Neutral Test Gray Card - Why doesn't it work for color correction?

johnmeyer wrote on 9/9/2004, 8:36 AM
I take lots of video in gymnasiums. The lighting color is very odd. So, I thought, why not use my Kodak 18% reflectance Neutral Test Card? I shot video of the card, with the card on the ground facing straight up at the lights in the gym ceiling. Later, in Vegas, I loaded the Color Corrector fX, clicked on the complementary eyedropper for the middle wheel, and then clicked on the middle of the image of the gray card.

When viewed on my calibrated external NTSC monitor, the color balance wasn't even close!

I tried various combinations of using the other two wheels (the shadow and highlight wheels), but finally gave up.

I would have thought that using a calibrated gray card would be the ultimate way to balance color, but I guess there must be a flaw in my reasoning, or my technique, or in the way the color correction tool works.

Am I missing something?

Comments

mcgeedo wrote on 9/9/2004, 8:41 AM
Hi John,

Why don't you hit your color balance button on your cam while pointing at the card in the gym?

When you say "it wasn't even close," what did it look like? I would have thought your idea (with Color Corrector fx) would work...
Former user wrote on 9/9/2004, 8:46 AM
Most cameras use a White balance instead of gray so there must be a reason.

Dave T2
Bill Ravens wrote on 9/9/2004, 8:47 AM
I think your idea was good, but, the implementation was flawed. Compementary color correction will generate the opposite color as the lighting. Now, if that was applied as a filter or color mask, you'd be all set to go. Unfortunately, that's not my understanding of how it works. It applies that color to the image. If you could subtract the RGB color correction info from neutral gray and apply the difference to the corrector, you'd be good to go. I've found, the best way to do white balance in post is to manually adjust the color corrector until things look right. If you want to be more technical about it, go to the curves plugin and adjust the curves to match the RGB values between the known card values and what you measure with the eyedropper.

Hope this helps.
musicvid10 wrote on 9/9/2004, 8:49 AM
If each of the primary color curves was linear and had identical slopes, your method would work.

However, this "ideal" situation doesn't exist, so even if your nuetral gray is perfect, the near-whites and near-blacks can be different colors entirely. If you shoot a grayscale you will see this immediately.

There are filters made for still cameras to compensate for the skewed curves caused by flourescent, sodium, and mercury vapor lights. You should be able to find one to fit your camcorder. These work pretty well, but are expensive and reduce the amount of available light.

You can also do a manual white-balance on a white card, shoot a grayscale, then in Vegas adjust the color balance and tweak the Gamma adjustments to even out the curves, but learning to make precise Gamma adjustments involves a lot of time and a rather steep learning curve.

IF you can live with some anomalies, just a good white balance and minimal color correction will suffice.
Spot|DSE wrote on 9/9/2004, 10:36 AM
There are also color-balanced cards available for various lighting conditions.
We use the WarmCards for ours. I don't like shooting through filters except as a very last resort. Too hard to fix in post if I forget (which I sometimes do) to take them off.
Chienworks wrote on 9/9/2004, 12:03 PM
Another problem you'll have in the gymnasium department, aside from the non-linear color spectrum of the lamps, is that it's quite possible that each lamp has a radically different spectrum from the others. As fluourescent and sodium vapor lamps age they change color drastically. What is a "white" mix where you shoot the card can be very different just a few feet away under different lights. My high school gymnasium also had a mixture of a few large sodium vapor lamps to light the court, and all fluourescent around the sides to light up the bleachers and for cleaning crews. Gyms also tend to have huge colored banners or posters hanging up here and there. Anything near such a banner will take on the color cast of that banner. So, depending on where you stand in the gym, which direction you are facing, and a multitude of other factors, the color balance can vary tremendously.
mjroddy wrote on 9/9/2004, 12:12 PM
We also use the WarmCards (Got mine from Markertek). They make an amazing difference. The hype you find on these guys is not overrated. We have the blue warm cards and the green as well. Highly recommended.
Randy Brown wrote on 9/9/2004, 12:21 PM
So Kelly, do you just use an "auto" white balance? I'm shooting football (w/ Canon XL1s) for a team with purple and white uniforms. I recently read that purple is inherently a problem showing up blue on DV but now think it has something to do with the type of lighting (as you mentioned). At their home games I finally resolved to trying to fix it in post because after several manual white balances and trying "auto" it shows up blue. However I shot their first away game last week (under the lights) and the colors are perfect on both cams using "auto".
Randy
musicvid10 wrote on 9/9/2004, 12:23 PM
BTW, I have a few interesting resolution and stepwedge charts zipped here......
ftp://shell.dimensional.com/users/musicvid/pub/images
musicvid10 wrote on 9/9/2004, 12:29 PM
Spot, mjroddy,
Please give a little background on Warm Cards and what their uses are. Sounds very interesting.
rs170a wrote on 9/9/2004, 12:36 PM
musicvid, read all about them at http://www.warmcards.com/
Their site goes into a lot more detail than a post here could.

Mike
busterkeaton wrote on 9/9/2004, 12:56 PM
Also, shouldn't grey cards be used not pointed directly up at the light source, but held at the same angle as your subject. That is, if your subject is standing up the card should be held up vertically perpendicular to the overhead light. This is because you are looking for the reflected light.
Spot|DSE wrote on 9/9/2004, 1:27 PM
http://www.warmcards.com
Ask for Doug, tell him you're a VASST student and get a discount.
They are GREAT tools, handy for any shoot. We use them religiously.
rs170a wrote on 9/9/2004, 1:32 PM
I've shot video under all kinds of different lights and my single rule of thumb is always white balance with a good white card (mine is a piece of white plastic that I clean constantly). If I go into a gymnasium or factory with multiple (ie. different colour temp) light sources, I find the brightest area I can, light if I can/have to, white balance and shoot.
Same thing in an office with lots of windows. I light the specific area I want, do a white balance and forget about the fact that the windows will be blue and the office lights green. As long as the talent/subject looks good, that's really all the viewer will care about.
There's no way, short of having a fully-loaded grip truck at your disposal, that you'll be able to colour correct every single light fixture to your satisfaction.
Finally, it really does help to have a good colour field monitor with you so you know what you're getting on tape.

Mike
Bill Ravens wrote on 9/9/2004, 2:21 PM
eu contraire....gray works just as well as white....the only difference is the luma signal is lower than white. The RGB values are all equal in both and gray doesn't blow out the video image.
rique wrote on 9/9/2004, 2:25 PM
Maybe I'm missing something here but I've never heard of a gray card being used for color balance, only exposure. When I shot film, an 18% gray card at the subject could be used to get a proper exposure if you were using a reflective light meter.
johnmeyer wrote on 9/9/2004, 2:52 PM
When I shot film, an 18% gray card at the subject could be used to get a proper exposure if you were using a reflective light meter.

That's actually why I have this card. However, in my still photography work, the color correction tools ask that you find a "neutral color" (meaning gray) somewhere in the image, and if you click on that, then if it is truly neutral, and if it is lit the same as the subject, any color cast will be removed. It works great in my photo editing program, so I though it would work in Vegas.

I'll try correcting the individual color curves and see how that goes. I think I can use the Vectorscope for this.

[Edit 5 minutes later]: Nope, the color corrector does pretty much the same thing to the color values as what I can do with the color curves.
farss wrote on 9/9/2004, 2:59 PM
John,
film has a much larger latitude than video. If the color balance is off more than a little then in 8 bit video systems too much info is lost for color correction to pull it back. In the camera the image is adjusted in a between 10 and 14 bit (depending on thge camera) space prior to downconversion to 8 bit.
This why things like cinegamma need to be done in the camera. not in post.

Bob.
rmack350 wrote on 9/9/2004, 3:13 PM
Maybe I'm piling on but grey cards were never designed for color calibration-just exposure. In film we'd shoot a color chip shart. Many people use a packaged folder of test charts. They keep it in a camera bag or accessory bag.

Rob Mack
wolfbass wrote on 9/9/2004, 3:39 PM
Johnmeyer:

As I sit reading the forum items during a quiet time at the Gym I own, Iwas wondering:

What sort of stuff are you shooting in Gyms?

Cheers,

Andy
rs170a wrote on 9/9/2004, 3:59 PM
Bill Ravens wrote:

You're right, it doesn't. Neither will white though - if you do it according to the manual (at least with the gear I use) which says "set the camera to auto iris and fill as much of the frame as possible with a white card." I then dial the iris back to my desired setting. Mind you, I'm talking about 3-chip cameras like the JVC-550 or a BetaCam of any kind. YMMV.

Mike
johnmeyer wrote on 9/9/2004, 7:35 PM
What sort of stuff are you shooting in Gyms?

Volleyball and basketball.

Basketball Video
NickHope wrote on 9/10/2004, 12:48 AM
Back to the color corrector... Billyboy in another thread has suggested to me that I use the complementary eye droppers for the lowlights and highlights first, and only use the midtones eyedropper if necessary after those. This might be a better approach for the footage you've already shot, forgetting the grey card and using tones from the action itself.

I hadn't heard of WarmCards but this concept of shunting the color balance with a complementary colored card is an interesting one. I white-balance underwater by pointing at white duct tape on my fins. I zoom in so the whole frame is filled with the tape before pressing the white balance. However a lot of my footage has been coming out a little too red for my liking and I'm now thinking of painting one of my fins with pale pink nail varnish to shift the balance back towards blue-green!

On a similar Heath Robinson note, I'm off to Photoshop... pale blue backgounds.... print.... laminate..... job done (sorry Mr Warmcards).