From: Glenn Randers-P. <gl...@gm...> - 2011-07-29 21:48:35
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My test file was a PPM file (i.e., no gamma or any kind of color information): program redgreen c write an ASCII-formatted PPM file with all c possible combinations of red and green and four c levels of blue. integer i,j,k write (*,'(a15)')'P3 256 1024 255' do k=0, 255, 85 do j=0,255 do i=0,255 write (*,*)i,j,k enddo enddo enddo end On Fri, Jul 29, 2011 at 5:27 PM, <jb...@ac...> wrote: > *From:* Glenn Randers-Pehrson [mailto:gl...@gm...] > >I did some quick tests, could have got them wrong, > >that show that ImageMagick and GraphicsMagick > >with -colorspace Rec709Luma produce different > >numbers, and pngcrush -c 0 produces still another > >result, even when I changed the "-1" parameters > >in png_set_rgb_to_gray() to 0.21260, 0.71520. > > **** > > Bear in mind that if you want libpng to do the ‘luma’ calculation you must > call:**** > > ** ** > > png_set_gamma(pp, 1.0, 1.0)**** > > ** ** > > What is more you must call this at the right point – after the gAMA chunk > has been read – otherwise the results are undefined (i.e. I can’t remember > what happens in the different versions.)**** > > ** ** > > Otherwise you will only get luma if the file doesn’t contain a gAMA chunk. > (In the absence of any information all versions of libpng default both the > file and screen gamma to 1.0, but if gAMA is present the screen gamma will > default to 1/gAMA.)**** > > ** ** > > I think this is actually documented in 1.5, but it’s a weird consequence of > the way the libpng code is written so it’s basically plain confusing. I’m > pretty sure the documentation doesn’t describe (at all) how to get luma as > opposed to real grey (CIE Y) values, I’m not sure it should; it’s a feature > bordering on a bug.**** > > ** ** > > John Bowler jb...@ac...**** > > ** ** > > > ------------------------------------------------------------------------------ > Got Input? Slashdot Needs You. > Take our quick survey online. Come on, we don't ask for help often. > Plus, you'll get a chance to win $100 to spend on ThinkGeek. > http://p.sf.net/sfu/slashdot-survey > _______________________________________________ > png-mng-implement mailing list > png...@li... > https://lists.sourceforge.net/lists/listinfo/png-mng-implement > > |