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Old 10-16-06, 03:49 PM   #1
asbestoshead
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Question TV out - colorspace clipping or scaling?

Hi,

I'm using NVIDIA 1.0-8756, Linux kernel 2.6.11.11, and a PCI Express GeForce 7300 LE (WinFast PX7300 128MB LE TDH).

I'm doing TV output (NTSC, 640x480).

According to this page:

http://people.csail.mit.edu/tbuehler/video/titles.html

I should limit RGB values to between (16,235) when doing NTSC output to avoid saturation and clipping artifacts. Does this apply equally when I'm using NVIDIA?

In other words, does the hardware just clip output to (16,235), or does it automatically rescale between (16,235)? If it clips, I have to rescale myself; if it doesn't, I shouldn't be rescaling.

Thanks,
Mike
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Old 10-16-06, 04:23 PM   #2
Thunderbird
 
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Default Re: TV out - colorspace clipping or scaling?

A while ago someone did meassurements on the tvout signal of nvidia boards and if I remember correctly the meassurements illustrated that the tvout encoder uses (0,255). Try the search function though it might be hard to find.
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Old 10-17-06, 08:30 AM   #3
asbestoshead
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Default Re: TV out - colorspace clipping or scaling?

Thanks. No luck finding it so far, but maybe I can do a bit of trial and error to double-check that.
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Old 10-20-06, 02:57 AM   #4
pgs
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Default Re: TV out - colorspace clipping or scaling?

Quote:
Originally Posted by asbestoshead
Hi,

I'm doing TV output (NTSC, 640x480).

According to this page:

http://people.csail.mit.edu/tbuehler/video/titles.html

I should limit RGB values to between (16,235) when doing NTSC output to avoid saturation and clipping artifacts. Does this apply equally when I'm using NVIDIA?

In other words, does the hardware just clip output to (16,235), or does it automatically rescale between (16,235)? If it clips, I have to rescale myself; if it doesn't, I shouldn't be rescaling.

Mike
That page is not really correct. The luminance channel has the limit (16-235), but the chrominance has (16-240), and white is (235, 128, 128), since the channels are offset.

Second, if the chroma encoder (RGB->YUV or YCbCr) is done properly, it will use full RGB range (0-255) and convert to correct (16-235, 16-240...) range, so, in principle, no RGB clipping is necessary. Also because is not really clear which kind of clipping should be done (16-235 or 16-240?).

Related to the title problem, it is always reccomended to choose, in any case, proper colors, since many combination (extreme case is red on blue) do not work well.

Hope this helps.
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