# What Is a View Frustum?

The XNA Framework uses a projection matrix to calculate a vertex position in the view frustum. The matrix reverses the y-coordinate (so it is top down) to reflect a screen origin at the top-left corner. After the matrix is applied, the homogenous vertices (that is, the vertices contain (x,y,z,w) coordinates) are converted to non-homogeneous coordinates so they can be rasterized.

The most common type of projection is called a perspective projection, which makes objects near the camera appear bigger than objects in the distance. For a perspective projection, the view frustum can be visualized as a clipped pyramid whose top and bottom are defined by the near and far clipping planes as shown in the figure.

A view frustum is defined by a field of view (fov), and by the distances of the front and back clipping planes, which are specified in z-coordinates. Set up a perspective fov using the CreatePerspectiveFieldOfView method.