# Projection matrix

If any clip coordinate is less than -wc, or greater than wc, then the vertex will be discarded. All xe, ye Projection matrix ze components in eye space are linearly mapped to NDC. The clip coordinates, Projection matrix, yc and zc are tested by comparing with wc.

Top View of Frustum Side View of Frustum From the top view of the frustum, the x-coordinate of eye space, xe is mapped to xp, which is calculated by using the ratio of similar triangles; From the side view of the frustum, yp is also calculated in a similar way; Note that both xp and yp depend on ze; they are inversely propotional to -ze.

First, it transforms all vertex data from the eye coordinates to the clip coordinates. But we need unique z value for the clipping and depth test. The distance between n and f should be short as possible to minimize the depth buffer precision problem.

To solve the equations for A and B, rewrite eq. See more details on OpenGL Transformation. And we set wc to -ze earlier, and the terms inside parentheses become xc and yc of the clip coordiantes.

If the viewing volume is symmetric, which is andthen it can be simplified as; Before we move on, please take a look at the relation between ze and zn, eq.

Perspective Projection Perspective Frustum and Normalized Device Coordinates NDC In perspective projection, a 3D point in a truncated pyramid frustum eye coordinates is mapped to a cube NDC ; the range of x-coordinate from [l, r] to [-1, 1], the y-coordinate from [b, t] to [-1, 1] and the z-coordinate from [n, f] to [-1, 1].

The MathML version is available here.

Mapping from xp to xn Mapping from yp to yn Then, we substitute xp and yp into the above equations. Finding zn is a little different from others because ze in eye space is always projected to -n on the near plane.

In eye space, we equals to 1. Note that the frustum culling clipping is performed in the clip coordinates, just before dividing by wc. Note that the eye coordinates are defined in the right-handed coordinate system, but NDC uses the left-handed coordinate system.

In other words, they are both divided by -ze. Overview A computer monitor is a 2D surface.

Then, these clip coordinates are also transformed to the normalized device coordinates NDC by dividing with w component of the clip coordinates. If the range [-n, -f] is getting larger, it causes a depth precision problem z-fighting ; a small change of ze around the far plane does not affect on zn value.

It means there is very high precision at the near plane, but very little precision at the far plane. The following sections describe how to build the projection matrix from 6 parameters; left, right, bottom, top, near and far boundary values.

Then, OpenGL will reconstruct the edges of the polygon where clipping occurs. Plus, we should be able to unproject inverse transform it.I tried to understand the mathematics behind the projection matrix and I found this page.

The matrix from this page: I found this matrix is similar to the matrix of Xna. I understood how they got. Projection matrix We’d like to write this projection in terms of a projection matrix P: p = Pb. aaTa p = xa =, aTa so the matrix is: aaT P. aTa Note that aaT is a three by three matrix, not a number; matrix multiplication is Lecture Projections onto subspaces Author.

A projection matrix is an square matrix that gives a vector space projection from to a subspace.

The columns of are the projections of the standard basis vectors, and is the image of. A square matrix is a projection matrix iff.

A projection matrix is orthogonal iff. The following sections describe how to build the projection matrix from 6 parameters; left, right, bottom, top, near and far boundary values.

Note that the frustum culling (clipping) is performed in the clip coordinates, just before dividing by w c.

Projection matrix
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