6

I know this almost touches some other questions already asked here, but it's actually aligned a bit different. This is not about DNG vs RAW, rather just about the demosaicing step in the conversion. It seems like this answer implies that some irreversible demosaicing of the bayer patter takes place, but other sources around the Internet states that it does not. I'm not referring to the linear DNG, but the one that Lightroom converts RAW files to.

I've also tried to dig into the specification from Adobe, but so far I've not been able to find a definite answer.

  • 2
    The biggest loss when many (but not all RAW) files are converted to DNG is that the black point is baked in and the data from the masked pixels used to derive the black point are stripped. This assumes the RAW format of the original file in question includes the data from masked pixels. Some proprietary RAW formats from some camera makers include that information and others don't. The other major thing stripped from RAW files converted to DNG are the "maker note" section of the EXIF information. Adobe products ignore this information anyway, but many other EXIF tools and RAW converters don't. – Michael C Jan 24 '14 at 4:32
  • As with any third party RAW conversion software, the algorithms used to perform demosaicing are often reverse engineered best guesses since the developers and marketers of such software may or may not have access to the proprietary information about the sensors characteristics and encoding processes. – Michael C Jan 24 '14 at 4:35
  • See this late answer to the same question referenced above. photo.stackexchange.com/a/45827/15871 – Michael C Jan 24 '14 at 4:39
  • @MichaelClark you should post that as an answer – Matt Grum Jan 24 '14 at 11:45
7

Partly. Some of the steps normally taken during demosaicing, such as setting black point, are performed when files are converted to DNG and can't be reversed later on. Other steps, such as interpolating the monochromatic luminance values of each pixel into RGB values based on the color mask characteristics of a particular sensor, are not.

Converting an image file from the manufacturer's raw format to .dng will strip all of the information in the maker notes section of the EXIF data. Most all Adobe products ignore the maker notes, many other EXIF tools and RAW converters don't. If you only use Adobe products to convert and edit your images you will not see a difference in this respect.

There are additional things that the conversion strips as well. For example, data from masked pixels used to determine black point are not carried over into the .dng file. Instead black point is computed and 'baked in' during the conversion process. This assumes the RAW format of the original file in question includes the data from masked pixels. Some proprietary RAW formats from some camera makers include that information and others don't. As with all raw convertors that do not use the manufacturers own proprietary algorithms on data that may be encrypted (again, dependent upon manufacturer), there is no guarantee that the conversion of the data remaining in the DNG file by the third party software will be the same as conversions that use the manufacturer's algorithms.

Since each sensor design is different, the output from the sensor must be interpreted based in the design of that sensor. As new cameras are released with new sensor designs, updates to the DNG convertor must be made to properly convert the output from the new sensor. Not all Bayer masks, for example, use the same exact colors for each of the R,G, and B filters. Some, such as newer designs from Fuji, even alter the pattern of which pixels are filtered by R, which by G, and which by B. Without the specific information of the sensor's unique design, the convertor will misinterpret the data from the sensor.

  • Thanks! This is very informative of an answer, but it doesn't really answer if the DNG is demosaiced or not. If the image is not demosaiced future updates might solve problems of incorrect bayer patterns, but if it is demosaiced incorrectly it's not correctable. Do you get what I'm asking for? Cheers – Hugo Jan 25 '14 at 3:27
  • @Hugo Some of the steps normally taken during demosaicing, such as setting black point, are performed when files are converted to DNG. Other steps, such as interpolating the monochromatic luminance values of each pixel into RGB values based on the color mask characteristics of a particular sensor, are not. So the answer is "Partly". – Michael C Jan 25 '14 at 3:47
  • 1
    Also there are quite a few 'flavours' of DNG. All DNG's are definitely not equal. Some are more lossy than others and it's very much a container format like TIFF (where multi-page Group 3 fax is as standard as the high bit depth content photographers are more used to and think of when they discuss TIFFs.) One of the DNG formats is an 8-bit discrete cosine transfer (essentially a jpeg, just built the unprocessed raw data!) – James Snell Jan 25 '14 at 14:49
  • 1
    Unknown maker data is not stripped. It's copied as-is to a tag for that purpose. – JDługosz Oct 28 '14 at 23:19
  • If it is undocumented maker note data, it is stripped. See the Adobe representatives comments at forums.adobe.com/thread/743437?start=0&tstart=0 – Michael C Oct 29 '14 at 3:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.