Eli5: How do photo restoration artists know the supposed colors of greyscale images?

Question

Are the colors based purely on their assumptions/imagination, or do the greyscale images retain some sort of data that tells what color on what part?

In: 641

Answer 1

There is something called LAB colour which is the scientific way (for want of a better explanation) of how humans see colour.

It can be demonstrated in Photoshop by choosing LAB mode and then looking at the channels.

Basically the detail of an image is represented as black, white greyscale in one single channel and the colour is completely separated in to the other channels.

So you have the L channel which is the Lightness channel and is purely black and white grayscale image. So no colour information at all.

Then you have the A channel which contains Reds and Greens.

And the B channel which contains Blue and Yellow.

Already this becomes difficult to explain. But anyway, because the detail in the L channel doesn’t change you can adjust the colours to your hearts content and you don’t lose detail.

In contrast the RGB mode doesn’t have blacks grays or whites without involving all three channels together. Thus you can mess things up really easily trying to do colour restoration without switching to LAB mode.

So that’s the secret. Switch to LAB mode in Photoshop and the detail is unaffected so long as you only affect the A and B channels with colour changes.