RawTherapee vs Darktable: invert scanned B&W negatives


(Hermann-Josef) #41

@ggbutcher
Hi, thanks for describing your method for negative inversion. This showed me, by the way, the answer to my question on how to set the shadow and highlight points in RT for the three channels :slight_smile: .

As far as I understand the white balance algorithms, the most common use is the grey world approach, which aims at the same “centre of gravity” for the three histograms in R, G and B. Setting the white and black point accordingly is what I usually do, just like you described.

The problem is, that you need a patch on the negative, which is not exposed, i.e. exhibiting the clean orange mask. Otherwise it is only an approximation what you get.

Hermann-Josef

PS: The method for inversion used by ColourPerfect is described in a paper by David Dunthorn.


(Glenn Butcher) #42

Edit: A patch from the sprocket edge would be a correction similar to the black/white point operations (original post was flat wrong), and really wouldn’t address white balance on the residual positive image.

Edit1: So, my thinking is that white balance needs to be done on the positive image, as the negative conversion just deals with the color cast and inversion. Film stocks were formulated to capture colors appropriate to the incident light’s color temperature, so that difference needs to be handled subsequently. You can do each image with a patch, but a good starting point that’s image-agnostic might be just a full-image ‘gray world’ calculation. I’m programming a whitebalance tool for rawproc now, and I’ve been surprised at how well full-image gray world actually works.


#43

@Jossie
That particular inversion method has been mentioned here previously.

All it boils down to is negate with 1/x instead of 1-x (or equivalently use negative gamma). There is of course some extra information about colour balance and choosing the particular gamma values.


(srowed) #44

I haven’t scanned a lot of colour negs, mainly because I don’t have many to work with. I have been able to get decent results, however, using a relatively simple method with RawTherapee on DSLR scans. Here’s a sample from a mid 1990s 6x7 Fuji neg, cropped for the privacy of the models. Basically just inverting using the curves, getting a white balance of the orange mask, then adjusting the colour temperature and tint. In this case, the light source was electronic flash through a diffuser.


(Morgan Hardwood) #45

I would appreciate for documentation purposes if someone could send me a color negative, preferably with the following requirements:

  • The negative’s colors are in good condition, not like in the Pepsi-Cola slide.
  • It can have dust and scratches, that’s fine.
  • There is something white or neutral in the frame, like the three blocks in @ggbutcher’s image from the net.
  • There are colors which a human would pick up as clearly wrong if they were off, such as the skin tones in @troodon’s scan above.
  • You own the slide, so you can release it under CC0.
  • The negative is available in 16-bit format (TIFF/PNG/DNG) and/or as a raw file.

(srowed) #46

I can send you the Nikon .NEF file of the two girls. It has good skin tones but no whites that are completely neutral. the blacks and grays in the gloves might work though. The original was a Fuji pro negative film (can’t remember which one) and I scanned the negative with a Nikon D7100 using diffused electronic flash.


(Morgan Hardwood) #47

@troodon I don’t think I can use that without a signed release from them, so we would need a different photo, one without faces.


(Hermann-Josef) #48

@Morgan_Hardwood
just recently I have made scans with SilverFast and VueScan available for the community to test and compare the software. You can find the three “raw” files here (Kodachrome, Ektachrome and Kodacolor X). I could supply other scans of negatives should the horse image be not ideal.

Hermann-Josef


(Morgan Hardwood) #49

@Jossie great!

Why are the horse scans from CanoScan, SilveFast and VueScan so different? Which one would you consider the typical RGB no-bells-or-whistles version?


(Hermann-Josef) #50

@Morgan_Hardwood
First of all, there are hardware differences: In the folder “CanoScan” the scans were done with my flatbed scanner CanoScan9950F using VueScan. Folders “SilverFast” and “VueScan” are the scans with my slide scanner DigitDia6000 with the software specified by the folder name. Folder “CyberView” are the scans from the slide scanner obtained with the CyberView software.

The “raw”-scans also have a different format. VueScan saves 4 channels/pixel whereas SilverFast produces a 3-page TIF: full resolution RGB, reduced resolution RGB and IR scan. The scans are not gamma-corrected, thus are dark. Only the CyberView-version is gamma-corrected since this software does not produce “raw”-scans.

Looking into this I realized, that VueScan seemed to have filtered the orange mask out already. This was not my intension, on the contrary! I will check and replace the image. Thanks for pointing this out!

Let me know if you need negative scans with more colours.

Hermann-Josef


(Hermann-Josef) #51

@Morgan_Hardwood

The VueScan-image has now been replaced.

Hermann-Josef


#52

Dear all,

sorry for the late reply, had been travelling a lot…

I now understand my misstake: I selected an EXPOSED area instead of an UNEXPOSED. I think I mixed it up because in Vuescan you need to take the exposed area.

Nevertheless I was playing around with colorPerfect, what gave me quite good results in quite little time. I am trying to find a solution to find a way with a good quality-to-time-ratio and colorPerfect seems to deliver it. Even with the picture chris was working on (indeed the film seems to be misshandeld by the lab) I could minimize the frustration and dissapointment.

Anyway - Thank you very much!
Alex

Alex