[luatex] luatex doesn't set /Smask in pdf 2.0

[luigi scarso]

On Wed, Jun 30, 2021 at 3:02 PM Ulrike Fischer <luatex at nililand.de> wrote:

>
> I lost transparency in a png used in a pdf 2.0 document.
>
> It looks as if luatex decides if to use a /Smask by looking only at
> the minor version. So 1.4-1.9 work, 2.0-2.3 fail, 2.4 works again
>
>
> \pdfvariable compresslevel=0
> \pdfvariable objcompresslevel=0
> \pdfvariable majorversion =2
> \pdfvariable minorversion =0 % or 4
> \saveimageresource {transparent.png}
> \useimageresource\lastsavedimageresourceindex
> xxx
> \bye
>
> Result
>
> minor 0:
> << /Type /XObject /Subtype /Image /Width 50 /Height 50
> /BitsPerComponent 8 /ColorSpace /DeviceRGB /Length 7500 >>
>
> minor 4:
> << /Type /XObject /Subtype /Image /Width 50 /Height 50
> /BitsPerComponent 8 /ColorSpace /DeviceRGB /SMask 7 0 R /Length 7500
> >>
>
> transparent.png is an arbitrary image with transparency.
>

yes,  we assume   majorversion=1
490             /*tex alpha channel support */
491             if (pdf->minor_version < 4
492                 && png_get_color_type(png_p, info_p) |
PNG_COLOR_MASK_ALPHA) {
493                 png_set_strip_alpha(png_p);
494                 png_copy = false;
495             }

What are the rules for png  when  majorversion=2 ?

-- 
luigi
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