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Problem with dng file from Pentax K5-II

Submitted by David Partridge on

I'm seeing some odd results when processing a DNG from a Pentax K5-II. Exiftool reports:

======== IMGP1593.DNG
Black Point                     : 513 513 515 516
Black Level                     : 513 513 515 516
Exposure Time                   : 1/8000
ISO                             : 800

Yet after calling LibRaw.unpack() I see 

+		cblack	0xcadc10d8 {0, 0, 0, 0, 2, 2, 513, 513, 515, 516, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...}	unsigned int[4102]
		black	0	unsigned int

So my code didn't do black subtraction (because black was reported as 0).

When processing an equivalent PEF file

======== IMGP1596.PEF
Black Point                     : 513 513 516 515
Exposure Time                   : 1/8000
ISO                             : 800
 
+		cblack	0xc4ecb0d8 {0, 0, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...}	unsigned int[4102]
		black	513	unsigned int

Odd to see that both black, cblack[2] and cblack[3] are filled in this case so I'd much appreciate some explanation of how cblack really works and how I should process these when operating on the raw image data?

Thanks

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