[Webkit-unassigned] [Bug 223799] New: Rendering lines with more than 256 points on a canvas

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=223799

            Bug ID: 223799
           Summary: Rendering lines with more than 256 points on a canvas
           Product: WebKit
           Version: Safari 14
          Hardware: iPhone / iPad
                OS: iOS 14
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Canvas
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: james.offwhite at gmail.com
                CC: dino at apple.com

Created attachment 424361

  --> https://bugs.webkit.org/attachment.cgi?id=424361&action=review

two lines drawn on canvas, the bottom line has 267 points

Lines rendered to canvas element with more than 256 points are rendered incorrectly.

If I render a single line with 256 points the line renders as expected.

If I render a single line with more than 256 points the line is broken into two or more separate lines.

The following JS fiddle shows two lines, the top line has 256 points, the bottom line has 257 points. They are otherwise identical.

The bottom one consistently renders in 3 parts on ipad.

https://jsfiddle.net/g26t8adL/2/

For clarity, the code to reproduce is below

HTML:

<canvas id="c"></canvas>

JS:

(function() {
  // prep canvas
  let canvas = document.getElementById('c')
  let ctx = canvas.getContext('2d')
  canvas.width = 700
  canvas.height = 300

  // prep stroke
  ctx.globalAlpha = 0.5
  ctx.lineWidth = 30
  ctx.lineJoin = ctx.lineCap = 'round'

  // draw 256 point on canvas
  ctx.beginPath()
  for(let i=0; i< 256; i++){
    ctx.lineTo((50+(i*2)), 100)
  }
  ctx.stroke()

  // draw 257 point on canvas
  ctx.beginPath()
  for(let i=0; i< 257; i++){
    ctx.lineTo((50+(i*2)), 200)
  }
  ctx.stroke()
})();

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