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"Linearize working space" and gamma: a conceptual doubt

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Paulo Jan
"Linearize working space" and gamma: a conceptual doubt
on Aug 28, 2011 at 12:01:03 am

Hello:

I used to think I knew what gamma was. Then, the other day, I clicked the "Linearize working space" option to try out this new workflow (working in linear color space, that is)... and now I'm hopelessly confused. I hope somebody is able to clarify some basic concepts that I'm probably missing.

(This is not about the practicalities of working in linear; I've read the Adobe white paper about color management, and it all sounds quite easy and sensible. My doubts are more on a conceptual, theoretical level).

Let's see:

1) I know that the human eye has a nonlinear response to light, so that it is more sensitive to the darkest parts of the visible spectrum, and it gets less discriminating the brighter an image is. That's why a card that is only 18% grey will look as if it was middle grey.

2) I know that gamma correction exists to compensate for the nonlinear response of monitors, and that what it does is to "brighten" the image (not really, but you get what I mean), "pulling the curve up" to make up for the fact that CRT monitors, when displaying the image, tend to darken it excessively.

Now, given the above, I had thought that, when one checks the "Linearize working space" option, and thus removes the gamma correction in the image, the resulting image would look darker, but it's the opposite: with "Linearize working space" on, the image looks brighter. Not only that, but also, with that option on, if you create a solid that is 18% grey (0.18 brightness), it will look like a perfect middle grey onscreen, while, without said option (i.e., working in a normal gamma-encoded way), a 18% grey solid will look... well, dark grey.

It's clear that "LInearize working space" doesn't do what I thought it did (remove the gamma correction from the image), and it's also clear that I must be mixing different concepts, but I don't really know which ones. I'd be grateful if someone could explain to me my error.

(The books I've read to get to where I am: "After Effects CS5 Studio Techniques", by Mark Christiansen, and "Digital compositing for film and video", by Steve Wright).


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Darby Edelen
Re: "Linearize working space" and gamma: a conceptual doubt
on Aug 28, 2011 at 1:47:57 am

[Paulo Jan] "It's clear that "LInearize working space" doesn't do what I thought it did (remove the gamma correction from the image), and it's also clear that I must be mixing different concepts, but I don't really know which ones. I'd be grateful if someone could explain to me my error."

It removes the gamma correction from the working color profile.

The gamma correction that most color profiles have makes the profile "correct" for human perception. That is that 50% gray appears 50% gray. When you remove that gamma correction by going to a linear profile, a value of 18% gray in the profile now appears to the human eye to be 50% gray.

The monitor's profile, which you also mentioned, is specific to the monitor. In a color managed workflow there can be several different profiles involved. 1) Profile of the source image 2) Working Profile 3) Display/Monitor Profile 4) Profiles used to convert the image to simulate display on another device.

Whenever you import footage into a color managed AE workflow, you'll have the option of converting that footage from a color profile to the current working profile. Depending on what you choose at this stage, this will result in a change to the perceived colors of the footage, or a change to the actual stored values of the colors (which will result in minimal perceived shift of colors).

You can also remove or add gamma correction using the Utility > Color Profile Converter effect.

Darby Edelen


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Paulo Jan
Re: "Linearize working space" and gamma: a conceptual doubt
on Aug 28, 2011 at 9:12:21 pm

Thanks for your answer. But in that case, I take that there's still some other gamma corrections going on in the system? The books I've consulted all explain it in a relatively simplistic way: "oh, the monitor has a certain gamma value, so the software adds a gamma correction to compensate for that", which makes it sound as if there was only one gamma correction working in the entire OS/software stack. But judging from AE's behaviour, it's clear that there must be something else going on. After all, even if you click "Linearize working space", the monitor will still be darkening the images, so AE will still have to compensate for that somehow...


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Darby Edelen
Re: "Linearize working space" and gamma: a conceptual doubt
on Aug 30, 2011 at 12:08:32 am

Linearizing the working space does what it says and only that. The working space is linearized and has a gamma of 1.0. The working profile is the profile that all compositing is done in.

When AE renders the result of the composite to your display it usually takes your display profile into account (as you've noted). You can change that by disabling View > Use Display Color Management. Now AE will not convert the linearized working profile colors to your display profile and your image should look considerably darker if you're using a linearized working space.

Darby Edelen


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