javascript - HTML5 canvas toDataURL sometimes returns "data:," instead of the image -

i have created canvas document.createelement("canvas"):

var canvas = document.createelement("canvas"); canvas.width = width;     // width = 4000 or more canvas.height = height;   // height = 5000 or more 

then use canvas.todataurl() base64 string:

var str = canvas.todataurl(); 

but 'str' returns "data:,", no image data in it. 6 chars.

sometimes returns correct string "data:image/png;base64,ivborw0kggoaaaansuheugaaeggaabpscayaaabg/ah3aaagaeleqvr4xuzcqreaaajdm..."

i canvas.width large ....

you should try create 2d context explicitly , draw in empty shape, like:

var canvas = document.createelement("canvas"); canvas.width = 4000; canvas.height = 5000; var ctx=canvas.getcontext('2d'); ctx.fillrect(0,0,0,0); canvas.todataurl(); 

edit: seems canvas contains data exported datauri:

var canvas = document.createelement("canvas"); canvas.width = 400000000; canvas.height = 500000000; var ctx=canvas.getcontext('2d'); ctx.fillrect(0,0,0,0); canvas.todataurl(); // outputs : "data:,"