poppler-subpixel

Subpixel glyphs rendering support to poppler and others

This repository contains some patches I made in order to make my evince support subpixel rendering.

Gallery

Left side: subpixel rendering result (after patch)

Right side: gray-scale rendering result (before patch)

Click to Enlarge:

cairo

The important patch is cairo-lcdfilter-make-default-default.patch. This patch switches the default lcdfilter of FreeType rendering backend of cairo from FT_LCD_FILTER_LEGACY to FT_LCD_FILTER_DEFAULT. This is necessary because cairo does not provide a convenient way to specify the lcdfilter other than fontconfig, which is not suitable for poppler. In my opinion, this patch should be merged into upstream as FT_LCD_FILTER_LEGACY is only provided for comparison purposes, and might be disabled or stay unsupported in the future, according to the document of FreeType.

Without patching cairo, the subpixel rendering still works but you may notice more severe color fringes in glyphs due to the bad performance of FT_LCD_FILTER_LEGACY.

poppler

The patch poppler-subpixel.patch provides a new function called poppler_page_support_subpixel_rendering for glib/cairo backend for poppler. This function checks whether a PDF page could be subpixel-rendered. The reason that subpixel rendering is not implemented in may PDF viewer is that glyphs subpixel rendering cannot easily support transparent background, i.e., the background color much be known when the glyphs are rendered. By default, PDF has a transparent background. In most case, one can fill white to the background before the rendering of glyphs to workaround this problem. However, PDF file format has a feature called blend modes. Example can be downloaded from [https://github.com/mozilla/pdf.js/blob/master/test/pdfs/blendmode.pdf]. Some blend modes will generate different results when drawing on transparent or white background. Therefore, filling color into background is incorrect in this case. The new added function poppler_page_support_subpixel_rendering checks whether it is safe to fill some color into background by traversing current page and check whether transparent background and colored background are distinguishable by all the blend modes used before rendering. If it returns true, the application can call cairo_paint to fill the background and cairo_font_options_set_antialias to enable the subpixel rendering for cairo.

See glib/demo/render.c for an example how to use the subpixel rendering feature.

evince

The patch of evince uses poppler_page_support_subpixel_rendering to check whether current page supports subpixel rendering. If it is, fill white background to the cairo surface and enable subpixel glyphs rendering in the cairo engine. Otherwise, insert white background afterward using CAIRO_OPERATOR_DEST_OVER to guarantee the correctness.