[dvipdfmx] bug with inclusion of a PNG

[Paulo Ney de Souza]

On Wed, Jan 13, 2021 at 8:30 AM Shunsaku Hirata <shunsaku.hirata74 at gmail.com>
wrote:

> I don't know if it is the right way for dvipdfmx to ignore gAMA and
> cHRM settings as in other applications.
>

I would love to hear the answer to that question, and on that line, I'll
follow up with LibreOffice
guys to see what is their reasoning -- since they are the only other ones
that I see doing this.

And meantime I tried "pngtools" (specifically pngcheck and  pngchuncks) on
a large collection
of PNG's and NONE of them report the RGB data as a "sRGB" chunk, and the
command

convert *.png txt:-

puts out the pixels and the "srgba" values.

In my particular PNG, the RGB values can be seen easily on the HaexDump,
but not in a "chunk"
and the other ones.

Is it possible that the RGB values are commonly not part f a "chunk" in a
PNG?

Do you have a PNG that any of these tools report an sRGB chunk?

Paulo Ney




>
> But for the moment, I would suggest choosing one of the following
> solutions:
>
> 1. Add the following special command somewhere (probably at the beginning
> of the
> first page)
>
> \special{pdf:pageresources  <<
>    /ColorSpace <<
>     /DefaultRGB [ /CalRGB <<
>       /WhitePoint [.95046 1 1.08906]
>       /Gamma [2.19998 2.19998 2.19998]
>       /Matrix [.41239 .21264 .01933 .35758 .71517 .11919 .18048 .07219
> .95053]
>     >> ]
>    >>
> >>}
>
> This declares that DeviceRGB color space should be adjusted similarly as in
> PNG file A.png and B.png.
> Note that this will cause slight changes of all colors used in your
> PDF document.
>
> 2. Remove gAMA and cHRM chunks from your PNG files.
>
>
> Thanks,
> Shunsaku Hirata
>
> 2021年1月14日(木) 1:10 Shunsaku Hirata <shunsaku.hirata74 at gmail.com>:
> >
> > Hi Paulo,
> >
> > > I did see that XeTeX has company in Libre Office, but that is about
> it. All other programs I tried -- Adobe InDesign, Photoshop, Illustrator,
> MS Word, MS Publisher do it just like pdfTeX and LuaTeX, and ignore the
> calibration data on the PNG.
> >
> > Even Chrome which utilizes cHRM when displaying seems to ignore
> > it when printing to PDF. I don't know the reason behind this...
> >
> > > And on another point, reading the PNG specification (
> http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html) I saw the
> following text:
> > >
> > >         An sRGB chunk or iCCP chunk, when present and recognized,
> overrides the cHRM chunk.
> > >
> > > that seems to imply the calibration chunk should be ignore in the
> presence of an sRGB chunk -- which I believe is our case, isn't it?
> >
> > No, it isn't. As pointed by Philip, it doesn't contain sRGB nor iCCP.
> > And also, the presence of an sRGB chunk does not imply using DeviceRGB.
> > The sRGB and iCCP chunk settings take precedence over cHRM.
> >
> > Thanks,
> > Shunsaku Hirata
> >
> > > Many thanks for your help in understanding this,
> > > Paulo Ney
> > >
> > >
> > > On Tue, Jan 12, 2021 at 8:10 AM 平田俊作 <shunsaku.hirata74 at gmail.com>
> wrote:
> > >>
> > >>
> > >>
> > >> 2021/01/13 0:05、Paulo Ney de Souza <pauloney at gmail.com>のメール:
> > >>
> > >> 
> > >> Thanks Shunsaku, I am sorry I have more questions that on the first
> e-mail...
> > >>
> > >> On Tue, Jan 12, 2021 at 6:37 AM Shunsaku Hirata <
> shunsaku.hirata74 at gmail.com> wrote:
> > >>>
> > >>> Sorry there is a correction.
> > >>>
> > >>> 2021年1月12日(火) 21:10 Shunsaku Hirata <shunsaku.hirata74 at gmail.com>:
> > >>> >
> > >>> > Hi Paulo,
> > >>> >
> > >>> > Your PNG images A.png and B.png actually use a calibrated color
> space
> > >>> > as specified in gAMA and cHRM chunks of PNG images. Hence CalRGB
> > >>> > should be used.
> > >>
> > >>
> > >> What do you use for checking the ColorSpace of an image?
> > >>
> > >>
> > >> This time I just used TweakPNG program to inspect the PNG files. I
> don’t know if there is any tool which can be used to check the color space
> of images in general.
> > >>
> > >>> > Colors specified in the DeviceRGB color space may look differently
> on
> > >>> > different devices.  Indeed, your color.pdf doesn't show a color
> difference on
> > >>> > my smartphone whereas it does on my laptop screen.
> > >>>
> > >>> This is wrong. I was checking a different file.
> > >>
> > >>
> > >> I think that now it isincorrect. The file "color.pdf" as processed by
> xelatex under TL'20
> > >> show differences in tonality on laptops and tvs, but not on phones.
> > >>
> > >>
> > >>>
> > >>> Further inspecting it I found that the calibrated color space used
> in the
> > >>> provided PNG files seems to be using an approximated sRGB space.
> > >>> This seems to be causing a subtle difference in the color.
> > >>
> > >>
> > >> Would you mind go into a bit more detail? The differences in color
> are completely
> > >> gone after one opens and export it in GIMP. How is GIMP able to
> calculate the
> > >> correct color? THat ought to be part of the original image, no?
> > >>
> > >>
> > >> This is because GIMP removed the color calibration information
> originally stored. I think the correct color was not calculated but the
> calibration information was simply ignored and was omitted when the image
> was exported.
> > >>
> > >> Although your original PNG files contain some extra information which
> is required for reproducing the same color across different devices, PNG
> files exported by GIMP only contain RGB values with no associated
> information required for color calibration.
> > >>
> > >> As you have specified the background color as device dependent RGB
> color values and now GIMP exported PNG also uses the same color
> specifications, the color must look exactly the same as long as the same
> RGB values are specified.
> > >>
> > >>
> > >>>
> > >>>
> > >>> > If this is not as you intended, please remove the PNG chunks
> mentioned
> > >>> > above.
> > >>>
> > >>> I think dvipdfmx output is correct and I suggest doing the above.
> > >>
> > >>
> > >> Would you mind go into a bit more detail here as well? If dvipdfmx is
> correct here then
> > >> latex,  pdflatex and lualatex are wrong.
> > >>
> > >>
> > >> I think pdflatex and lualatex don’t fully support  color calibration
> information in PNG format and ignore such information as in GIMP export.
> > >>
> > >> The problem occurs because dvipdfmx tries to preserve the color
> calibration information stored in PNG files and uses device independent
> color space CalRGB for images while the background color is specified in
> the device dependent DeviceRGB color space.
> > >>
> > >>
> > >> Thanks,
> > >> Shunsaku Hirata
>
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