Adobe DNG HDR Format, Part 2

This is a follow-up to my previous post about Adobe’s new HDR DNG format.

I had another conversation with Eric Chan, a developer on Adobe Lightroom CC / 6, and one of the key people involved in the new Photo Merge features.  It was mentioned that the data in an HDR DNG is demosaiced, linear data, so I had some questions as to how closely the file resembles a true RAW file, and how this stacked up against sRAW or mRAW formats which are also demosaiced.

Eric:

Probably the easiest way to think of it is that the merged HDR DNG does not have any of the Develop processing controls applied to it yet.  For example, white balance, camera profiles, lens profiles, clarity, etc. — none of that stuff is yet applied to the HDR DNG.  So you have just as much freedom to adjust these HDR DNG files as you do with regular raw files.

If you’re more comfortable thinking of image processing stages, then the correct analysis here is that HDR DNGs have indeed been interpolated/demosaiced, but are still ’scene-referred’ in that they have not yet been further processed.  This means that the pixel values in the image still have a direct linear correlation with the amount of light that was actually captured by the lens and the camera’s sensor  (this is the meaning of ’scene-referred’ by the way).

Thus, technically, you could say that the HDR DNGs are not exactly “raw” (since they’re not in mosaic form) and they’re not yet “rendered” (because the Develop controls haven’t been applied to them) …  Instead, they’re somewhere in between, but much closer to “raw” than to “rendered”.  Perhaps we should call them “rare”?  Ok, I’m joking …

Regarding M-RAW / S-RAW:  You could say those files are also “rare” or “partially processed” but much more so than HDR DNG files are.  For instance, M-RAW/S-RAW files have white balance already baked in, so if you need to change the WB later, you may introduce some artifacts since some colors may already be clipped.  M-RAW/S-RAW files also sub-sample the color information to save space, so there will be small amounts of color bleeding at edges.  HDR DNGs don’t have these limitations.

So there you have it.  In addition to being reassured about the editability of an HDR DNG file, we also learned a little about the limitations of the mRAW/sRAW format.  Again, if you missed the previous conversation about the team’s decision to go with 16-bit floating point data rather than 32-bit data, be sure to check out Part 1 of this conversation.

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