[poppler] XYZ white point correction patch

[Koji Otani]

From: Albert Astals Cid <aacid at kde.org>
Subject: Re: [poppler] XYZ white point correction patch
Date: Tue, 23 Jun 2009 00:22:17 +0200
Message-ID: <200906230022.18717.aacid at kde.org>

aacid> A Dissabte, 23 de maig de 2009, Hal V. Engel va escriure:
aacid> > On Saturday 23 May 2009 02:55:29 am Albert Astals Cid wrote:
aacid> > > > According to the PDF spec. the Y white point value should always be
aacid> > > > 1.0. So it might be possible to just do the correction to X and Z since
aacid> > > > this would make the code slightly more efficient.  Also I now think it
aacid> > > > would be better to do this in GfxCalGrayColorSpace::getXYZ() and
aacid> > > > GfxLabColorSpace::setXYZ() instead of in the locations right after
aacid> > > > calls to these functions.
aacid> > >
aacid> > > When i mean no difference i mean "diff" says the files are exactly the
aacid> > > same, not that i'm not able to see a difference in them.
aacid> > >
aacid> > > May it be because i'm not using any color profile?
aacid> > >
aacid> > > Albert
aacid> >
aacid> > It could be but the current code base defaults to sRGB as the output
aacid> > profile if one is not specified by the calling app.  So if poppler was
aacid> > built using USE_CMS then by default it should be using sRGB as the output
aacid> > color space for CIELab, Cal* and ICC objects.
aacid> >
aacid> > To confirm that you are using a profile you might consider modifying your
aacid> > code so that it explicitly sets the output profile by calling
aacid> > setOutputProfileName() or setOutputProfile() before opening the document.
aacid> > For additional testing I am attaching a profile intended for testing that
aacid> > causes the colors to be transformed in a way that is unmistakable because
aacid> > it transforms blues into reds and reds into blues.   Using this profile it
aacid> > should be easy to confirm that your code is actually using an output
aacid> > profile and also which parts of the rendered output are using the output
aacid> > profile.
aacid> >
aacid> > No matter what output profile is being used there should be at least some
aacid> > difference in the resulting RGB values of CIELab and CalGray objects if the
aacid> > white point of the object is NOT X = Y = Z = 1.0.  I have placed printf()
aacid> > statements in the code to confirm that this is altering the XYZ values of
aacid> > the CIELab objects in the altona pdf and it is.  I have also confirmed
aacid> > through testing that this makes a visible difference in the rendered output
aacid> > at least if the white point is significantly different from X = Y =Z = 1.0.
aacid> >
aacid> > Also I cleaned up the white point code so that the white point correction
aacid> > is now taking place in the getXYZ() functions which is where is really
aacid> > belongs. I have attached a patch with this set of changes.
aacid> 
aacid> Ok, i've tested the patch and yes, i need to load a displayProfile, otherwise 
aacid> it seems to do nothing, the problem is that the differences i see are so minor 
aacid> i can't say if it is correct with the patch or without it.
aacid> 
aacid> Do you have other PDF that shows a more visible difference?
aacid> 
aacid> Koji is that inside your area of expertice and can say yay/nay to the patch?
aacid> 

I'm afraid that I don't have enough knowledge about this to say the
patch is correct or not.

-------
Koji Otani.