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DNG as a container


my experiments show two interesting things
1) Some DNG implementations use camera specific processing paths. I.e. one way for black calculation for Canon cameras and another way for 'xxx' one. So, these DNG engines are not camera independent.

2) Most common DNG converter calculates wrong Black Level tag. This is not a specification problem, but implementation one. Anyway, from practical point of view, DNG specs are a theory, but Adobe DNG Converter is the reality.

So, in current reality it is not possible to build good RAW processor for DNG files which uses only Adobe-documented DNG tags. At least for DNGs produced from Canon .CR2 good RAW engine should ignore DNG-specific Black level tags and calculate black level by itself. If application relies on DNG tag, it will result in very bad rendering in Zone V and below (Also, dynamic range will drop from 8-11 stops to 5-6).

Of course, DNG is more documented than usual (undocumented by vendor) RAW format. This is definitely a step in the right direction. But this step is not big enough: DNG processors must use vendor-specific EXIF fields for good rendering, vendor-independent DNG-fields are not enough.

Some DNG problems are related not to the specs itself, but to particular Adobe DNG implementation. On the other side, there are not many DNG converters on market, so Adobe DNG converter problems and DNG format problems are not different in mass mind.

-- Alex Tutubalin @LibRaw LLC