Re:A Pixel Is Not a Little Square (1995) [pdf]
by CodesInChaos on Wednesday April 28, 2021 @05:30AM
(#26967472)
> and makes no sense for the alpha channel (wave-based sharpening filters create opaque halos).
Alpha channels are generally problematic, since if alpha is small, big changes in the rgb channels result in small changes in the actual color. You can't linearly interpolate in this color space, just like you can't in a polor coordinate space like HSL.
Pre-multiplied alpha should fix most of these problems (though you still need to be careful with fancy filters, no not end up with values outside the valid range)
In image formats and image processing you can define pixel to be whatever you like. In many cases it's useful to define/interpret it as an infinitesimal point.
In other algorithms, it's not useful. For example, pixels of textures are filtered (mipmaps, AF, MSAA), because an actual ideal point sample of a texel being read as an ideal point sample of a pixel on the screen has awful aliasing artifacts.
Pixel samples are not the same as audio samples. Screen isn't a 2D wave. There's no gibbs phenomenon for pixels on screen. Treating samples as a wave works for resampling to a large degree, but breaks down in a few places like crisp edges (see font hinting, pixel art) and makes no sense for the alpha channel (wave-based sharpening filters create opaque halos).
And when a pixel is connected to a physical device, it's clearly not a point. Camera sensors' pixels aren't points. LCD pixels aren't points. Printed pixels aren't points. Our eyes actually see the difference. Pixel-grid-aligned lines are clearly sharper than the same lines shifted by half of a pixel. If sampling theorem applied to pixels, it would make no difference.