Re:Image Dithering: Eleven Algorithms and Source Code
by dahart on Sunday October 08, 2017 @06:33PM
(#15430394)
It's because with 8 bits you can't really see the dither pattern itself once a 1-bit difference is at the human threshold of perception.
8 bits is not just an arbitrary threshold, it's the number of bits you need so that a 1 bit difference is just about the "just noticeable difference", assuming you've handled gamma and color reasonably well. 9 bits is more lightness resolution than you can normally see, and 7 bits is too few. If you want to geek out super hard, check out figure 238 here: http://www.telescope-optics.net/eye_intensity_response.htm
The JND of Weber's law is 0.4 - 0.5% for print and monitors most of the time, which is between 1/200 and 1/250. This is why 8 bits is so super popular, it has a 1-bit step of 1/256, which is normally just barely below what we can see with our eyes.
You may need to dither at 6 bits, banding could start to become pretty obvious. Whether you need error diffusion, I don't know, I doubt it would always useful at 6 bits, but I'm sure it would sometimes. I'd say try it on some images and compare to random dithering. Random dithering only sucks on really small palettes like 2 or 16 total colors. It's not that bad on a 256 color gif, and it becomes hard to tell the difference when you're above 4 bits per channel or 12 bits per pixel. 6 bits per channel is decent color resolution, and random will sometimes be good enough. If it's a photograph from a digital camera that's older than a year or two, chances are pretty good you can threshold the image without even dithering, because it's already noisy enough.
8 bits is not just an arbitrary threshold, it's the number of bits you need so that a 1 bit difference is just about the "just noticeable difference", assuming you've handled gamma and color reasonably well. 9 bits is more lightness resolution than you can normally see, and 7 bits is too few. If you want to geek out super hard, check out figure 238 here: http://www.telescope-optics.net/eye_intensity_response.htm
The JND of Weber's law is 0.4 - 0.5% for print and monitors most of the time, which is between 1/200 and 1/250. This is why 8 bits is so super popular, it has a 1-bit step of 1/256, which is normally just barely below what we can see with our eyes.
You may need to dither at 6 bits, banding could start to become pretty obvious. Whether you need error diffusion, I don't know, I doubt it would always useful at 6 bits, but I'm sure it would sometimes. I'd say try it on some images and compare to random dithering. Random dithering only sucks on really small palettes like 2 or 16 total colors. It's not that bad on a 256 color gif, and it becomes hard to tell the difference when you're above 4 bits per channel or 12 bits per pixel. 6 bits per channel is decent color resolution, and random will sometimes be good enough. If it's a photograph from a digital camera that's older than a year or two, chances are pretty good you can threshold the image without even dithering, because it's already noisy enough.