction using the quaternion inverse and the inverse const direction (Always Z+).
relativeRight = glm::normalize(glm::cross(localForward, UP)); //This is relative because we use a const direction (Always Y+), so this isnt a local rotation.
localUp = glm::normalize(glm::cross(localForward, relativeRight)); //Now we calculate the local up based on local forward and relative right.

frustum->SetCameraInfo(position, localForward, relativeRight, localUp);

And to extract planes I just setup:

nearCenter = position + (forwardVector * nearDistance);
farCenter = position + (forwardVector * farDistance);

nearLeftTop     = nearCenter - rightVector * nearWidth + upVector * nearHeight;
nearLeftBottom  = nearCenter - rightVector * nearWidth - upVector * nearHeight;
nearRightTop    = nearCenter + rightVector * nearWidth + upVector * nearHeight;
nearRightBottom = nearCenter + rightVector * nearWidth - upVector * nearHeight;

farLeftTop      = farCenter - rightVector * farWidth + upVector * farHeight;
farLeftBottom   = farCenter - rightVector * farWidth - upVector * farHeight;
farRightTop     = farCenter + rightVector * farWidth + upVector * farHeight;
farRightBottom  = farCenter + rightVector * farWidth - upVector * farHeight;

/*

            +-----------+                        
            | \       / |
            |   +---+   |
            |   |   |   |
            |   +---+   |
            | /       \ |
            +-----------+ 
*/

planes[ePS_NEAR].SetPointAndNormal(nearCenter, forwardVector);
planes[ePS_FAR].SetPointAndNormal(farCenter, -forwardVector);

planes[ePS_TOP].CalcPointAndNormal(farLeftTop, (nearCenter + upVector * farWidth), farRightTop);
planes[ePS_BOTTOM].CalcPointAndNormal(farRightBottom, (nearCenter - upVector * nearWidth), farLeftBottom);

planes[ePS_LEFT].CalcPointAndNormal(farLeftBottom, (nearCenter - rightVector * nearWidth), farLeftTop);
planes[ePS_RIGHT].CalcPointAndNormal(farRightTop, (nearCenter + rightVector * nearWidth), farRightBottom);

Hope this help someone else. Thanks.