设置round lineJoin的canvas线条在非整数缩放时出现异常渲染问题

Canvas 线条特定缩放比例下渲染异常问题 #

设置了lineJoin="round"的canvas线条在特定缩放比例下会出现不符合预期的渲染效果，缩放比例为1.403619311470477和无缩放的对比可通过以下代码复现：

<canvas id="canvas" width="2000" height="2000"> </canvas>

<script>
const points = "45.13869240187139,481.020616681278,45.13869240187139,480.72980663647314,44.84788235706651,479.5665664572536,44.70247733466407,477.9671112108268,44.84788235706651,477.53089614361943,44.84788235706651,477.385491121217,44.993287379468946,476.80387103160723,45.28409742427383,476.22225094199746,46.44733760349335,475.059010762778,47.319767737907995,474.9136057403755,47.90138782751775,475.34982080758283,48.19219787232263,475.7860358747902,48.483007917127516,476.5130609868024,48.62841293952995,476.94927605400966,48.33760289472507,477.67630116602186".split(",")

const ctx = canvas.getContext("2d")
ctx.strokeStyle = "black"
ctx.lineWidth = 2
ctx.lineCap = "round"
ctx.lineJoin = "round"

ctx.beginPath()
ctx.moveTo(points[0], points[1])
for (let i = 2; i < points.length; i += 2) {
  ctx.lineTo(points[i], points[i + 1])
}
ctx.stroke()

ctx.scale(1.403619311470477, 1.403619311470477)
ctx.beginPath()
ctx.moveTo(points[0], points[1])
for (let i = 2; i < points.length; i += 2) {
  ctx.lineTo(points[i], points[i + 1])
}
ctx.stroke()

window.scrollBy(0, 400)
</script>

复现现象 #

预期得到无缩放描边的等比缩放版本，但在Chrome 94.0版本中会出现一条从线条中延伸出来的异常射线，效果如下图：

内容的提问来源于stack exchange，提问作者Vladiator