graphicsrenderingrasterizing3d-rendering

Need help understanding top-left rasterization rule

I need help in understanding top-left rasterization rule. It sounds simple, but I have trouble understanding certain example cases.

Why one triangle has its corner pixel filled but other doesn't? (outlined in magenta)

The same question for ones outlined in cyan. I get it that rightmost one is "top", so it's filled, but shouldn't the other one also be filled since it's "left"?

examples

Solution

  • The answer is in the poorly worded rule: "If two edges from the same triangle touch the pixel center, then if both edges are "top" or "left" then the sample is inside the triangle."

    That means that corners are, by default, NOT drawn unless the segments forming it are "top and left" or "left and left". (Top and top isn't possible as that would make a degenerate triangle).

    Your left-most magenta point is a corner for a left and bottom segment--so, not drawn.

    Your right-most magenta point is a corner for two left segments on the upper triangle--so, drawn.

    Your upper-most cyan point is a corner for a left and a right segment--so, not drawn.

    Your lower-most cyan point is a corner for a left and a top segment--so, drawn.

    Do be careful, though. You will note that the diagram does not include Y-coordinate direction (presumably on purpose) as that can muddy what the test for "top" is.

    Hope this helps.