We are mostly concerned with this to eliminate banding and give nice smooth gradients.
bpc = bits per channel.
RGB = 3 channels, so 8bpc is 24bit colour. This is what we’re used to, and most codecs work in 8bpc.
8bpc gives 256 levels of brightness in each channel. (2X2X2X2X2X2X2X2=2^8=256)
10bpc gives 2^10 levels, which is 1024 levels of gray in each channel.
16 gives 2^16, and so on.
Imagine a grayscale gradient in 8bpc. If the gradient is 256 pixels wide, it will look smooth, with one level (gray value) per pixel.
But if the gradient is now 1024 pixels wide, you will see banding, with one gray level for every 4 pixels. You’ll see stripes 4 pixels wide.
But if you switch to work in 16bpc mode, that gradient will smooth out, since you’ll now have more levels of gray available.
But then it’s time to render. If you’re rendering to an 8-bpc codec, the codec will quantize the grayscale image back to 256 levels, and you’ll see banding again. However, rendering to a 10bpc codec will give you a smoother image.
In practice, if you see banding, you can work in 16bpc and render to an 8bpc codec, and it might look okay. Oftentimes, you have to render to an 8bpc codec, because that’s what the editor uses.
But if you’re going to DVD yourself, and you have banding issues, consider working in 8bpc, switching to 16 before rendering, then render to a 10bpc codec. That’s the movie that you then compress to DVD.
32bpc is even smoother, but you should check the help for that — I haven’t needed it yet.
Steve’s answer gives a good start regarding 8bpc, 10bpc, and 16bpc color.
32bpc color does give some additional smoothness compared to 16bpc, but the difference is perceptible only under rare circumstances. The big difference is that 32bpc color values are floating-point numbers and can go under 0 (black) and over 1 (white). This is what is meant by ‘high dynamic range color’. This means that you retain over-range values when applying effects and such. The canonical example is applying a glow and then a blur; the result looks much better and more realistic when you work in 32bpc color. You can still render for output in 8bpc color and get this benefit.
Thanks a lot both Steve and Todd, for clearing up my doubts. But …lets say..Im editing a video on Premiere Pro 2, I cut paste the sequence in a comp and do my effects stuff and color correction in 16bpc..I then render out an uncompressed AVI file…I still have to render it on the time line in Premiere to give a tape out ???…so I have a loss in quality then ? Is this loss perceptible…?
And in future shall we have 16bpc mode in the NLE’s ?