{"id":1611,"date":"2024-04-16T07:06:02","date_gmt":"2024-04-16T01:36:02","guid":{"rendered":"http:\/\/codetheory.in\/?p=1611---2121a2b6-4375-4465-929e-cae49d719ac3"},"modified":"2024-04-16T07:06:02","modified_gmt":"2024-04-16T01:36:02","slug":"why-clearrect-might-not-be-clearing-canvas-pixels","status":"publish","type":"post","link":"https:\/\/codetheory.in\/why-clearrect-might-not-be-clearing-canvas-pixels\/","title":{"rendered":"Why clearRect Might Not be Clearing the Canvas Pixels"},"content":{"rendered":"<p>Sometimes the <code>clearRect(x, y, width, height)<\/code> method on the canvas context might not erase the previous graphics drawn. This usually happens when we&#8217;re drawing paths using methods like <code>lineTo()<\/code>, <code>arc()<\/code>, <code>rect()<\/code>, etc. and then stroking them with <code>stroke()<\/code> or filling their content area using <code>fill()<\/code>. <a href=\"http:\/\/cssdeck.com\/labs\/ulixuuxi\" target=\"_blank\" rel=\"noopener\">Here&#8217;s<\/a> an example of what I&#8217;m trying to convey.<\/p>\n<p><!--more--><\/p>\n<p>The solution to this problem is actually quite simple but the issue itself can cause major headache if you&#8217;re unable to nail it. We basically have to call the <code>beginPath()<\/code> method on the 2D context. This method will create a new path and all the drawing functions called later will be directed to it. If we do not call <code>beginPath()<\/code> (and then preferably closing that using <code>closePath()<\/code>), then all the drawing commands called will stack up in the memory and every time <code>stroke()<\/code> or <code>fill()<\/code> is called, it&#8217;ll draw all the graphic paths.<\/p>\n<p>The stacking up of all the previous path drawing functions due to not closing the paths (with <code>beginPath()<\/code> and <code>closePath()<\/code>) and hence not getting cleared (with <code>clearRect()<\/code>) can lead to unpleasant results in animations. Here&#8217;s an image of what you can expect for a <code>rect()<\/code> call with <code>stroke()<\/code> in such a situation.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" src=\"http:\/\/codetheory.in\/wp-content\/uploads\/2014\/04\/canvas_path_stack.png\" alt=\"HTML5 Canvas Path Stack\" width=\"226\" height=\"298\" class=\"aligncenter size-full wp-image-1614\" \/><\/p>\n<p>Finally, here&#8217;s the <a href=\"http:\/\/cssdeck.com\/labs\/w8gytaqu\" target=\"_blank\" rel=\"noopener\">fixed version<\/a> of the demo linked above:<\/p>\n<pre class=\"brush: jscript; title: ; notranslate\" title=\"\">\r\n(function () {\r\n  var canvas = document.querySelector('canvas');\r\n  var ctx = canvas.getContext('2d');\r\n  \r\n  canvas.width = window.innerWidth;\r\n  canvas.height = window.innerHeight;\r\n  \r\n  ctx.beginPath(); \/\/\r\n  ctx.rect(50, 50, 100, 100);\r\n  ctx.stroke();\r\n  ctx.closePath();\r\n  \r\n  ctx.beginPath(); \/\/\r\n  ctx.moveTo(350, 50);\r\n  ctx.lineTo(400, 100);\r\n  ctx.stroke();\r\n  ctx.closePath();\r\n  \r\n  ctx.clearRect(0, 0, canvas.width, canvas.height);\r\n\r\n  ctx.beginPath(); \/\/\r\n  ctx.rect(100, 100, 100, 100);\r\n  ctx.stroke();\r\n  ctx.closePath();\r\n  \r\n  ctx.beginPath(); \/\/\r\n  ctx.moveTo(450, 50);\r\n  ctx.lineTo(500, 100);\r\n  ctx.stroke();\r\n  ctx.closePath();\r\n  \r\n  ctx.clearRect(0, 0, canvas.width, canvas.height);\r\n  \r\n  ctx.beginPath(); \/\/\r\n  ctx.rect(150, 150, 100, 100);\r\n  ctx.stroke();\r\n  ctx.closePath();\r\n  \r\n  ctx.beginPath(); \/\/\r\n  ctx.moveTo(550, 50);\r\n  ctx.lineTo(600, 100);\r\n  ctx.stroke();\r\n  ctx.closePath();\r\n  \r\n}());\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"<p>Sometimes the clearRect(x, y, width, height) method on the canvas context might not erase the previous graphics drawn. This usually happens when we&#8217;re drawing paths using methods like lineTo(), arc(), rect(), etc. and then stroking them with stroke() or filling their content area using fill(). Here&#8217;s an example of what I&#8217;m trying to convey.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[40],"tags":[14,15],"_links":{"self":[{"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/posts\/1611"}],"collection":[{"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/comments?post=1611"}],"version-history":[{"count":8,"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/posts\/1611\/revisions"}],"predecessor-version":[{"id":1620,"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/posts\/1611\/revisions\/1620"}],"wp:attachment":[{"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/media?parent=1611"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/categories?post=1611"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/codetheory.in\/wp-json\/wp\/v2\/tags?post=1611"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}