javascript - clearRect does not clear the canvas when drawing vertical lines

Question

I hit a weird edge case building something with canvas at work. clearRect does not clear the canvas when drawing vertical lines that go from the top to the bottom of the canvas. When rendering other stuff, clearRect works fine.

I'm not sure if I am missing something obvious, if this is intentional behavior, or a browser bug (unlikely since the behavior is identical in chrome, safari, firefox and opera on mac). If it is intentional behavior, does anybody know the rationale behind it and/or can perhaps point to some documentation?

I made a small test case that shows the behavior clearly, only the combination clearRect/vertical lines does not clear the canvas: http://jsfiddle.net/kZj6F/

Thanks!

Answer 1

Your issue is with a missing beginPath causing subsequent lines to be added to the same path that was stroke drawing all lines.

If you switch to dots and back to lines with the clearRect option choose you can see the last arc added to the path being drawn too. Just add a call ctx.beginPath(); prior ctx.moveTo(randomX + 0.5, 0); ctx.lineTo(randomX + 0.5, canvas.height); and the problem is solved.

You can check http://jsfiddle.net/kZj6F/1/ to see it working.

Bwt it will with other shapes too if they got added to the path and the path was not cleared.