Color: Gamma, Gain, Offset What's the diff?

Sidecar2 wrote on 7/27/2007, 12:14 PM

Need to brighten 720-30p Sorenson3 video before rendering out to Windows Media HD. Using the Color Corrector, should I shift Gamma, Gain or Offset upwards to hold the blacks but brighten the overall look?

What's the difference between Gamma, Gain and Offset?

Thanks.

PS
Yeah, I know I should have taken the color correction class when it was here in LA a few months back, but my company only reimburses if the class takes place at a university, not at a hotel seminar. Bummer.

Comments

GlennChan wrote on 7/27/2007, 1:26 PM

Gamma, gain, and offset have different definitions depending on what application you're looking at and even what filter. For example, "Saturation" in Vegas has different maths behind it (Y'CbCr / chroma gain, HSL math, etc.)... it's different in the color corrector versus HSL adjust.

The key thing behind gain is that all the values are *multiplied* by a particular number. It may have an offset (add / subtract a particular value) to keep black level the same.
Offset just means you add/subtract a particular value.
Gamma applies a power function (curve) to everything. If you are playing around with a color curves filter, it makes the curve concave/convex. If that makes sense.

2- Sometimes your encodes will have the wrong levels if the encoder/codec wants to see studio RGB or computer RGB levels (16-235 range or 0-255 range) but you are feeding it the other. In Vegas, applying the Color Corrector preset that converts from one to the other.
I think both sorenson3 and windows media HD do computer RGB levels, so that probably isn't your problem.

If you want to brighten something, don't use the Color Corrector in Vegas. It applies corrections onto the Y' channel, which can make things look whacked out.
Use the levels filter and use its gamma setting. (If you are working with studio RGB levels, which you probably aren't, you may need to 'juggle' the other settings to maintain the same black and white level.)

Sidecar2 wrote on 7/27/2007, 2:32 PM

Thanks, Glenn, for the quick response. Very helpful.

Seems I've read elsewhere on this forum to use Color Corrector but never Brightness and Contrast. They didn't mention the Levels filter.

Many of the filters seem to overlap capabilities and I guess some do the job better than others, even if they are named the same. Odd.

vicmilt wrote on 7/27/2007, 3:06 PM

Hi Sidecar -

There are many different ways to correct color - and many folks are absolutely ADAMENT about the "best" way.

But some filters require more knowledge and practice than others. Needless to say, in skillful hands, the more complex filters can deliver better results. But how much better is a question of skill, application and end result required.

For "down and dirty" corrections, Levels and/or Brightness and Contrast, can't be beat. And they will be FINE in dozens of situations.

The Color Correctors and Curves filters allow a HUGE amount of more possiblities with equal opportunity to mess things up, when/if you don't know what you're doing.

The nice thing about digital editing is you really can't do any permanent harm, if you simply make a copy of your clip for all of your experiments. So feel free to try things out.

There are many great books and DVD's covering color correction and I suggest you check out the best available (in my opinion) by Douglas Spotted Eagle and Glenn Chan at the VASST website; www.vasst.com

RE your original question - here's a way easier way to understand ( and WAY more simplistic, as well )
- this is not a definitive explanation (see above), just one "dumbed down" for newbies.
GAIN - handles the overall brightess - but especially at the "top end", that is, near the whites
OFFSET - handles the "low end" near the blacks
GAMMA - shifts the overall center of your image (brightnesswise) without moving either the "top end whites" or the "low end blacks".

So if your picture is too dark - raise the gain (then adjust the offset a little lower to get decent blacks back)
If your picture is too light, try lowering the gain, and then raise the offset to regain detail in the blacks.
OR - simply "play around" with the gamma to lighten or darken your shot

THERE ARE NO RULES in ART except - if it looks good to you, then that's alright... (and screw what everybody else sez!!)

Now if you're producing broadcast TV commercials, you'll need more help and DO have rules to apply. But that's the subject of another question :>))

best,
v

GlennChan wrote on 7/27/2007, 6:26 PM

Some other potentially useful information (this is on using Levels to adjust exposure):
http://www.vasst.com/resource.aspx?id=a7a8c403-64dc-420d-97d0-90d2f8de9fc1

Using levels in that manner is usually a good place to start. There are some rules of thumb in there... and you can break em if you like.