Can RT be used to create grayscale images?

As best I can tell all of the built-in “Output (Color) Profiles” supplied with Rawtherapee (RT) are RGB profiles. Even though RT seems to provide tools to convert to Black & White the resulting files are in RGB format which makes them much larger than they would be using grayscale profiles.

In fact, I even notice that I can input a file that uses grayscale profiles then do no editing and when that file is output it has been converted to RGB.

You can set the output profile to be whatever you like. If you have a grayscale profile, then use it!

Do you have an example of such a grey scale profile?

If you mean that RT does not output RGB triplet that are identical eg (100,100,100), then I don’t think this is possible atm.

I remember, we discussed an automatic solution to the one channel grayscale export some years ago but dismissed the proof-of-concept code because it was too slow for a one-in-a-hundred case. A manual solution would sure be possible, but you can always use other tools to strip the two redundant channels for archival.


I do use some custom ICC profiles. When I add a grayscale ICC profile to the folder that contains these custom profiles (which seem to work fine) it does NOT show up on the list of output profiles that can be selected (which appears to also include the profiles built-in to RT).

It sort of looks like RT might be able to tell that this is NOT an RGB profile and therefore excludes it from said list.

Possibly there is something I don’t understand about this suggestion.

afaik, RT does always output 3-channel files

Ok, I’m adding my opinion here from the PoV of someone who has relied on selling their images for the last 40 years.
There are two possible destinations for an image - internet/web 500px etc, or print.

Not all browsers understand all grey spaces, and neither do a lot of printer drivers!

The only time I would entertain converting a greyscale image to a grey space would be for sending to a dedicated greyscale printer/ink setup, and that grey space would be unique to the printer/inkset combo.

Greyscale images suffer zero defects if tagged with an RGB space, and striving to do otherwise is a little like trying to re-invent the wheel; technically interesting but in the end somewhat futile.

Definitely don’t want to send a RGB image tagged with a grayscale profile to the wild.

That said, I think using such a profile for an intermediate, internal conversion prior to export is viable. But, I don’t think it’s the best approach; I believe there’s more aesthetic control to be had in doing the RGB devolution to L manually.

FWIW, I also keep my grayscales in three-channel RGB, as I find re-introducing a bit of color ("seipia, “cold-tone”, etc…) to be fun…

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That is certainly consistent with what I’m finding.

Very helpful replies from both Andy & Glenn.

Converting some of my photos to black&white (monotone might be more correct) is something I’ve considered doing for some time now but have yet to find the time. The current experience came about as a result of digitizing (i.e., scanning) a very old and degraded monotone photo received from a friend. Without knowing any of the above thoughts it seemed fairly natural to choose the “grayscale” option for the scanner output. However, purely from a curiosity point of view I also produced an rgb version with the same scanner. As it turned out the rgb version came out as a very faithful representation of the degraded color of the photo whereas the grayscale version simply looked better (to me).

As it turns out browsers are NOT the only software to have difficulty with grayscale. Because of some difficulties encountered printing the result I wanted to use the more sophisticated software associated with my photo printer. However it is supplied as a plugin for software used to develop raw image files. That software was worse than RT in the sense that it couldn’t even open a grayscale file. I’ve had similar problems in the past with the same software failing to recognize multi-layer .tiff files. I used to solve that problem by using RT to read (import) the multi-layer files and write (export) a file that could be processed by the software used to invoke this printer plugin. As it happens RT was also similarly successful at solving this problem by producing a file (in rgb format) that could be processed with the plugin. I just happened to notice that it did this by converting to rgb and being ignorant of the above considerations thought it would be better to retain the grayscale format produced by my scanner if only because of the smaller size.

The above pretty well explains why I should want rgb format when it comes to working on my photos and I now pretty well understand why this software (e.g., RT and others) that is primarily intended for developing photographs has NO need to produce grayscale files.

Thanks for the enlightenment.

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