I really love this article, it is one of the most exhaustive and precise overviews of digital B&W conversion that one can find on the net.
The only suggestion I have is related to the use of Lab “L” channel in the B&W conversion. The method proposed in the article, which is based on the
Colors → Components → Decompose, produces in my opinion a “wrong” result: the converted image is too light, because the gamma encoding of the “L” channel is different from sRGB and therefore cannot be used “directly”.
The correct way of producing a B&W image from the “L” channel is from my experience the following:
convert from the input RGB colorspace to Lab (using a color management system like LittleCMS)
fill the “a” and “b” channels with a uniform 50% gray to completely discard the color information
convert back from Lab to any suitable RGB colorspace, again using LittleCMS or something equivalent
Applying those steps to Frank Kovalchek’s portrait produces this result:
to be compared with the conversion shown in the article and based on GIMP’s decompose. One can see that the explicit colorspace conversion to Lab and back to RGB produces a darker image that better matches the expected tonal range.
However, I have no idea if and how such a colorspace conversion to Lab can be realized with GIMP…
The “L” channel is actually a very powerful conversion method, particularly in the case of portrait images, as it tends to produce “natural” tonal variations in the skin, eyes and lips.
That’s strange. Coz after reading your message, I retried a couple of time. In my setup (I am using Ubuntu and I installed GIMP directly from the main repository (version 2.8.16)), the script only works when I have the underscores like this “YCbCr_ITU_R470_256”. Otherwise, it keeps popping up error message.
That is odd. I’ll fire up another Ubuntu box and see what’s going on. I should also probably move that script over to github so others can hack at it and submit PR’s as needed. Thanks for the heads up!
Beware & sadly C2G does not feature in the latest GIMP 2.10. I shouldn’t have let 2.10 uninstall 2.08 because now I don’t have access to C2G. Really annoying.
I’m having a problem with pseudogrey. It generates the following error
Execution error for ‘Pseudogrey’:
Error: eval: unbound variable: NORMAL
Gimp 2.10.12
I find it very useful for some B&W conversions and will feel a bit lost without it. Hopping some one can sort. Tried the version of Pat David’s site. Seems to be the only copy around.
There’s actually a version bundled in G’MIC as a filter as well that even has more options - might be worth trying that one instead (plus you get a ton of neat G’MIC filters as well… ).
Wow @patdavid, great thorough article…apparently I screwed up by not poking through all of the history of this forum prior to the time I joined. Wonder what other nuggets I will discover…
Obviously I shouldn’t have stuck curves on the top of the layer stack but wanted to see if it functioned as the others. The controls if any for all of the layers that are added remain active. Very photographer friendly. Just wish it had gimp levels as being able to set the output span can be very useful.
I had to edit the make file to get it to compile on Linux 64bit by forcing the selection. Just commented out the if else around it.