I was reading about some big name brand LCD displays which don’t actually show 24-bit colour. Instead, to save costs, they have 6 bits per channel: 18-bit colour.
It would be fairly obvious (to someone who knows what they’re looking for, at least) if a screen was simply taking the nearest 18-bit colour to its 24-bit input colour. Think of a gradient from full red to black – with 24-bit per colour (8 bits for red) there are 256 shades but with 18-bit colour (6 for red) there are only 64.
So they don’t do it that way (or I’d be interested to know if sometimes they do) – they employ dithering algorithms to make patterns of the closest two shades or, sometimes, temporal dithering to switch rapidly between the closest shades.
My screens are fairly high quality (or they were a few years ago) but I do see colour banding if a gradient is shallow enough. I wanted to determine whether I was seeing real 24-bit colour or some imitation.
I haven’t gone all the way from black to full intensity so it is easier to see the banding (the bands are wider – 50 pixels each in this case). There are 24 bands of each colour, and they should be visible in red and green if your eyes are sharp. It’s easier in the dark. Blue is more difficult because our eyes are less sensitive.
If banding is obvious but there are significantly fewer than 24 bands visible for each colour, it looks like you have an 18-bit display. I’d be very interested to see how it looks – please take a photo. (Not screenshot!)
If it looks grainy, your display is probably 18-bit but automatically dithering the colours. Again, I’d like to see it.
Finally, it might be very hard to tell, but if the colours are subtly flickering, you may have temporal dithering.
I get some odd behaviour on my display (NEC LCD1760NX) – the banding seems even for green and (as far as I can make out) blue, but with red there seems to be a bigger difference between certain shades: 0x38 to 0x39, 0x40 to 0x41 and 0x48 to 0x49. I doubt it’s coincidence that these are at evenly spaced intervals – I’m hoping it’s a software thing and I’ve got gamma set up badly somewhere, but it could be a flaw in my monitor model. Whatever it is, it’s common across all three of my monitors.
Let me know what you see, and what kind of monitor you’re using.
Bart Nagel – bart at tremby.NOT (net, not not)