Computer / Studio RGB ?

[Lucius Snow]

Hello,

When i render a video, either in AVI DV or MPEG2, i get a different display between a computer and a TV. The picture is much contrasted and luminous on a PC. I found back this difference into an effet in Vegas which is called "studio RGB to computer RGB". So i deduced that the RGB norms (and so those of luminance?) were different between a computer and a TV. In the weeks coming, i have to burn 300 DVD but i'd like to get the original colours not only on TV but also on PC in case the consumers play the DVD on a computer.

Here are two captures. The first one is done by Vegas (which displays as studio RGB for TV). The second one is captured in WinDVD.

http://poubellelqt.free.fr/cap/Studio_RGB.bmp
http://poubellelqt.free.fr/cap/Computer_RGB.bmp

You can notice well the overexposure on Computer_RGB.bmp (which doesn't exist with the same video played on TV).

So my question is : Is it possible to avoid this difference by making a DVD ?

I render in frameserving from Vegas (through the frameserver by Debugmode in RGB24) to Canopus Procoder 2.0 (in "mastering quality" preset). By the way, i've been adviced to apply the filter "601 correction - Schrink Color Space" in Procoder. It reduces well the overexposure on PC but the original colours get very diminished. I've made two other captures to compare :

http://poubellelqt.free.fr/cap/Sans_filtre.bmp
http://poubellelqt.free.fr/cap/601_correction.bmp

The first one is without the filter, the second one is with the filter. Both captures were done in Vegas so as studio RGB for TV.

Thank you.

[kschang]

Not really, not without some sort of a color calibration tool.

You can try using the THX Calibration tools (like the one included on the T2 Extreme Edition) on both the PC and the TV, and see if that helps align your expectations.

[mikiem]

There's a lot of details & stuff if you want to look further on-line, but the shortest, most basic version is that the broadcast TV spec does not include the color range 0 - 255 that your PC does. Further complicating things, too much color saturation is one of a number of things that can throw signals to the TV off.

Very Generally, retail DVDs are in the color space for TVs, and player software can then compensate on a PC. The other way around can be pretty risky if you have no control over the TV(s) used for playback.

Converting from one space to another should be done on a graduating basis -- you want the full color spectrum increased or decreased. Later versions of Vegas can do this -- read the docs. *601* refers to a video standard using smaller color range, as does I believe DVD spec (DVD comes from D1 comes from 601). You definitely don't want to go up to PC range, then back down in Procoder -- lose quality for no reason.

Edit: adding on in case it helps... the colorspace of a video, like the colors in a video, are first determined by the camera & it's settings. It's a whole other issue if you want to explore color correction itself -- something that Vegas can also accomplish -- and a worthwhile effort I think.

[vhelp]

In a nutshell..

sRGB = 16-235 ... (or studio RGB)

I found back this difference into an effet in Vegas which is called "studio RGB to computer RGB".

When you were deciding on your Vegas app feature earlier in your post,
the conversion process is basically a 16-235 -to- 0-255 color mapping.
When this is done, there is a certain amount of loss in this, because
it is a lossy step.

Also, if you do choose to use this filter, and then framerserve this
to another editor (or specially, MPEG Encoder) it will go through
*another* color space conversion (or mapping) and make things worse.
If the editor or MPEG encoder does not auto-convert to 0-255 range,
it will probably do some other weird step to your source. In any case,
this just might be your (added) issue(s) with your color quality.


By the way, i've been adviced to apply the filter "601 correction - Schrink Color Space" in Procoder.

(by the way, I was the first to discuss this and make the suggestion
to use this filter--later, I made the part 2 findings that even with
this filter (procoder) that it is not enough to equalize or reproduce
to the same color quality as the source -- bummer)

[re Procoder..]
The 601 correction filter in procoder (PC) is the proper filter
to use for all your sources. But, it seems to be either broken or
buggy, because it is not fully matching the source's color quality.
But its the next and best closest method for your process. (I would use it,
IMHO)


Last.., in your final process, your source avi to your MPEG encoder
should always receive 0-255 color mapping, not 16-235 range. The
reason for this is because most MPEG encoders will auto-convert if
the source is not 0-255 range. But, I could be wrong on this, or
it may be on an MPEG to MPEG basis - (though you could test this
out for yourself and the MPEG encoder(s) you are using)

Anyways.


My suggestion would be to pay more attention to your source, and know
what you are serving to other sw utilities.

For instance, if your source is from a DV cam, like home footage,
then it's good to know the following:

** is YUV color space, and is 4:1:1 sampling [NTSC], or 4:2:0 [PAL]
** is 16-235 color range [IRE 0.0]
.
** if using TMPGenc, let it convert to RGB and 0-255 range, or
** if frameserved to TMPGenc, make sure editor is RGB w/ 0-255 range.
.
** if using AVIsynth, make sure you set it to RGB and 0-255 range
throughout, and don't use other conversion filters during useage.
TMPGenc uses RGB24, so be sure you have it set that way, so that
it does not go through another conversion.

Also, don't convert color ranges. It's not necessary and bad for
most MPEG encoders, because of the way they require source to be
in 0-255 color range.. I think.

For instance, if your source is DV, then don't convert to sRGB at
any stage and then frameserve it to your MPEG encoder. But instead,
convert to RGB, and then frameserve to TMPGenc, or let TMPGenc
convert for you. It will do it correctly the first time.

If your source is MPEG ...

I would suggest to convert it to a full RGB color space, and then
frameserve it to TMPGenc, or let TMPGenc do the conversion. It
will do it correctly.

If your MPEG encoder is CCE, I would suggest to use AVIsynth and
convert to YUY2 color space and frameserve to CCE, because it uses
YUY2 color space. (don't convert to sRGB or 16-235 range)

If your MPEG encoder is Procoder, I would suggest to convert to full
RGB color space and frameserve to Procoder.


Brief FAQtoid..

DV = YUV 4:1:1 / 29.970 fps NTSC or 4:2:0 / 25 fps PAL
MPEG = YUV 4:2:0 (NTSC and PAL) is beleived to be in YV12 format
TMPGenc = RGB24 color space - [outputs YUV 4:2:0]
CCE = YUY2 color space - [outputs YUV 4:2:0]
Procoder = RGB/YUV conversions - [outputs YUV 4:2:0]
.
TV = YUV 4:2:0 or 4:2:2 [but is probably 4:2:0]
PC = RGB and 0-255 color range
.
RGB = 0-255 color range
sRGB = 16-235 color range (aka, studio RGB)
.
sw dvd players = UPsample 4:2:0 to 4:2:2

-vhelp 3746

[mikiem]

@ vhelp
FWIW & all... Hoping it interests

Reading the old Snell & Wilcox docs, they talk about re-constructing the signal going from 4:2:0 back to 4:2:2 in a lossless or very near lossless manner. Anyway, perhaps an interesting read if you're into that stuff.

& FWIW, I've had some success tracking color range changes by checking frames & screen captures in an image editor... Know you can do it in video software, but like the extra tools.