Automattic/node-canvas

SVG Path - elliptical arc not drawn correctly

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#1377 opened on Mar 1, 2019

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BugHelp wanted

Description

Created this test app to demonstrate the issue: https://github.com/matthewbillienyc/node-canvas-svg-test/

Node Canvas SVG to PNG issue

So this repo is to show how between node-canvas versions 2.0.0-alpha.181 and 2.1.0 a bug with rendering SVG arcs was introduced.

Environment Info
Ubuntu 18.04
Nodejs 6.11.0
node-canvas 2.0.0-alpha.18

The png image that is created looks correct. Imgur

node-canvas 2.1.0

The png image that is created does not look correct. Imgur

SVG data

Here is an online SVG rendering of the SVG data below. The offending data is this: A 10 10 0 1 1 16.499995000000418 10.49000000166666. Specifically it looks like the 10.49000000166666 in the arc path is what causes it to break.

    <svg xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" style="font-family:RobotoCondensed;font-size:12px;" xmlns="http://www.w3.org/2000/svg" width="400" height="400" viewBox="0 0 400 400">
        <g class="translate" transform="translate(200,200)">
            <path fill="#6833c9" class="flag" d="M 16.5 10.5 A 10 10 0 1 1 16.499995000000418 10.49000000166666 Z M 6.5 20.5 L 6.5 29.531480378285778" stroke="#6833c9" stroke-width="1"></path>
        </g>
    </svg>
Why is it breaking? Great question!

I am not 100% sure. But we spent some time looking into by comparing all the commits between 2.0.0-alpha.18 and 2.1.0. You will see that it looks like this Pull Request (1288: Adapt to V8 7.0) is the offending change. More specifically I believe it is these lines in CanvasRenderingContext2d.cc that caused the breaking of the arc path.

Contributor guide

Tech stack
javascriptcppnodejs
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Knowledge of SVG arc commandsC++ debugging skills
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the changes in CanvasRenderingContext2d.cc around lines 2813-2826 from PR #1288 that adapted to V8 7.0. The bug affects elliptical arc path rendering; reproduce using the provided SVG test case. Examine the arc calculation logic and verify against the correct rendering from version 2.0.0 alpha.18.