Re: [PATCH] cairolatex: respect transparency setting

[Henri M. <hen...@gm...>]

On 20/08/20, 21:31, Ethan A Merritt wrote:
> On Thursday, 20 August 2020 20:17:44 PDT Henri Menke wrote:
> > On 20/08/20, 20:01, Ethan A Merritt wrote:
> > > On Thursday, 20 August 2020 18:34:54 PDT Henri Menke wrote:
> > > > Currently the cairolatex terminal does not respect transparency options
> > > > given in the terminal setting.  In this minimal example the
> > > > notransparent option is ignored:
> > > > 
> > > >     set terminal cairolatex png notransparent
> > > >     set output "test.tex"
> > > >     plot sin(x)
> > > > 
> > > > Running this through gnuplot and then using `file' reveals that the
> > > > alpha channel is still present:
> > > > 
> > > >     $ gnuplot test.gnuplot && file test.png
> > > >     test.png: PNG image data, 1500 x 900, 8-bit/color RGBA, non-interlace
> > > 
> > > I think the raionale for the current code is that the png image created for
> > > inclusion in a LaTeX document must have a transparent background in order
> > > to support the "back" and "behind" attributes for text elements, since they
> > > will be drawn by the latex engine and then covered by inclusion of the
> > > graphics image.  The structure of the latex processing is:
> > > 
> > >     render text elements that are "behind" or "back"
> > >     \includegraphics{gnuplot_figure}
> > >     render text elements that are "front"
> > > 
> > > If you use a solid color background in the graphics image, it will hide
> > > any previously rendered text elements.
> > 
> > The reason why I'm looking into this is that currently it is impossible
> > to generate PDF/A when using plots generated by gnuplot.  PDF/A doesn't
> > allow transparency, so I'm trying to remove all of it (or at least make
> > it optional).  As noted in the patch, the default behaviour of using
> > transparency remains, so this is entirely optional.
> 
> I am researching this as I type, so please bear with me.
> 
> From Wikipedia I see
> 	Transparent objects and layers (Optional Content Groups) are 
> 	forbidden in PDF/A-1, but are allowed in PDF/A-2.
> PDF/A-1 was published in 2005; PDF/A-2 in 2011; PDF/A-3 in 2012;
> So maybe this is no longer a limitation?
> 
> From my own knowledge, there are commonly available tools that 
> render the transparency in a PDF document once, then save the result
> as an embedded image that no longer contains transparency.
> For example if you use gnuplot to create a PostScript file directly it cannot
> use transparency because PostScript itself does not support transparency.
> But if you use gnuplot to create a PDF file with transparency and then
> convert it
> 	pdf2ps input.pdf output.ps
> you get a PostScript file in which the transparency is "baked in" rather
> than interpreted.   Logically then you could take that PostScript file
> and convert it back to PDF, keeping the pre-rendered image that no
> longer uses an alpha channel. 
> 	ps2pdf output.ps new.pdf
> Whether that particular pair of tools produces a PDF/A compliant
> file I do not know.

This is actually a pretty good proposal.  In my actual plot I now have
added

    set terminal cairolatex pdf standalone header '\usepackage[a-1b]{pdfx}'
    set output "test.tex"
    # ...
    unset output
    !pdf2ps test-inc.pdf test-inc.ps
    !ps2pdf -dPDFA=1 test-inc.ps test-inc.pdf
    !pdflatex --interaction=nonstopmode test.tex

and the resulting document validates correctly with VeraPDF.  It's a bit
ugly and I don't know whether the ghostscript roundtrip would correctly
preserve fonts, but since the background image doesn't contain fonts, I
don't care.  Note that adding the -dPDFA=1 flag will make the PDF
structurally PDF/A compliant, but will not add metadata or color
profiles, so test-inc.pdf does not validate.  These extras are handled
by the pdfx LaTeX package.

To get my plots with PNG background compliant I use the patch from this
thread together with your suggestion `set tics front'.  Another option
would be to run mogrify to remove the transparency.

    set terminal cairolatex png standalone header '\usepackage[a-1b]{pdfx}'
    set output "test.tex"
    # ...
    unset output
    !mogrify -alpha remove test-inc.png
    !pdflatex --interaction=nonstopmode test.tex

So bottomlined my thesis now validates as PDF/A-1b, which is really
nice.

Cheers, Henri

> 
>    best,
> 
> 	Ethan
> 
> > 
> > Kind regards,
> > Henri
> > 
> > > 
> > > 	Ethan
> > > 
> > > > 
> > > > With this patch the transparency setting will be honoured.  The current
> > > > default setting of implicitly assuming transparency is unaltered.
> > > > ---
> > > >  term/cairo.trm | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/term/cairo.trm b/term/cairo.trm
> > > > index ee748dfe2..3ef76ecc6 100644
> > > > --- a/term/cairo.trm
> > > > +++ b/term/cairo.trm
> > > > @@ -849,7 +849,7 @@ void cairotrm_graphics()
> > > >  	plot.background.g = cairo_params->background.g;
> > > >  	plot.background.b = cairo_params->background.b;
> > > >  	gp_cairo_set_background(cairo_params->background);
> > > > -	if (ISCAIROLATEX || cairo_params->transparent)
> > > > +	if (cairo_params->transparent)
> > > >  		gp_cairo_clear_background(&plot);
> > > >  	else
> > > >  		gp_cairo_solid_background(&plot);
> > > > 
> > > 
> > > 
> > > 
> > > 
> 
> 
>