Replacing Luminance channel with the luminance channel from another image

[tiresias]

I have a low-ish resolution greyscale image that I had recoloured. The person recolouring it upscaled the image before they started working on it, but now I have just discovered a much higher resolution version of the same greyscale image. The new high-res image contains a lot more detail than the upscaled colourised image.

It is too much to expect the colourist to start all over again on the high res image. But it struck me that he applied the colours to the upscaled image so the colours are effectively in high resolution. So if I was able to replace the low-res upscaled image's luminance channel with the new high-res greyscale luminance channel (taking care to align it exactly) then this might be a quick way of getting a high-res colour image. I suspect that it might involve converting the RGB image to Lab and then somehow replace the L channel with the L channel from the high-res image, but I'm not sure how to go about this.

Any suggestions on how to do this, or maybe a better method of achieving it?

[NotMyFault]

This is not too complicated.

1. convert to lab mode
2. have both layers aligned in one document (e.g. using new stack with alignment)
3. use channels panel to create spare channel from high-res layer
4. use channels panel to overwrite lab channel of of colorized image

 

[tiresias]

Great thanks. I wasn't too experineced with channels, but got there in the end.

Here's a simplified step-by-step method of what I did:

1. Open the upscaled-colourised image (it is the background layer by default)
2. Load the hi-res greyscale image on a layer above the low-res (upscaled) colourised background layer
3. Make the high-res layer 50% opaque then stretch and nudge it until it is perfectly aligned with the underlying colourised background layer.
4. When it is aligned, change the opacity to 100% again
5. Convert the colour format to LAB (Document > Convert Format)
6. Select the high-res layer
7. In the Channels panel, right click the "Composite Lightness" channel and click "Create Spare Channel"
8. Select the colourised background layer
9. In the Channels panel, right click the "Spare Channel", and Click "Load to Background Lightness"
10. Delete the high-res layer