Created
March 13, 2014 00:47
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Two ways to detect if an image generated via canvas is blank (transparent PNG). (canvas.getImageData vs canvas.toDataURL)
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| <!doctype html> | |
| <html> | |
| <head> | |
| <title>Check transparent canvas</title> | |
| </head> | |
| <body> | |
| <canvas id='editor' style='border:solid'></canvas> | |
| <canvas id='blank' style='display:none'></canvas> | |
| <script> | |
| canvas = document.getElementById('editor'); | |
| blank = document.getElementById('blank'); | |
| ctx = canvas.getContext("2d"); | |
| w = canvas.offsetWidth; | |
| h = canvas.offsetHeight; | |
| // uses getImageData and compare all pixels | |
| f1 = function() { | |
| var data = ctx.getImageData(0, 0, w, h).data; | |
| for(var i = 0, l = data.length; i < l; i += 4) { | |
| if(data[i + 3] !== 0) { | |
| return false; | |
| } | |
| } | |
| return true; | |
| }; | |
| // uses toDataURL and compare just the urls | |
| f2 = function() { | |
| return canvas.toDataURL() === blank.toDataURL(); | |
| }; | |
| console.log('Return of getImageData method', f1()); | |
| console.log('Return of toDataURL method', f2()); | |
| </script> | |
| </body> | |
| </html> |
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Based on this thread on stackoverflow: http://stackoverflow.com/q/7991139/2252829
Check this JSPerf test to compare performance between methods: http://jsperf.com/check-transparent-canvas