View Full Version : Getting a pixel in webgl area
09-07-2010, 04:35 PM
I know behind webgl is a canvas object. However, when I attemp to use getcontext('2d'), realizing it does not work for webgl canvas. So, how am I going to get the pixel information from a Webgl canvas?
09-16-2010, 02:59 PM
1) Draw it into 2d canvas
a = document.getElementById("a");
b = document.getElementById("b");
gl = a.getContext("webgl");
ctx = b.getContext("2d");
// now use ctx to get the pixels
2) Use readPixels
buf = new Uint8Array(gl.canvas.width * gl.canvas.height * 4);
gl.readPixels(0, 0, gl.canvas.width, gl.canvas.height, gl.RGBA, gl.UNSIGNED_BYTE, buf);
3) If all you want is a PNG or JPG canvas.toDataURL still works.
10-31-2010, 07:52 PM
Hey thanks for your reply. I'll try it out. Nearly lost the hope of getting the answer for this question, thank goodness that I have come back to have a look (after a month)
01-28-2012, 08:30 AM
When using ReadPixel, I found some problems when my Canvas was not square (i.e. when Canvas Width and Height did not match). As soon as they matched (regardless if they were a power of 2) Read Pixel returned good values.
So if you have problems reading back pixel values try making you Canvas size square.
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