/drawing-with-pycairo

Drawing with pycairo

Cairo paints paths with solid colours, gradients, mesh gradients and images, and composites them with 20-odd operators. It writes PNG, PDF, SVG and PostScript. It does not shape text, filter pixels, combine paths, outline strokes, or do depth. scripts/cairokit.py fills each gap; scripts/selftest.py exercises all of it. Both were verified on pycairo 1.29.1 / cairo 1.18.0 / Python 3.11.

Setup

pip install --break-system-packages pycairo numpy uharfbuzz fonttools skia-pathops
python3 /mnt/skills/user/drawing-with-pycairo/scripts/selftest.py /tmp/cairo-selftest

The self-test prints the cairo version, whether Pango is usable, per-panel timings, and the path of a 2×2 contact sheet. Read that sheet: panel A shows joined Arabic in green next to broken toy-API Arabic in red, B shows four boolean results and an outlined wave, C shows glow and a soft shadow, D shows a smooth-shaded torus. Then:

import sys; sys.path.insert(0, "/mnt/skills/user/drawing-with-pycairo/scripts")
from cairokit import *

In the environment where this was built, pycairo was not preinstalled. Pango was also absent: the system PyGObject was compiled for Python 3.12 while python3 was 3.11, and there were no Pango typelibs. pango() reports which case you are in.

Render, then look

Every render ends with a Read of the PNG before the next edit or before reporting. Exit codes and timings don't show visual bugs. In the session that produced this skill, looking caught five defects that ran without error:

• a painter sort drawn near-to-far
• inward normals combined with a light pointing away from the camera, which left a black knot
• a scene scaled in absolute pixels, cropped once the frame size changed
• glyphs crowding on a squashed spiral, because arc length was measured on the unsquashed curve
• a stray hook where a polyline's first move_to wasn't its first sample

For several outputs, contact_sheet() tiles them so one Read covers all of them. For animation, extract two or three frames with ffmpeg -ss T -i out.mp4 -frames:v 1 f.png and read those.

What cairokit provides

Find a font for a script with fc-match -f '%{file}' 'DejaVu Sans:lang=ar' (or fc-list :lang=hi family file). DejaVu Sans covers Arabic and Hebrew; FreeSans covers Devanagari.

Measured behaviour (800×600 unless noted)

Cairo renders on one CPU core. For large batches or long animations, spread frames across processes with multiprocessing rather than threads.

Behaviour that trips people

Text. show_text() / text_path() map codepoints to glyphs one by one, left to right, with no joining, reordering, ligatures or kerning. They are fine for ASCII labels. For anything else, use Pango if pango() returns modules, otherwise Shaper. Shaper does no line wrapping, no bidi across mixed-direction runs, and no font fallback: one font per call, and you split runs yourself.

Paths and the CTM. Path coordinates are transformed when they are appended, and the path is not part of the saved state. save(); scale(); <append>; restore(); fill() fills the transformed shape without distorting anything later. Line width, however, is transformed at stroke() time. Stroking inside a non-uniform scale() gives calligraphic, uneven lines, so restore first.

Pixels. ARGB32 memory is B,G,R,A per pixel, alpha-premultiplied, with rows get_stride() bytes apart. Call flush() before reading and mark_dirty() after writing. pixels() and from_array() handle this. Blurring premultiplied data is correct; un-premultiplying first gives dark fringes.

Seams. Two antialiased fills sharing an edge leave a faint line of background between them (conflation). Fixes: fill_preserve() plus a 0.5–0.7 px stroke in the same colour, or a single MeshPattern for the whole surface.

MeshPattern. Neighbouring Coons patches must use the identical curve for their shared edge. Generate each edge's control points in a canonical direction and reverse them for the other patch, or seams appear. Patches paint in insertion order, which is what makes gouraud_mesh() a painter's-algorithm renderer.

3D. There is no depth buffer. Sorting by mean depth fails when polygons interpenetrate or are long and thin across depth; subdividing helps. Use two-sided normals (flip toward the camera) unless the mesh orientation is known. Screen y points down, so a light "from the top left, toward the camera" is (-x, -y, -z) with camera-space +z pointing away.

skia-pathops. Round caps and joins produce conic segments, and simplify() rejects them; stroke_to_path() converts them to quadratics first. Any other pathops output passed to append_pathops() needs convertConicsToQuads() too.

Output. SVGSurface / PDFSurface must be finish()ed, or the file is truncated. RecordingSurface records once and replays at any scale or to any backend. Operators such as ADD, SCREEN, MULTIPLY and DIFFERENCE are on the context (set_operator). Float formats FORMAT_RGB96F / FORMAT_RGBA128F exist from cairo 1.18 for HDR-ish accumulation.

Route elsewhere