[tlbuild] writepng.c: No transparency with PDF/A-1b

[Florian Breitwieser]

Hi!
[I hope I do not close this window once again with an emacs inspired
'ctrl-w' before finishing writing. 3rd time now..]

I am trying to create PDF/A-1b compliant documents with (pdf)LaTeX.
PDF/A-1 is based on PDF v4, however unlike the former it does not
allow transparency (and is very strict about all its rules).

./texk/web2c/pdftexdir/writepng.c only writes the alpha channel when
pdfminorversion is >= 4, and excludes it otherwise. Eg in line 523:

    /* alpha channel support */
    if (fixedpdfminorversion < 4
        && png_get_color_type(png_ptr(img), png_info(img)) |
PNG_COLOR_MASK_ALPHA) {
        png_set_strip_alpha(png_ptr(img));
        png_copy = false;
    }

Would it be feasible to add a 'notransparency' or 'pdfa1' flag to
generate a no-transparency PDF 4 document?

Best,
Florian