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important comment documenting a difference in this triangle algorithm

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the difference is important to note since most articles discussing
implementations of barycentric triangle rasterization show a test
for positive w0/w1/w2 to determine if you're inside the triangle or not.
they also usually use a slightly different formula in the `cross`
function if they assume counter-clockwise vertex winding.

since i'll admit i still don't *fully* grasp the math behind the edge
functions and that stuff that is used to calculate the a01/b01/a12/...
values, i wasn't sure how to adjust these for counter-clockwise winding
to keep a positive w0/w1/w2 check. so the simple solution is to use
the `cross` function as it is now, calculate a01/b01/a12/b12/a20/b20
as we are now, and do a negative w0/w1/w2 check and we get to keep
counter-clockwise vertex winding which i prefer. hooray!
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Gered 2023-04-01 14:57:17 -04:00
parent 6689f31146
commit eef0b4f7a4
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ggdt/src/graphics/bitmap/triangles.rs
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@ -90,6 +90,8 @@ impl<PixelType: Pixel> Bitmap<PixelType> {
let row_pixels = unsafe { std::slice::from_raw_parts_mut(pixels, draw_width) }; let row_pixels = unsafe { std::slice::from_raw_parts_mut(pixels, draw_width) };
for pixel in row_pixels.iter_mut() { for pixel in row_pixels.iter_mut() {
// note that for a counter-clockwise vertex winding order with the direction of Y+ going down instead
// of up, we need to test for *negative* area when checking if we're inside the triangle
if w0 < 0.0 && w1 < 0.0 && w2 < 0.0 { if w0 < 0.0 && w1 < 0.0 && w2 < 0.0 {
pixel_fn(pixel, w0, w1, w2); pixel_fn(pixel, w0, w1, w2);
} }