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How does invert work in terms of luminance?

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Dear all,

I wanted to invert a photo and apply a correction curve in order to create a digital film.

But, if I invert a test grey chart using an invert adjustment layer, photoshop actually applies some correction curve. So my L=0 becomes L=100, my L=100 becomes L=0 (so far, so good), but my L=53 becomes... L=69.

Can somebody explain me why and how I could invert so that my L= 53 becomes L=47 and so on? I did not see any option on the invert layer to choose this.

Thank you!

Laurent

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davescm

Community Expert

I've just tried, and it seems that if you change your Image mode to Lab Colour then Invert works the way you want - with 50 being the centre point

Dave

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D Fosse

Community Expert

Interesting. I'm going to look more into this when I get time.

For a long time I thought Photoshop luminosity was Lab L. It would make perfect sense, since Lab is used as profile connection space in color management, and Lab numbers therefore are always easily available under the cover.

But then testing revealed that they wen't absolutely identical - there is a small difference in the tone curve. Since then, I've kept wondering what Luminosity really is, and never came up with a good answer.

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Laurent SimonAuthor

Participating Frequently

Thanks!

It seems GIMP offers several ways of inverting, so I will use that instead I guess. Thank you!

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davescm

Community Expert

Invert works in RGB and takes the RGB values from 255,255,255 so 180,170,0 becomes 75,85,255

Because of the gamma curve in RGB, the 128 does not translate to L 50

Dave

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Laurent SimonAuthor

Participating Frequently

Thank you! Would you know a way to invert the luminance then?

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D Fosse

Community Expert

I think you would need to work in linear gamma to get entirely predictable and mirrored numbers (gamma 1.0).

Aside from that, all of this is a bit of a murky area. Photoshop luminosity is a custom curve that is independent from the document's color space, and it doesn't match any of the standard tone response curves. It's likely based on Lab L, but it's not identical. I suspect its main purpose is to give a visually consistent result, and nevermind the numbers.

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