Keep rasterized pixel amount constant

Question

I have a scene that is rendered from the point of view of the light using an orthographic projection matrix.

For an arbitrarily shaped and oriented object that doesn't change its shape or size in any way, the amount of pixels that get rasterized change with its position. I guess it's better to explain this with an images:

The depth of the quads (their y coordinates) are constant. Their x and z coordinates are slightly different. However, since it's an orthographic projection matrix, their "world space" floating point sizes are the same as seen from the camera. Just in one case, one additional lines of pixels is rasterized (I drew the two white lines with mspaint to illustrate their size difference).

This is highly undesirable for my application. Is there a way to keep the amount of rasterized pixels consistent? How would I go about implementing it using OpenGL?

Answer 1

Snap the quad sizes to integer multiples of the light's-eye-view camera's pixel size.

What's happening is the bottom vertices in the right quad have just barely crossed the y-threshold of the bottom row of pixels, so that row gets included in the rasterized quad.

If the height is exactly an integer number of pixels, then this will only happen when the top vertices have slipped out of the top row at the same time, so the total rows covered remains the same.

As long as the quad is strictly between n and n+1 pixels tall, there will be some positions where it will rasterize to n rows and some positions where it will rasterize to n+1 rows.