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Check if canvas pixel is black

This function checks if a canvas pixel is black.

This is very useful for making pixel-perfect collisions in a javascript game.

argument x and y are for the coordinates of said pixel.

argument c is for the canvas context.

function(x,y,c){return window[c].getImageData(x,y,1,1).data[1]<20}
function(x,y,c){return window[c].getImageData(x,y,1,1).data[1]<20}
{
"name": "PixCheck",
"description": "This function checks if a certain canvas pixel is black",
"keywords": [
"canvas",
"collision",
"pixel",
"data",
"html"
]
}
<!DOCTYPE html>
<title>Foo</title>
<div>Expected value: <b>undefined</b></div>
<div>Actual value: <b id="ret"></b></div>
<script>
// write a small example that shows off the API for your example
// and tests it in one fell swoop.
var myFunction = function(){ /* the code here should be identical to the entry. */ }
document.getElementById( "ret" ).innerHTML = myFunction()
</script>
@maettig

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commented Apr 3, 2012

Doesn't this fail for all red, blue and magenta colored pixels? Also (I'm sure you know) it's very slow to call getImageData over and over again.

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