On 30-Jan-2003 Matto Marjanovic wrote:
| I was under the impression that mpeg2enc does *not* clip out-of-range
| values and does simply encode them. Although values outside of
| [16,235] do not map to R'G'B' codes, they are still valid values
| --- and necessary even, if you want to encode things like
| standard colorbars.
|
There is a simple scenario I used to test theese things: I created a jpeg
with colorbars of known RGB-values ( grey0(=black), grey8, ...,
grey255(=white), and similar for red, green, blue ). Then I duplicated the
file many times and made a YUV stream of it using jpeg2yuv. I piped this
stream to some mjpeg-tools and checked the value range of the YUV stream
after each tool by inserting a selfmade yuvfilter. At last I encoded the
stream with mpeg2enc, decoded it with mplayer to jpegs again and checked
the colorbars with gimp.
Result: Everything works well if YUV ranges are kept within
[16-235]-range. mpeg2enc then encodes [black-white] and after decoding the
original colorbars are replicated. When YUV-ranges broaden before
mpeg2enc, then the resulting colorbars have an increased contrast and the
dark/light bars are replicated as black/white.
So it's indifferent where in mpeg2enc/dec the clipping/whatever takes
place. The main thing is, that out-of-border-values finally get lost.
This is bad. It's not intended.
So why not accept standards and try to cleanup software?
Regards,
Ralf
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