site_archiver@lists.apple.com Delivered-To: pro-apps-dev@lists.apple.com Domainkey-signature: a=rsa-sha1; q=dns; c=nofws; s=dk20050327; d=earthlink.net; b=fHE13oS9IpPvUDn+dN1pNW5nOkB3D4a3DhXqRLpJ2CP44sriaphw+DX1MdVdnkZh; h=Received:References:In-Reply-To:Mime-Version:Content-Type:Message-Id:Content-Transfer-Encoding:Cc:X-Mailer:From:Subject:Date:To:X-ELNK-Trace:X-Originating-IP; I've had this problem a lot. Especially when working on a movie that involves a lot of cross-platform post production work. Metadata carrying gamma, or clues to the correct gamma, tend to get stripped out by some applications or during format conversions. (If the info was ever there.) Are there any tips, tricks, or docs which aid people in guessing the correct gamma? I assume that certain equipment have an associated gamma, and that certain software tends to either create certain gamma, or will default to assuming certain gamma and (mis)act accordingly. Has anyone ever compiled a list of these? So far, I find myself using Processetatus-Eliminatus far too much. ^_^ -- Brian Sent from my iPad On Dec 24, 2010, at 3:01 PM, Robert Monaghan <bob@gluetools.com> wrote:

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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