How to process DNG files from CHDK

Software darktable
1

Hi! I have some photos taken with my Canon Powershot S5 IS. They are in RAW format (.DNG).

When I open it to edit, the colors appear to be wrong…

A general question was asked on photography stackoverflow: canon - Why do some DNG pictures from chdk have a purple tint? - Photography Stack Exchange

How can I edit such DNG images in darktable properly? Can “demosaicing” module be customized more?

JPG:

IMG_1857
IMG_18573264×2448 2.58 MB

DNG:
IMG_1857.DNG (9.9 MB)

DNG in darktable:

image
image788×591 308 KB

2

It is the White Balance module that changed it. If I change it from “As Shot to Reference” to “As Shot”, it looks like the top photo (JPG) of your post. Personally, I prefer it as in the bottom photo (DNG in darktable).

So, look into the White Balance module and set things the way you want.

3

For me I definitely had trouble getting an identical color to the JPG image. By default my image opened with a slight blue/magenta tint which is the opposite effect to what you are seeing. Could you supply your xmp file for people to check if you have a strange setting for white balance or color calibration modules? I could only replicate a similar color to you if I kept WB at the scene refer default but disabled color balance module.

It may also be good to have a nice sunlight scene for comparison as the forest is tricky lighting.

4

First, I think that camera only outputs .jpg and no raw format. Second, DNG is not really a raw format. It can hold unprocessed raw data, or processed data.

So how did you create the dng?

5

It’s a firmware hack.

1 Like
6

Yeah, in this photo the DNG is okay, but I was wondering, if wrong demosaicing could cause more noice… Or the camera is too old

7

Hmm, there were some settings that darktable applied automatically… And it made it look like this…

Well, I don’t have many photos, but if I’ll find some strange variants, I’ll send them here.

But thanks for mentioning “White balance” and “Color calibration” settings

8

If you upload a raw and the xmp sidecar (but at least the sidecar), we could have a look. Until that, it’s just guessing, so let us help you.

9

UPDATE: Actually, Darktable has applied some “scene-related default” Color calibration module, and it made the magenta tint.

JPG:

IMG_1895
IMG_18953264×2448 2.3 MB

DNG look:

image
image1508×1136 1.23 MB

DNG look without any default settings:

image
image1502×1132 1.06 MB

DNG:
IMG_1895.DNG (9.9 MB)

XMP:
IMG_1895.DNG.xmp (6.1 KB)

10

There’s just the basic processing. In color calibration, you can sample e.g. the road to get a reading.
color calibration depends on darktable’s camera profile having a good ‘camera reference’ white balance. Maybe the values for your camera + CHDK are simply wrong (I would not be surprised, as the camera normally does not support raw).
Possible solutions:

  • use white balance only, setting it to ‘as shot’.
  • pick colour from a neutral spot in color calibration
  • if your screen is properly calibrated to D65, create an override for the camera’s D65 reference – see darktable 4.9 user manual - color calibration
11

Yes: darktable applies some default processing, depending on the input file and your settings.
Your dng files are supposedly raw files, and scene-referred is the default workflow setting (iirc).

So you will have applied a few modules, some of which are absolutely essential and cannot disabled (shown with a ‘◉’ symbol) and some which are technically not required, but you’ll want them most of the time (marked with ‘⏻’)

If you want as few modules applied as possible, set your workflow to “none” (see the manual)

As @kofa said, in your case some of the values provided in the dng metadata may be wrong. If you can figure out a suitable default for some of the modules, you can create a preset for those with the better defaults, and have those auto-applied (or you can create a style with them, which you will then have to apply “by hand”)

12

Sorry. From your top post, I didn’t understand your issue.

13

When I looked at the meta data I think it said the DNG version was 1.0…its seems to be single illuminant and the wb data was a bit weird…the green channel was 1.1644 or something like that and the red and blue were fractional which I have seen before but usually green is 1 or maybe some variants have this…in any case maybe there is weird wb information because going from memory I don’t think DT used values for as shot that matched the meta data but it was a day or two ago when I looked …

14

thanks for the answers! omg, you are such professionals

(Looking forward to understand all of this :smile:)