How do I save a file as a linear TIFF from Raw Therapee?
I still haven't figured this out. Anybody?
And is the process I described in my previous post (exporting from RawDigger) giving me a linear TIFF file?
I sort of don't think so. I installed Canon DPP. This has an obvious way to save as linear (a checkbox named "linear" that defaults to being unchecked.) Files I export from DPP don't look like the files I exported from RawDigger. And don't look like the "linear" files I get from DCRAW (above.)
But files from DPP seem to convert better than anything else I've tried. I spent the afternoon revisiting camera scanned negatives and converting them again. In case the original poster is still paying attention, (based on my understanding of Bart's advice) here is my workflow for converting camera scanned negatives to color corrected positives, using Canon DPP and PhotoShop:
1. Load RAW file in DPP. Click on the "Linear" checkbox in the "RAW" tab. Then click on the White Balance adjustment and then click on a neutral part of the negative. In between sprocket holes or in between frames is a good place to click.
2. Switch to the "RGB" tab. Adjust the endpoints to the black and white point of the histogram. (I don't know if here is a good place to adjust the individual R, G, and G. black/white points. I haven't been because it is difficult to see what you are doing when adjusting R, G, and B separately in the DPP "tone curve adjustment." I adjust R, G, and B white/black points in Photoshop, later.)
3. Optional. Go into "Tools/Preferences/Color Management" and change to "Wide Gamut RGB" I've found in the past that colors on negatives sometimes don't fit into sRGB so I use a large color space until I am all done with the conversion and am ready to save. YMMV.
4. Use "File/Convert and Save..." Change the "Save as type" to "TIFF 16bit" Assuming that you are using something like PhotoShop that can deal with 16 bit TIFFs. The following steps will involve some radical curve twisting and anything 8-bit is likely to end up posterized. Again, YMMV, but I use 16 bit TIFFs.
I use Photoshop. Adapt instructions as needed for any other editing program.
5. Open the image file you just exported from DPP in PhotoShop. Change the little triangle thingie on the status line so it shows "Document Profile". It should be showing "Wide Gamut RGB (16bpc) if you followed my recommendations. Straighten and crop away the edge of the frame so all you have is image.
6. Open Curves (Ctrl-M) and choose "Negative (RGB) from the Preset dropdown. Click "OK." (The image should change from negative to positive. But it will be very bright and the colors will probably look wonky at this point.)
7. Open Levels (Ctrl-L). Click "Options" and choose "Find Dark & Light Colors" Maybe Click "Snap Neutral Midtones". Click "OK" in the Auto Color Correction Options". You are still in the "Levels" dialog. Check each of the R, G, and B channels to see that the endpoints match each histogram. Then select RGB and adjust the gamma (brightness) by sliding the middle slider to somewhere around 0.6. Adjust to taste. Click on "OK" in the Levels dialog.
(You can do step 7. from the Curves dialog if you want. But checking the fit of each R, G, and B histogram is easier in Levels than it is in Curves.)
At this point it should look close. But let "Auto" in curves take another whack at it. "Auto" works differently now that you've adjusted the gamma.
8. Open Curves again and click on "Options..." Try each of the top three algorithms, with "Snap Neutral Midtones" selected and deselected. When you find the one that looks best, click on OK to exit the "Auto Color Correction Options dialog." You will still be in the "Curves" dialog.
9. At this point it should look real close. But you can tweak the individual R, G, and B curves and see if you can improve it.
10. After this use any other PS tool that will help. Shadows/Highlights, etc. Change the color space to sRGB (or whatever you need) as a final step (Edit/Convert to profile). Change to 8 bits (Image/Mode/8 bits per channel.) I usually keep the 16 bit TIFF but you probably should also save a copy as 8 bit JPG.
Most of this is mechanical and doesn't need any fine judgement until you get to step 9. By the time you get to step 9., it should be very close to correct.
Corrections and suggestions are welcome! Please.