Re: Retrieving impossible colour from Lab
I’m not sure there’s a basis for that statement. The file may be in LAB and it may contain “colors” that are outside the gamut of (say) sRGB but the monitor that displays the LAB file is an RGB device, so the LAB file, with rare exceptions, closely resembles what you’ll get upon conversion to sRGB.
The usual cause for concern is that somebody is trying to use LAB to force more color into a file destined for some other colorspace, not realizing that the LAB file already specifies something OOG for that space. Do this, and all that happens is that detail starts to go away.
So if you recover the parts of the file that are OOG chances are they will display differences in detail. The differences in color would be difficult to evaluate except by artificial instrument.
If you must see a close approximation of the difference, here is the procedure; for the sake of argument I assume you are concerned about an LAB>sRGB conversion.
Starting with the LAB file believed to contain OOG colors,
1) Make one copy and convert it to sRGB. Chances are you won’t see a difference.
2) Make a second copy of the LAB file and convert it to ProPhoto RGB, which technically speaking contains almost all the LAB gamut and almost certainly won’t show a visible difference.
3) Convert the sRGB version to ProPhoto RGB.
4) Put one of your two files that are now ProPhoto RGB as a layer on top of the other.
5) Change layer mode from Normal to Difference. Chances are, it will look black.
6) Duplicate this file, flattening it. Still looks black.
7) Auto Tone. This will display the differences that were previously hidden by black, showing the differences between the LAB>ProPhoto and the LAB>sRGB>ProPhoto files, which are basically those colors outside of the sRGB gamut.
That should be close enough, but for more accuracy you should turn off the automatic dithering in the conversion as that will produce some fine noise that really isn’t OOG. Also, rather than Auto Tone, it’s slower but more accurate to apply a straight-line curve to the Difference file, moving the highlight point inward until detail starts to appear out of the blackness.