How Would You Create Your Own Video Wipe Transition from the Top-Left

Essay

How would you create your own video wipe transition from the top-left corner of the viewport down to the bottom-right corner - a diagonal transition? Fig. 2.1 shows such a video transition.

Fig. 2.1: Wipe transition, at $\mathrm{t} / \mathrm{tmax}=0.66$
Here, we wish to simply take pixels from either the first or the second video, depending on whether they are above or below the moving diagonal line.
Write some pseudo-C or pseudo-Premiere pseudocode to produce correct pixel values during the transition.
Hint: for any x and y position, we can determine where the line that {x, y} is on, which is parallel to the wipe, cuts the main diagonal from top-left to bottom-right of the frame, simply using similar triangles. To do so, it's easiest to calculate the y-intersept of that line (where it hits the y-axis).