4 ms·

Curious: is the significant palette shift purely a byproduct of the limited bits per color? Or is that somehow baked into the high color format? For example the

Curious: is the significant palette shift purely a byproduct of the limited bits per color? Or is that somehow baked into the high color format? For example the anime image gets much bluer than I would have expected. Is that a tunable parameter or just how well you can possibly do within the constraints of the format?

raphlinus 5y ago
You can absolutely make hi-color images which are close to visually indistinguishable from the 24 bit originals, by using a good dither matrix. GdkRgb did this well over 20 years ago, back when "CRT" meant a display technology, and the fuzziness made the dither patterns melt. If you were doing it really carefully, you'd use a corrective gamma function, as dithering in a nonlinear colorspace is not an identity transfer function. Alternatively, you can do error diffusion in a linear space. But at 5 bits, the effect is fairly subtle.
- dmbaggett 5y ago
  Thanks — that accords with my intuition but your detailed explanation is excellent.
- retrac 5y ago
  Even 256 colours at high DPI can look pretty good with an intelligent dither.
networked 5y ago
The palette shift is an artifact of the current dithering (quantization) algorithm [1]. It becomes visible when you apply quantization a few times to the same image. (50 in the example. I've updated the image and replaced the "After" label with "Gen 50" to make this more obvious.) It won't be too visible to the human eye after a single quantization pass. Edit: The result of a single pass for the anime image looks like this: https://i.imgur.com/dTmzRwF.png https://i.imgur.com/dTmzRwF.png. I've changed "visible" to "too visible" above, because the image is visibly a little brighter. [1] https://github.com/dbohdan/hicolor/blob/f00eac41417b0e989e1e4363393994a1c7a02939/hicolor.h#L282 https://github.com/dbohdan/hicolor/blob/f00eac41417b0e989e1e...

raphlinus 5y ago

You can absolutely make hi-color images which are close to visually indistinguishable from the 24 bit originals, by using a good dither matrix. GdkRgb did this well over 20 years ago, back when "CRT" meant a display technology, and the fuzziness made the dither patterns melt. If you were doing it really carefully, you'd use a corrective gamma function, as dithering in a nonlinear colorspace is not an identity transfer function. Alternatively, you can do error diffusion in a linear space. But at 5 bits, the effect is fairly subtle.

dmbaggett 5y ago
Thanks — that accords with my intuition but your detailed explanation is excellent.
retrac 5y ago
Even 256 colours at high DPI can look pretty good with an intelligent dither.

networked 5y ago

The palette shift is an artifact of the current dithering (quantization) algorithm [1]. It becomes visible when you apply quantization a few times to the same image. (50 in the example. I've updated the image and replaced the "After" label with "Gen 50" to make this more obvious.) It won't be too visible to the human eye after a single quantization pass. Edit: The result of a single pass for the anime image looks like this: https://i.imgur.com/dTmzRwF.png https://i.imgur.com/dTmzRwF.png. I've changed "visible" to "too visible" above, because the image is visibly a little brighter. [1] https://github.com/dbohdan/hicolor/blob/f00eac41417b0e989e1e4363393994a1c7a02939/hicolor.h#L282 https://github.com/dbohdan/hicolor/blob/f00eac41417b0e989e1e...