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Re: [Openexr-devel] Half samples & DPX

From: Bob Friesenhahn
Subject: Re: [Openexr-devel] Half samples & DPX
Date: Tue, 6 Mar 2007 22:35:09 -0600 (CST)

On Tue, 6 Mar 2007, Florian Kainz wrote:

Of course you'd handle DPX 16-bit float samples the same way you treat
32-bit and 64-bit samples...

Seriously, how you treat the 16-bit float samples for viewing, color
grading etc. depends on what the samples mean.  Three possibilities
come to mind immediately:

A) The samples might represent the amount of light emitted by the screen.
  In this case, the conversion for viewing is straightforward: make the
  screen emit the amount of light specified by the RGB samples you get.
  Defining the primaries and white point of the DPX image would probably
  be enough to make this work.

This seems useful and since DPX already has a way to specify primaries and white point, not difficult to incorporate.

B) The samples might represent light in the depicted scene.  In this
  case you need a rendering transform that converts to light on the
  screen.  The problem is no more or less difficult than with OpenEXR
  or any other scene-referred floating-point format.  (In my opinion,
  the Academy's Image Interchange Framework committee is pretty close
  to resolving the issue.)

This approach does not seem to be common for DPX, which is usually output referenced. DPX is usually used for telecine/datacine or for grading and output to a film printer.

C) The samples might represent the density of a scanned Film negative.
  In this case you treat the images the same as DPX files with 10-bit
  integer samples.

This sounds valuable to vendors who want the extended range of floats but want to preserve the density approach. I think that this is supportable with existing metadata as well.

I suspect there is no _simple_ answer that will satisfy everyone.  If
someone does find a simple solution to the HDR color management problem,
it should apply equally to DPX, OpenEXR and floating-point TIFF.  "Exotic"
techniques such as CTL, Truelight, ICC profiles, and the in-house lookup
table formats used by various production companies exist because so far
nobody has found anything better.


Bob Friesenhahn
GraphicsMagick Maintainer,

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