From the (defunct) web page of Barry Pearson on DNG, specifically Linear DNG:
What is Linear DNG?
DNG has two varieties that deserve different names on this page:
* "Raw DNG": The familiar variety of DNG, containing the raw image data plus added-value metadata. Almost all of the products that support DNG support at least this variety. (There are exceptions). This variety of DNG will gradually become the de facto standard raw file format.
* "Linear DNG": A rarer variety of DNG, containing RGB image data (or not just RGB!) arranged in a rectilinear format. (Most practical examples of Linear DNG hold RGB image data, but it isn't a constraint, and 4 or more colours are allowed). This RGB image data may have come from demosaiced raw image data, or from another source such as TIFF or JPEG or something else. (There is little difference between "demosaiced" and "never mosaiced").
Most other pages on this site focus on Raw DNG. This page focuses on Linear DNG. This variety of DNG may become an alternative to TIFF in a new set of digital image processors.
Benefits of Linear DNG
Support for unusual sensor configurations
Whether or not a raw converter can handle the raw image data from a particular camera depends partly on whether that raw converter can handle the sensor configuration concerned. It is easier for a raw converter designed to handle Bayer sensors to handle raw files from another Bayer sensor camera than from a camera that doesn't use a Bayer sensor. (This is not a flaw in DNG - it is simply a comment on the relative difficulties of handling different sensor configurations).
* Some raw converters only handle Raw DNG files from camera whose raw file formats they support anyway.
* Some raw converters can handle Raw DNG files from cameras whose native raw file formats they don't support, but only where they support the sensor configuration concerned, for example Bayer sensor.
* But - some raw converters support Linear DNGs from cameras that have sensor configurations that they don't support either via the native raw formats or the Raw DNGs. (Eg. Silkypix).
When ACR handles a Linear DNG file, all of the tabs, sliders, and drop-down boxes still work, and do the sort of things expected of them. It is a digital image processor without any demosaicing to do, yet it is still useful. It can be used to set the white balance, correct for chromatic abberation, reduce noise, sharpen, apply a curve, etc. And that is true even if the "true" raw conversion was done before it was invoked, or was never needed because the image data didn't come from a digital camera, but perhaps started as a TIFF or a JPEG from some other source!
Pano (and HDR) converts a raw image to Linear DNG (still sort of, kind of raw) and then does the DNG/Pano blend. So the image in HDR is 16-bit floating point but the processing is done in 32-bit floating point. The Pano blend is 16-bit Linear DNG.