How can I prevent cairo from rasterizing my pattern fills?

Question

Some time around 2011, I wrote a Pycairo script to generate a PDF that included several fills of custom vector patterns. Today I re-ran it (Python 3.5.2, Pycairo 1.10.0) and was surprised to see that all these patterns were rendered as low-resolution rasterized bitmaps. I reduced my script to this minimal example:

#!/usr/bin/python3

import cairo

def main():
    surface = cairo.PDFSurface("test.pdf", 100, 100)
    ctx = cairo.Context(surface)
    pattern = make_pattern()
    ctx.rectangle(10, 10, 80, 80)
    ctx.set_source(pattern)
    ctx.fill()
    surface.finish()

def make_pattern():
    pattern_surface = cairo.PDFSurface(None, 32, 8)
    ctx = cairo.Context(pattern_surface)
    ctx.set_line_width(.5)
    ctx.set_source_rgb(0,0,0)
    ctx.move_to(5, 6)
    ctx.line_to(27, 2)
    ctx.stroke()
    pattern = cairo.SurfacePattern(pattern_surface)
    pattern.set_extend(cairo.EXTEND_REPEAT)
    return pattern

if __name__ == "__main__":
    main()

The resulting PDF, heavily zoomed, renders the pattern like this:

Eyeballing the text of the PDF file confirms that this is a bitmap. Using an SVGSurface produces similar results. Is there a way to revert to the old behaviour whereby PDF pattern fills were rendered as vector fills in the final PDF rather than being rasterized like this? The only reference I've found online to the problem is this unanswered question on the cairo mailing list from January 2012.

score 3 · Accepted Answer · answered Feb 25 '19 at 15:30

I still haven't found a way to do this strictly using Pycairo, but I have found a solution using cairocffi, an improved, drop-in replacement for Pycairo. cairocffi offers the class RecordingSurface,

a surface that records all drawing operations at the highest level of the surface backend interface, (that is, the level of paint, mask, stroke, fill, and show_text_glyphs). The recording surface can then be “replayed” against any target surface by using it as a source surface.

I modified the script to use cairocffi and RecordingSurface:

#!/usr/bin/python3

import cairocffi as cairo

def main():
    surface = cairo.PDFSurface("test.pdf", 100, 100)
    ctx = cairo.Context(surface)
    pattern = make_pattern()
    ctx.rectangle(10, 10, 80, 80)
    ctx.set_source(pattern)
    ctx.fill()
    surface.finish()

def make_pattern():
    pattern_surface = \
        cairo.RecordingSurface(cairo.CONTENT_COLOR_ALPHA, (0, 0, 32, 8))
    ctx = cairo.Context(pattern_surface)
    ctx.set_line_width(.5)
    ctx.set_source_rgb(0,0,0)
    ctx.move_to(5, 6)
    ctx.line_to(27, 2)
    ctx.stroke()
    pattern = cairo.SurfacePattern(pattern_surface)
    pattern.set_extend(cairo.EXTEND_REPEAT)
    return pattern

if __name__ == "__main__":
    main()

This resulted in a non-rasterized pattern:

I translated this into C++ cairo code and it works as it should. However, I don't seem to be able to set the line cap or the line join style correctly. Any calls that I do to the ctx inside make_pattern do not seem to have an effect. Any ideas why that is? — huzzm, Apr 25 '23 at 14:17

How can I prevent cairo from rasterizing my pattern fills?

1 Answers1