Re [PD] pix_freeframe
padawan12
padawan12 at obiwannabe.co.uk
Thu Aug 31 10:23:36 CEST 2006
I'm not following this thread but nice explanation.
So YUV in anything other than 4 4 4 is basically
"compressed video" ?
On Wed, 30 Aug 2006 19:44:25 +0200
pär F <p.friden at gmail.com> wrote:
> Sorry about the double-post, wrong key sequence.
>
>
>
>
> I kind of get the feeling this thread is in need of some
> clarification regarding RGB, YUV, 4-1-1, 4-2-0, 4-2-2 and 4-4-4. I'll
> threat you all as idiots and try to explain it from scratch.
>
> A digital image is built up by pixels.
>
> If the picture is in gray-scale you only need one number to define
> the tone of each pixel. The defining number is somewhere in a
> predefined range, the greater the range the more possible tones. If
> the range is only 0 and 1 you can define only black and white. If the
> range consists of more than about 200 possible variations you can
> define a seemingly unbroken grayscale between black and white (8 bits
> equals 256 variations). If you need to make vast manipulations of the
> tones of your pixels you might need an even greater range of possible
> tones in your original pixels (that's why there is such things as 16
> bit and 32 bit grayscale images).
>
> In a RGB-format image the color is defined by 3 grayscale channels
> (that is three separate grayscale images really), one for red, one
> for green and one for blue. It is also possible to ad other channels
> to an image, such as alpha-channels for transparency or z-channels
> for depth or any other channel you find useful. Every pixel in your
> image could be defined by 1, 3, 4, 5 or even more channels, each
> consisting of 1, 2, 4, 8, 10, 16, or 32 bits.
>
> YUV also contains 3 channels, but the luminance (brightness)
> information (Y) of the image is separated from the color information
> (u and v). The human brain is much less sensitive for changes in
> color than for changes in luminance. Therefore if the color
> information is separated from the luminance you can save space by
> removing some of the information in the color-channels while
> retaining approximately the same visual qualities. 4-1-1 means you
> have only one pixel in every color channel per pixel in the luminance-
> channel (4-2-0 is a variant where the color-pixels alternate between
> u and v every frame (or field?), the compression is the same). 4-2-2
> means 1 color-channel pixel per 2 luminance pixels. In this respect
> RGB is actually always 4-4-4.
>
>
> 4-2-2 or 4-1-1 reduces space in a great way, but if you need to make
> manipulations that moves information from color to luminance the
> method might be a rather bad idéa. Worse even, if you need to use
> color-information for creating an alpha-channel (blue-screen/green-
> screen f.ex.) you might experience serious loss of quality.
>
>
> regards, Pär
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