[argyllcms] Re: Profile input white not mapping to output white

  • From: Iliah Borg <ib@xxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 22 Nov 2012 22:25:45 -0500

Dear Ben,

It looks like profiling for gamma = 1 adds posterization problems. Gamma around 
2 may be better.

Additionally, I see clipping of the green channel on the portions of the 
whitest patch of the target itself and general clipping on the darkest patch. 
That means you are stretching the dynamic range of the camera with this shot, 
stretching beyond usable - that is, getting into the unstable and non-linear 
zones of highlights and shadows. Maybe editing out the blown-out portion on the 
target may help?

Shooting against bright background for profiling may add unwanted flare to the 
shot. I recognize that those are your actual shooting conditions, still I would 
try to profile from a shot with a dark background and see if it helps. It 
certainly helped in a test I just made.

Last, but not least - raw converter you used seems not fully adequate for the 
task as it treats highlights in a slightly crude manner.

On Nov 22, 2012, at 9:00 PM, Ben Goren wrote:

> On 2012-11-22, at 5:42 PM, Iliah Borg wrote:
>> Dear Ben,
>> Have you tried RawDigger on the raw file to be sure all the four raw 
>> channels are clipped on the white?
> I wasn't familiar with RawDigger, but I just downloaded it...and, yes: all 
> four channels are clipped. Here's a sample on the diagonal midway between the 
> upper right edge of the disc and the corner of the photo:
> <PastedGraphic-1.tiff>
> The values are also clipped in the RGB TIFF that I'm feeding to scanin. 
> (*Only* the background is clipped, which I'm cropping out before I feed it to 
> acanin; none of the patches on the chart itself are clipped, best I can tell.)
>> If you can upload or email me the raw file I will try to see what is going 
>> on.
> You should be able to get to the file here:
> http://trumpetpower.com/files/trumpetpower.com/706C8559.CR2
> Thanks! I appreciate the help.
> Cheers,
> b&

Iliah Borg

Other related posts: