PDA

View Full Version : Color & Luminosity: LW > TV


narph
07-13-2005, 03:55 PM
I'm having some confusing color and luminosity issues when I look at my LW project on a TV monitor. Why confusing? Some scenes suffer, some don't. Also, I don't know why some colors appear significantly different on a TV monitor than on the computer monitor: example: grey surfaces which appear bluish or reddish on TV screen..colors which are literally shades of grey with no blue or red in them whatsoever.

Now, I realize that RGB colorspace is different from Video/TV colorspace, but what I'm looking for is to be able to see that I'm in the ballpark with my color and luminosity. I can deal with making tweaks in FCP, but I'm hesitant to do the serious rendering of my scenes until I gain understanding of the cause of my color problems.

Note: I'm operating on a shoestring here (a frayed one!) and don't have a 'real' NTSC monitor..my workflow is LW>DVView>Firewire>DV Cam>RCA>TV, or FCP>Firewire>DV Cam>RCA>TV.

As I mentioned, some of my 3D work looks reasonably close on the TV to how it looks on my RGB computer monitor..in other words, the colors look as they should as does luminosity. I know I'm not going to get a perfect match, but I need to get it reasonable..ie, gray should be gray, blue should be blue, black black, red red, etc.

I have done experiments such as with simple scenes containing simpleshapes and grey values. Test render: color OK. Change a surface to a color, test render: color ok. I use the same lights as the trouble scene contains. I've duped that scene's object file and changed all surfs to grey. Test render: color OK. When I begin adding back the real surfaces, that's when the color gets wonky.

I'm pretty confused and could use some help. thank you in advance.

Mylenium
07-13-2005, 05:50 PM
I'm having some confusing color and luminosity issues when I look at my LW project on a TV monitor. Why confusing? Some scenes suffer, some don't. Also, I don't know why some colors appear significantly different on a TV monitor than on the computer monitor: example: grey surfaces which appear bluish or reddish on TV screen..colors which are literally shades of grey with no blue or red in them whatsoever.

Now, I realize that RGB colorspace is different from Video/TV colorspace, but what I'm looking for is to be able to see that I'm in the ballpark with my color and luminosity. I can deal with making tweaks in FCP, but I'm hesitant to do the serious rendering of my scenes until I gain understanding of the cause of my color problems.

Note: I'm operating on a shoestring here (a frayed one!) and don't have a 'real' NTSC monitor..my workflow is LW>DVView>Firewire>DV Cam>RCA>TV, or FCP>Firewire>DV Cam>RCA>TV.

As I mentioned, some of my 3D work looks reasonably close on the TV to how it looks on my RGB computer monitor..in other words, the colors look as they should as does luminosity. I know I'm not going to get a perfect match, but I need to get it reasonable..ie, gray should be gray, blue should be blue, black black, red red, etc.

I have done experiments such as with simple scenes containing simpleshapes and grey values. Test render: color OK. Change a surface to a color, test render: color ok. I use the same lights as the trouble scene contains. I've duped that scene's object file and changed all surfs to grey. Test render: color OK. When I begin adding back the real surfaces, that's when the color gets wonky.

I'm pretty confused and could use some help. thank you in advance.

From a personal view as someone who does 3D, regularly does compositing and even does a little editing from time to time (Avid), I have to say your approach is quite crap. How in hell can you expect to properly judge colors on your stuff if it gets converted 3 times on its way to the monitor?

That aside, you need to understand the technical side of things much better. Luminosity is limited to values between 16 and 235 in YUV as opposed to 0-255 (it's actually more complicated and measured differently, but I'll spare you the details), anything that exceeds those limits gets clipped or warped. Colors are treated as differential values within certain ranges that also should not be exceeded. Those strict rules also apply to DV and that's where your stuff goes boom. Even a single wrong pixel will do the following:

a) It will confuse the DV compression algorithm (normal DV has no such thing as "Super Black" and "Super White")
b) on a strictly electrical level create values that will make proper digital to analog conversion impossible (one color component will shift out of sync trying to compensate for that which adds a tint to your image)

I guess in the test scenes you used you were just lucky 'cos all of the pixel values were within a certain range. Only the textures will introduce "illegal" values. No need to despair, though.

Dunno FCP except from some read-ups, but I'm sure it has a tool for video legal levels (an effect or color correction preset presumably) that will either adjust contrast automatically or mark the wrong pixels in a certain color as a warning. Similarly, LW has such a filter of its own, but it's rather useless since it does not natively work in YUV color space. In addition to that follow a few simple rules:

- stay away from bright reds
- minimize overall luminosity by adjusting ambient light values in your scene
- DV has a tendency to make your image look greenish so try to compensate for that by adjusting your lights and surfaces accordingly (i.e. turn down all green values just a tiny bit)

Mylenium

narph
07-13-2005, 06:37 PM
OK..when you say that my approach is 'quite crap,' are you referring to the path: computer>firewire>DV Cam>RCA>TV?

I get from your informative response that if I could improve the way I get signal to the TV, that color would improve..is some kind of Card the way to go? ie, computer>card>TV?

thanks!

plotz
07-13-2005, 10:42 PM
If you've got neutral grays taking on a color tint then check the cabling you're using to connect to your TV. Chances are good that the RCA connection might be loose. Wiggle that RCA cable a little and I'll bet you'll see the color flicker.

The other possibility is that your DV conversion software isn't properly transcoding the RGB information in the color and luminance information your TV uses. I haven't used the DV View thing you talked about, but it's possible that FCP might be set up to output the wrong format. When you transcode RGB to DV you're combining color and luminance information in a completely different way. If you try and transcode the wrong way (for instance choosing "8bit D-1 instead of "DV-YUV") you'll get a funky color shift. But that shift would be consistent. The fact that colors are changing leads me to believe it's a bad cable.

I the FCP scenario you should make use of the built in waveform and vectorscope. They'll allow you to monitor brightness (waveform) and color (vectorscope) information accurately. Here's a shoestring test:

1. In FCP drag a "color bars-NTSC" from the effects folder to the timeline. (I'm not exactly sure of the folder name or nomenclature, but it's in there somewhere. Just make sure to choose NTSC. This is the standard color and luminance information test signal for video.

2. Export that pattern as a still frame you can use as a texture in LW.

3. In LW create a 4x3 plane and map this texture to it. Make the plane completely luminant. Turn off "ambient" in the light properties tab and set the value of the default light to zero. Render.

4. Now compare what's on your computer monitor to what's output to your TV through FCP. They should be in the same ballpark color wise, but saturation (brightness) may be off.

5. Use Apples color setup utility to calibrate your monitor. When you get to the step where "gama" is adjusted you'll need to bump the value up to something more video-like. On Mac OSX hooked up to a standard (cheap) CRT that value usually ends up being between 1.6 and 1.8.

Once you've got the gama values worked out between your CRT and computer monitor then you can be confident your stuff is going to translate fairly accurately. Normally you'll need to tweak whites or blacks, but colors should NEVER be off or inverted. If they are then it's due to a hardware problem, or a bad transcoding operation in software.

Color consistency from the computer the tv screen is a real mother. Quantel used to offer a free booklet they'd send out on this stuff that was tailored toward computer graphics types...I don't know if they still do it.

Crap, I forgot to add the last step.
Render out a full frame of the color bar pattern from LW and import it into FCP. Then open the vectorscope and compare how much color shift there is between what you rendered and the color bar pattern from FCP. There should be very little shift in color values. If this is the case then you know your files out of LW are fine and the culprit lies in another (fixable) area.

narph
07-14-2005, 02:10 PM
The other possibility is that your DV conversion software isn't properly transcoding the RGB information in the color and luminance information your TV uses. I haven't used the DV View thing you talked about, but it's possible that FCP might be set up to output the wrong format.


Hey, plotz;

thanks for the suggestions; I'll try them. I have made sure that my rca connection is snug; I do have another cable and will try that.

I need to clarify that the color issues are having right now are occurring directly out of LW 8.3; I haven't gotten to the FCP point yet. I thought I'd get my colors under control in LW first, then use FCP for tweaking, rather than gross transformation of way-off wrong color.

n

plotz
07-14-2005, 02:30 PM
I'd strongly recommend doing your color comparisons on files in FCP rather than output from LW to a TV monitor. Once you're in the FCP environment you're in a "video standards" world where it's easier to make comparisons. Lightwave isn't geared for this task.

By rendering a file on your computer and looking at it's signal on a waveform and vectorscope you're going to get a a true comparison against a TV standard.

Outputting directly from the computer monitor to a TV is not an accurate test of image quality.

CGTalk Moderation
07-14-2005, 02:30 PM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.