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Chapter Three · Mixer (Two Sources)

Split

A wipe between two pictures along a straight line, at any angle, with a soft or hard edge.

The animation is the part with something to say. To scroll the line all the way across the frame and come back without a visible jump, the shader has to know exactly how far the line needs to travel, and that depends on both the angle and the shape of the canvas: a diagonal line across a wide frame has much further to go than a vertical one across a square.

So the reach is worked out from the angle and the aspect ratio rather than guessed, and the sweep runs across exactly that distance and no further. The direction then reverses each cycle, so the line arrives back where it started instead of teleporting. Running it one way only would wrap visibly the moment it reset.

Further reading Wipe (transition) Rotation matrix