AbsoluteDestiny wrote:OK I've updated the AMVapp to include a newer build of XviD that fixes the trellis and b-frame bugs.
The AMVapp basically runs the regular 1.0 installer and then copies the new dll's over the old ones. This allows all the cool stuff in the koepi installer to work fine and have an uninstaller etc.
After that, it's more logical to explain that UV channels have half vertical and half horizontal resolution in mpeg formats (except for mpeg-2 studio profile that works in YUY2)RGB uses the fact that three colour components can be added together to create any colour.
In contrast, YUV stores the colour the same way human brain (not eyes) works.
Now comes an explaination: the primary thing that human brain sees is brightness, aka "luma". (not neccessary inline: Luma can be relatively easly calculated from RGB channels by averaging them with some weights. Scientists have came up with weights that match these of human perception - green has high contribution, red half of that, blue - one third of red).
Luma is a non-negative value. Zero means black, high values mean white.
As for colour information, they are not so easy. They are called U and V, or Cr and Cb. They can have both positive and negative values, and completely match the way colour is processed in our brain.
Cr, when positive, means that the object is red.
Cr negative means that the object is green. Our brain understands these two colours as opposites - If you think about it, no object can ever be red-greenish.
Cb, when positive, indicates blue object. Cb negative means yellow. Again, they are the opposites for our brain.
So, why is it useful to store pictures in YUV? Two reasons:
- because you get one channel that is much more important, and two channels that are less, but equally important. You can do tricks with that.
- you get channels that are almost uncorrelated - if shadows move, only one channel is affected. This helps compression quite a lot, because the same information is not repeated for different channels.
- a historical reason: when colour TV was invented, it needed to be both backwards and forwards compatible with black-and-white TV. The old B&W channel became luminance, while two colour channels were added on top of that. Old TVs simply ignore the two extra channels while colour TVs just understand that chroma is zero in B&W signal.
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