Computer RGB - when to use?

douglas_clark wrote on 10/10/2007, 2:40 AM

I've read threads and Glenn Chan's article on color spaces in Vegas but I'm confused when to convert to Computer RGB for output.

I now understand for digital camera stills/slideshows, to add Color Corrector with Computer RGB to Studio RGB preset to all stills on the timeline.

But when rendering, I'm not sure if or when to use the reverse Color Corrector Studio RGB to Computer RGB preset as a video output fx.

Glenn first says "The Vegas default codecs will want to see studio R'G'B' levels." but then says "When encoding to most web streaming formats, the encoder will want to see computer R'G'B' levels." Exactly which codecs expect Computer RGB?

I primarily use the following encoder/display combinations for web, laptop-to-projector and PAL SD DVD output. Assuming everything on my timeline is in Studio RGB, should I convert video output to Computer RGB for any of these?
1. Flash for web, progressive download. I encode to uncompressed avi for input to Adobe Flash encoder or Camtasia Flash encoder;
2. WMV9 for web progressive download;
3. Cineform avi (1024x768 slideshow) for PC playback to projector via VGA cable, playback in Windows Media Player;
4. MainConcept MPG2 for SD DVD to SD TV.
5. (I will begin encoding to H.264 for flash playback when the new Flash 9 player update is released.)

Comments

Bill Ravens wrote on 10/10/2007, 6:57 AM

See Glenn's article entitled V8Color. I've found that the best workflow is to always include a colorbar pattern in my videostream. I use it to check the in/out colorspace. When I go to my final render, I remove the colorbars.

GlennChan wrote on 10/10/2007, 5:28 PM

"The Vegas default codecs will want to see studio R'G'B' levels."
Ack - that sentence wasn't quite clear.

The Vegas default video codecs (DV, HDV, MPEG2) usually want to see studio RGB levels. (But this is not true anymore in Vegas 8.)

As far as the particular codecs go:
1- AVI uncompressed (***from 8-bit project***): Computer RGB likely.

2. WMV9: Computer RGB, if encoding straight from Vegas (i.e. not from WME).

3. Cineform avi: To get the correct Y'CbCr values, you need to send studio RGB levels from Vegas (via VFW / video for windows).

I'm not sure what the decoding behaviour is when you play back via Windows Media Player (you need the codec to work outside Vegas; so I think you need to buy it / demo it).

4. MainConcept MPG2 for SD DVD to SD TV. From 8-bit project, send studio RGB levels.

douglas_clark wrote on 10/11/2007, 3:50 AM

Thanks Glenn. That helps a lot!

Regarding 3- CineForm, it is rendered in Vegas7 to avi with the built-in CineForm HD Codec V2.5, as custom 1024x768 25p preset. (this is a lossless codec, right?) I do not use any external software. I play the avi back in WMP with projector attached. It looked OK on screen with the projector set to sRGB...but I didn't have any bars (or time) to check calibration.

Where do you find these details about codec color space, Glenn? There doesn't seem to be much documentation on it in Vegas. Your article on http://glennchan.info/articles/vegas/v8color/v8color.htmColor Spaces in Vegas 8[/link] is a great help!! But what about 32 bit? You end your article with "Unfortunately, color spaces are a lot more complicated in the 32-bit modes of Vegas 8." Does that mean your table of codec color spaces does not apply in 32 bit?

Can we hope for a white paper from Sony to clarify these color space issues for VP8?

Bill Ravens wrote on 10/11/2007, 6:38 AM

as I said, the ONLY way to be sure is to bring a test pattern and monitor the pluges every step of the way.

GlennChan wrote on 10/11/2007, 12:11 PM

Regarding 3- CineForm, it is rendered in Vegas7 to avi with the built-in CineForm HD Codec V2.5, as custom 1024x768 25p preset. (this is a lossless codec, right?)
Cineform is not mathematically lossless. It is visually lossless. Considering the time you save by going with smaller files, I would prefer it over mathematically lossless codecs. Because you need to get work done and it's highly highly unlikely that anyone would be able to spot the difference.

I do not use any external software. I play the avi back in WMP with projector attached. It looked OK on screen with the projector set to sRGB...but I didn't have any bars (or time) to check calibration.
To check cineform, add color bars to your timeline and encode that to Cineform. Play that back in WMP on your projector... you want the leftmost PLUGE bar to be clipped on the projector.

If that doesn't happen then you should apply a studio RGB to computer RGB preset onto your whole project.
*Doing this is somewhat wrong as the levels in the Cineform file will/may be non-standard. But that is ok in a closed system.

Where do you find these details about codec color space, Glenn?
You can test things to figure out what conversions are going on.

But what about 32 bit?
The third column in the table in the Vegas 8 article has 32-bit mode information.

You end your article with "Unfortunately, color spaces are a lot more complicated in the 32-bit modes of Vegas 8."
IMO... there's no clear logic to it / the codecs are on a case-by-case basis. DV and Cineform don't change behaviours like the HDV and MPEG-2 codecs do. So you need to know what each individual codec does.

MH_Stevens wrote on 10/11/2007, 8:19 PM

........"3. Cineform avi: To get the correct Y'CbCr values, you need to send studio RGB levels from Vegas (via VFW / video for windows). "..............

Could you please explain this sentence. I assume some HDV has been captured with Cineform and converted to the Cineform avi files and then imported into the Vegas time-line. Are you saying that when rendering that time-line to SD DVD with DVDA or to HD with WMV9 you need have the Vegas project set to Studio RGB?

Glenn, you are one of the most knowledgeable guys on Video out there but don't forget some of us, me anyway, can't always read between your abbreviated lines or summarized work-flow descriptive.

GlennChan wrote on 10/11/2007, 10:56 PM

Sorry, what I meant by that is

when encoding something to Cineform, you need to send studio RGB levels to get correct Y'CbCr values in the Cineform file.

So I wasn't talking about encoding from cineform to other codecs.

Grazie wrote on 10/11/2007, 11:34 PM

What does the "Studio" mean in "Studio RGB"? Computer RGB, I kinda get - but Studio? Been wanting to ask for at least 4 years now.

TIA

Grazie

GlennChan wrote on 10/12/2007, 10:36 AM

Like studio video / in a television studio. I think so anyways.

It's to differentiate between the television/video world from the computing world (different needs, though perhaps definitely converging).