Re: Why is there a difference...
Re: Why is there a difference...
- Subject: Re: Why is there a difference...
- From: Robert Monaghan <email@hidden>
- Date: Fri, 24 Dec 2010 15:01:52 -0800
Hi Patrick,
You are looking at potentially two different problems. (or both.)
1.) Gamma Curve missmatch.
Gamma is a horribly misunderstood and often incorrectly applied to RGB imagery.
Wikipedia has a great set of articles to describe what Gamma is. I will skip the explanation and purpose.
But I will suggest that you read about it.
On the Mac, we are confronted with a bunch of different gamma curves. They aren't 100% compatible, too.
Unfortunately there is no way to know which one to use on a given image. Most often guesswork is required.
Here are some that is used:
- Gamma 2.2
- Gamma 1.8
- Gamma 2.2222
- Rec709
and
- sRGB
When you have an image that is (lets say) Rec709, and you want to apply a matrix for color correction,
it is easy to mistakenly use a inverse Gamma 2.2 curve to make the image linear, do the work, and then add a Gamma 2.2 curve
for use in RGB. Because Rec709 isn't the same as Gamma 2.2, you will have introduced an error, which will make the colors look wrong.
The only way to make an image properly linear, so that you can do proper image processing operations, is to the use the inverse Gamma, of the same
type that the image currently uses. That way, when your round-trip some effects, etc. The colors won't shift.
2.) BT-R.601 vs. BT-R.709 colorspace missmatch.
Another possibility, is that the footage was shot using a video colorspace that QuickTIme doesn't understand. When this happens,
QuickTime (or Final Cut Pro) will default to a "standard" matrix. This causes the imagery to shift color, too. This is typical with the recent bunch
of DSLR cameras that can shoot video. This tends to shift colors as well.
3.) A combination of 1 and 2.
I have hit this problem before and it is frustrating to have to troubleshoot.
My gut feeling is that problem #1 is your culprit, but hard to know for sure.
Good Luck!
bob.
On Dec 21, 2010, at 10:10 AM, Patrick Sheffield wrote:
>
> ...between how Quartz Composer processes the Screen blend mode and how Final Cut Pro and Motion do it?
>
> This is an image screened over itself in Final Cut Pro/Motion:
>
> <fcs1.jpg>
>
> And in Quartz Composer:
>
> <quartz1.jpg>
>
> In Quartz Composer it's flatter and considerably more red than FCP.
>
> If I perform a Screen Blend (the inverse of the multiplication of the inverse) mathematically, I get the same result as Quartz, but I need to figure out what FCP/Motion are doing differently.
>
> Does anyone have any insight?
>
> Thanks,
>
> Patrick Sheffield
> Sheffield Softworks
>
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