Friday, December 14, 2012

What about this Media White Point?

The "wtpt" (media white point) tag of a display profile should be D50 regardless of what the actual white point is. To quote from the ICC spec here:

Page 33:
"For displays, the values specified must be those of D50 normalized such that Y = 1,0 (i.e. 0,9642 1,0 0,8249)."

The actual white point of the display gets encoded into the "chad" chromatic adaptation matrix tag, and this is not something that you can easily reverse to see the value.

I believe there was a clarification on this issue by the ICC because early versions of the spec were open to interpretation that the actual white point of the display should be encoded into the "wtpt" tag.

If you want to find out what Photoshop is using for the white point of the display, you can always use the "Custom RGB..." trick. i.e.

1. In the Photoshop Color Settings dialog, *temporarily* select the "Monitor RGB - xxxxx" profile as your working RGB space.
2. Then select "Custom RGB..." in the list
3. The Custom RGB dialog should show the unadapted white point and chromaticities of the display.
4. Be sure to set your working RGB space back to whatever you were using previously.

Header white point is not D50?

At 10:15 AM -0800 12/16/03, Doug Walker wrote:

>While repairing Profiles with ColorSync Utility I notice many have the following notation:
>Header white point is not D50. Fixed.
>I should have only Verified, right? Before selectively repairing?
>Was this global repair not a wise and good thing to do if my workflow is 6500?

There are typically two white points specified in a profile.

The white point in the header refers to the white point of the Profile Connection Space (PCS). As the ICC has specified that the PCS white point is D50, then any profile that has a white point in the header other than D50 (or D50 encoded incorrectly) needs to be corrected - no questions asked.

The white point that a display may be calibrated to can obviously be different than D50. This is encoded in the 'wtpt' tag.

Now, in version 4 ICC profiles 'wtpt' tags need to be adapted to D50 so even a 6500K-calibrated display will have D50 in the 'wtpt' tag. This is going to cause some confusion.. Where should the calibration point of the display be recorded in the profile? in the viewing conditions tag. Whether or not this happens correctly in the future remains to be seen.

And yes, for those thinking this to the next step, Adobe RGB and sRGB should both have D50 as their 'wtpt' in v4 ICC profiles. Did I say confusion?


o Steve Upton CHROMiX
o (hueman) 866.CHROMiX
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