Saving Spoiled DNG 1.3 Images

ltraynor

15

« on: 20 / November / 2012, 15:50:11 »

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As reported on the CHDK Facebook page, there are issues with images taken with the DNG version set to 1.3, instead of 1.1. Images have darker, coloured spots on them, which are particularly visible in lighter portions of the image. It is unclear whether these spots result from a bug in CHDK, or one in DCRAW (and by extension, UFRAW, and other programs that use DCRAW as a basis for conversion). These are not bad pixels, which by definition are lighter spots more visible in darker regions of an image.

I have discovered the following workaround:

1) Use the free Adobe DNG Converter to convert your DNG files to new ones in a separate subfolder.
2) Open these files with Irfanview and convert them to a regular graphics format.
Before image (excerpt, with spots):

After image (same excerpt after conversion with Adobe and view in Irfanview):

Hope this saves someone from too much grief.

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philmoz

3450

Re: Saving Spoiled DNG 1.3 Images

« Reply #1 on: 20 / November / 2012, 16:42:20 »

Thanks for the tip.

The DNG 1.3 spec includes opcodes that can be used to tell software to fix bad pixels.
CHDK uses this to avoid the overhead of having to generate the bad pixel map and do the bad pixel fixing in the camera.

Some of the newer CMOS camera have tens of thousands of bad pixels.

Unfortunately only Adobe has bothered to properly implement the 1.3 extensions in their software.
If you aren't using Adobe software you can change the default in the camera back to DNG 1.1 ~~1.2~~ - or contact the software author and encourage them to add better DNG handling.

Just to clarify, in CHDK terms a bad pixel is one that comes out black (also referred to as a dead pixel). These are usually fixed; but may be ISO dependant. A pixel that shows as a bright spot is a hot pixel and these are generally temperature and ISO dependant - long exposures will have many more. CHDK only handles bad/dead pixels.

Phil.

« Last Edit: 21 / November / 2012, 02:26:32 by philmoz »

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