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.

How good is your color?

There’s a bit of a hubbub about the colors on Apple laptops. It seems they’ve been claiming to display millions of colors, but the LCD displays only support 8 bits of color. There has been a lot of talk about technical means to tell how good your color is, but none of these get at the core issue: can you tell the difference?

So here are a few pictures which can help you determine the visible quality of your display.

24-bit color test

Look at the 24-bit image first. If the colors look like a perfectly smooth gradation, you have a 24-bit display. If you see bands the size of the black bars, then your vision is better than a normal human’s. (Or so the conventional wisdom says.) If you see bands that are significantly wider than the black bars, then you probably have an 18-bit display, like the ones Apple apparently is using.

On 18-bit displays (6 bits each for red, green, and blue) the 24-bit color bands should look like the ones in the 18-bit test image. This is equivalent to the “thousands of colors” mode on Mac OS 9.

In case the color bands aren’t obvious in that image– which is typical in bright daylight and other adverse situations– here’s a 4-bit test image. Which brings up an important point: if you couldn’t see the color bands in the 18-bit image, you might want to turn out all the lights. Don’t do it right now, though, the after-images in your retina from looking at all these vertical lines will make you see bands where there aren’t any. That’s why I had you look at the 24-bit image first.

I haven’t tested this on any laptops yet, so it will be interesting to see what other people report. I can verify that my generic desktop 19-inch LCD is fully 24-bit. (I’m slightly red/green color blind, but that doesn’t matter for this test.)

So how did Apple come to be in the position of advertising 24-bit, but delivering 18-bit? This is pure speculation on my part, but here we go. Traditionally, laptop displays have been significantly worse than desktop displays. Desktops had bright cathode ray tubes (CRTs), while laptops had the most state-of-the-art liquid cristal displays (LCDs). State-of-the-art originally meant you could see beige-on-black, unless you were to one side, in which case you saw black-on-beige. Over the years LCDs have improved from awful to not so bad. My 7-year-old PowerBook has a slight greenish tinge, which you don’t notice unless it’s next to a better display. You wouldn’t notice the difference between 24-bit and 18-bit very easily on that.

It used to be that the video cards were the limiting factor in the color display, so once 24-bit cards became cheap, Apple started to discourage the use of the 18-bit color mode. From there, it became easy to forget that the laptop displays were still only 18-bit.

Assuming it really is 18-bit. Again, I haven’t tested it, but there are ways to cheat. Dithering across pixels (or sub-pixel, as some have suggested) won’t help in this case, but they could dither across time: you can cheat by flashing between two nearly-identical colors. If the LCD refresh is slow enough, the liquid crystal might actually remain between those colors– producing a real intermediate color. I’m skeptical that Apple would do this, since the LCD manufacturer has more interest in investing in these sorts of tricks, and I would suspect that true 8-bit-per-subpixel quality is easier for them to get at directly.

Ian is signing

It’s official. Ian has used sign language for “drink” and “more” (food). He also signs “more” in the mirror when I’m holding him, just for his own amusement–which doesn’t count. He’s actually communicating.

Signing is useful, not because it teaches language a little earlier, but because it allows us to communicate. I remember when Sylvia learned to sign, she let us know that she wanted that blue cup–the one she had just seen, not the identical one we tried to give her. Without language, we would have never known why she was crying. Signing keeps parental blood pressure low.

Ian is crawling well, he’s signing… Grandma Catherine is going to be really surprised when she gets back next week to see how much he has changed in the last three weeks.

Also on the topic of language, witness the power of storytelling. For the past few weeks, I’ve been getting Sylvia to let me brush her teeth by telling her a story about her whale toothbrush. The climax of the story is the whale saying, “little girl, little girl, may I brush your teeth?” (In the last few days, she’s been avoiding answering the whale’s question. I’ve had to add the threat of an ogre to the story. But we’re still way better off than the fussing and crying she was doing before.)