I've recently come across some Photoshop-generated PDFs with text layers. If I render them with the Ghostscript 8.62 with the -dGraphicsAlphaBits=4 -dTextAlphaBits=4 options set no smoothing is done on the text. The text is properly smoothed when using Adobe Acrobat, Photoshop, and even Evince with poppler-cairo. In general, the *AlphaBits options work fine for me with non-photoshop PDFs.
Created attachment 3923 [details] This is a photoshop PDF with a text layer.
Related to the pdf14 compositor disabling anti-aliasing?
Created attachment 3924 [details] Here's a screenshot of Acrobat Reader rendering this PDF.
Created attachment 3925 [details] Here's what ghostscript renders.
It probably is related to such an issue because, I forgot to mention, gs also fails to anti-alias Photoshop layer clipping paths (I'm not sure if that's the correct terminology, you know, the ones that let you make mutable shapes). I also tried saving down to PDF 1.3 and up to PDF 1.5 using Photoshop and this problem still happens.
Tagging this as a 'color' issue since this is a known deficiency (at least with text) in the way anti-aliasing is ignored by the pdf14 transparency device.
Looked at this and the issue has nothing to do with transparency. What is strange is that there is a rendering difference between having -dTextAlphaBits=4 and not but it fails to do any alpha like blending.
This is because the PDF file does not, in fact, contain 'text' in PostScript terms. The Text AlphaBits parameter only applied to actual text, which in PostScript terms means something which is drawn using the 'show' family of operators. The 'text' in this document is actually drawn using clipping and an image. While there is 'text' in a PDF sense, it is drawn using text rendering mode 7 (add text to path for clipping) and then used to clip a large pure black image. The image is named 'text layer.jpg' and is a CMYK DCT-compressed (!) pure black image. So TextAlphaBits will have no effect on this input, because the text is not really text, its a clip path. GraphicsAlphaBits also won't have any effect, because we don't anti-alias clip paths, as that leads to some highly undesirable effects under other circumstances.