results. Most likely you could have done your manipulations, seen the posterization, converted to 16-bit, then done a second conversion back to 8 bit and the posterization would have disappeared. This doesn't prove that taking an
Hi David,
that's not how dither works. It is not just drawing your attention from the edge between two separate tones by giving you noise to look at. If you just convert from 8 to 16 bits, and back again, you would still be able to see any posterisation you started with. Dither (introduced during conversion from 16 bits to 8 bits) only can fight colour banding that was not there in 16 bit and would turn up in 8 bit, especially if later amplified by curves, or other manipulations that increase B&W or colour contrast. Dithering (successfully) tries to express any 16 bit value between two 8 bit values by proportial amounts of pixels using those two adjacent 8 bit values.
B&W example (assuming 8 bit is 0.0-255.0, zero after the comma, and 16 bit means a few digits after the comma, like 5.123, resulting in a range of 0.0-255.99): Assume there is a patch of 133.2 grey, 16 bits. Then, in your first attempt, you convert by simple rounding, and you get a patch of 133 grey. 133.7 would become 134. In your second attempt, you use dither, and from your 133.2 patch you get a patch with 80% of the pixels at 133, and 20% at 134 - which is exactly 133.2 ON AVERAGE! 133.7 would be 30% at 133, and 70% at 134. Converting 133 to 16 bits, and back to 8, without doing anything in between, would still result in 133, because 16 bit 133.0 is 100% 133 and 0% 134 in 8 bit space. You wouldn't want anything else, because it would alter your image.
There are additional tricks, like trying not to dither where there is no value step, or where there is sufficient noise in the image to make dither unnecessary, or even to use more than two eight bit values, but those are details. (b.t.w.: digital audio would not be what it has become in the past two decades without dither)
All the best Otto
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