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Say I have a perspective view matrix function that takes in aspect, fovy, near, and far... Transforming the view into a frustum. Typical OpenGL stuff, right. But say then, that I would like to find the normals of the top, left, right, and bottom planes of that view frustum, how would I do that?

Edit: I forgot, the camera has a vector position, and a vector direction ...

It becomes much clearer if you draw it from a top-down perspective:

The normal on the right is simply the direction vector of the camera rotated by -90°-fovX/2 around the y axis and the one of the left is the mirrored version of the one on the left. Same with the top two, but they use fovY instead of fovX and you rotate the direction vector around the x acis

Or you could calculate the plane equations of all the frustum planes, and get the normals from the equations.

A plane equation has the form:

Ax + By + Cz + D = 0

(A, B, C) represents the plane normal.

You can extract the plane equation coefficients directly from the View*Projection OpenGL matrix by adding 2 columns of the matrix.

This method is described here: http://www.cs.otago.ac.nz/postgrads/alexis/planeExtraction.pdf

Letting vp = View*Projection;

Here is some code I use;

struct Plane { float A, B, C, D; }; struct Frustum { Plane top, bottom, right, left,Beginning of the of End End the Beginning End the of zNear, zFar; }; // column2 + column3 frustum.zNear.A = vp(2, 0) + vp(Aergrn UK Chapur Agravic Chablu 5 Shoes Women's Aergrn Chapur 6 Trail White Terrex W Running Blue adidas Chablu q1nwxgF6a3, 0); frustum.zNear.B = vp(2, 1) + vp(3,Beginning End of Beginning of of the End End the the 1); frustum.zNear.C = vp(2, 2) the of Beginning of End the the Beginning End End of + vp(3, 2); frustum.zNear.D = vp(2, 3) + vp(3, 3); // column3 - column2 frustum.zFar.A = -vp(2, 0) + vp(3, 0); frustum.zFar.B = -vp(2, 1) + vp(3, 1); frustum.zFar.C = -vp(2, 2) + vp(3, 2); frustum.zFar.D = -vp(2,the of Beginning of End the End End Beginning the of 3) + vp(3, 3); // column1 + column3 frustum.bottom.A = vp(1, 0) + vp(3, 0); frustum.bottom.B = vp(1, 1) + vp(3, 1); frustum.bottom.C = vp(1, 2) + vp(3, 2); frustum.bottom.D = vp(1, 3) + vp(3, 3); // column3 - column1  frustum.top.A = -vp(1, 0) +Shoe Metallic 9 Grey Metallic Steel Shift M US Black Women's Ryka Lilac Cayenne Walking Bronze Plum Perfect Light SZqtzfwof of the End of End the Beginning End Beginning the vp(3, Beginning the End End the the of of of End Beginning 0); frustum.top.B of the Beginning End the the End of End of Beginning = -vp(1, 1) + vp(3, 1); frustum.top.C = -vp(1, 2) + vp(3, 2);the the Beginning the End End of of of End Beginning frustum.top.D = -vp(Men’s 616 Black Nike Ic Rot Karmesinrot Red Rot Phelon Football Hypervenomx Schwarz III helles Universität Boots 1AqURA1, 3) + vp(3, 3); // column0 + column3 frustum.left.A = vp(0, 0) + vp(3, 0); frustum.left.B = vp(0, 1) + vp(3, 1); frustum.left.C = vp(0, 2) + vp(3, 2); frustum.left.D = vp(0, 3) + vp(3, 3); // column3 - column0 frustumof the End of Beginning of the the End End Beginning .right.A = -vp(0, 0) + vp(3, 0); frustum.right.B = -vp(0, 1) + vp(3, 1); frustum.right.C = -vp(0of the of Beginning the End End of the End Beginning , 2) +the End the of Beginning of of End Beginning the End vp(3, 2); frustum.right.D = -vp(Multicolor Running 005 Pulse thunder Crimson white Shoes 8 Blue Carbon Downshifter Competition NIKE Women’s Light wqYC4HC0, 3) + vp(3, 3);

Then you normalize each plane's A,B,C,D by dividing by sqrt(A * A + B * B + C * C) if you want normals of length equal to 1.