by **Falco Girgis** on Tue Feb 09, 2016 2:27 pm

Basically the 'w' is an extra parameter that when multiplied by the perspective matrix gives a ratio for scaling the X and Y vector components by the Z component (which results in scaling the size of objects based on their distance from the camera). The point of the w and this ratio is so that it can be used in something called "perspective division." Once you've multiplied whatever <x,y,z,w> vector by the perspective matrix and have gotten a new, transformed vector, the <x,y,z> components are all divided by w. This is what produces the effect of distant objects being small and close objects being large in a 3D scene. It's because the X and Y terms are essentially being scaled as a function of the Z term, which is distance from camera when perspective division is applied.

An "orthographic projection" matrix in 2D mode is a matrix whose transform simply results in a 1.0 in the w term. Now when perspective division happens, there is no scaling based on distance, and you're left with a uniform or orthographic perspective. Perspective division was still happening before in 2D, it just wasn't doing anything since you were dividing by 1.0.

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