Subpixel glyphs rendering support to poppler and others
This repository contains some patches I made in order to make my evince support subpixel rendering.
Left side: subpixel rendering result (after patch)
Right side: gray-scale rendering result (before patch)
Click to Enlarge:
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.
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.
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.