[cairo] Generating 32-bit PNG files

[Rob Fearnside]

I'm using cairo_surface_write_to_png() to generate PNG image sequences from 32-bit source surfaces (ie. CAIRO_FORMAT_ARGB32).  I need the resultant files to all have the same format (32-bit with an alpha channel), but cairo generates 24-bit PNG files for any source images that happen to be entirely opaque (and 32-bit files for those with transparency).

Is there a way of forcibly generating 32-bit PNGs? (and thus also avoid the performance overhead of _cairo_mage_analyze_transparency() )

Thanks in advance

Rob
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