Dithering?

Jussi Hagman from Finland writes:

I was just left wondnering whether the 18-bit test image should have
been dithered, the display manufacturers could perhaps use some kind of HW based dithering to give an illusion of a better color depth.

Good question, and if I get the time I’ll do a follow-up on exactly that issue.

I started by looking at my brother’s PowerBook, where he did a quick test gradient in Illustrator. We saw banding on the machine, so it looked 18-bit, not dithered.

I’ve since started to question this initial test, since everyone seems to report that their computer looks fine. Adobe has a long history of doing things their way, and it’s possible that Illustrator is 18-bit on an allegedly 24-bit laptop.

At some point, time permitting, I hope to post an 18-bit dithered test image. I’ve done a quick test on my desktop (Ubuntu Linux with a generic desktop LCD) which makes me suspect that hardware manufacturers are doing built-in dithering. But I’ll need a better test image to be sure.

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One thought on “Dithering?

  1. Yes, the MacBook displays *are* using dithering.

    Dave, unfortunately your color bar test is fundamentally flawed. It presents the ideal situation for the LCD to use dithering to simulate 24-bit color, and it does a pretty decent job (although with sharp eyes, you can see “grain” in the banding compared to a true 24-bit display). An 18-bit display with dithering is *never* going to show bars as you have created in your 18-bit test. That’s the whole point: they are using dithering to come up with the rest of the colors.

    There are various approaches to dithering on an LCD, temporal (flashing pixels between two colors) or spatial (checkerboard) but in either case, all such 18-bit displays will perform fairly admirably, especially with such small areas to cover.

    I have a 6-bit per color MacBook Pro, and the 24-bit image looks very nice on it because the dithering is doing a good job and there are only a few pixel widths to trick.

    If you do an image with large solid areas of color that specifically cause the dithering to enable, it will become *very* obvious the difference between these 6-bit displays and 24-bit displays. For example, start with a green and change the color one bit at a time until the dithering kicks in. Then you’ll see why people are so upset.

    One such color is the default color for tab backgrounds in the default Windows XP theme. When I reboot to XP through boot camp, I can see the dithering in gross clarity. The trick in getting this test to work on the mac is to ensure that the color doesn’t get “corrected” via colorsync to a non-dithering color. If you just copy the color directly, it gets translated by color sync, so you aren’t seeing the actual color Windows is using.

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