get pixel colour in javascript

Question

This might be a very basic question but as I am a newbie when it comes to javascript I can't figure out the right way to solve my issue. I need to do an if statement which checks the pixel colour and if it's black - do something if it's not - do something else. (It's a back-propagation project so that's why there are iterations)

for(i=1;i<iterations;i++) {
for(j=1;j<300;i+=2) { //canvas size is 300x300
for(k=1;k<300;i+=2) {
if (context.getImageData(j,k,1,1).data == [0, 0, 0, 0])
{/*code to be executed*/}
else {/*other code*/}
}
}
}

Sorry for asking easy stuff but I have not much time to do this so can't start from basics right now.

Answer 1

You need to compare each items in the array:

var pixel = context.getImageData(j,k,1,1).data; // cache array
if (pixel[0] === 0 &&                           // R
    pixel[1] === 0 &&                           // G
    pixel[2] === 0 &&                           // B
    pixel[3] === 0) {                           // A

    ...
}

Better yet, not use getImageData() for each pixel but cache it before starting the iterations:

var x, y,
    buffer = context.getImageData(0, 0, 300, 300).data;

for(y = 0; i < 300; y++) {
    for(x = 0; x < 300; x++) {
        var pos = (y * 300 + x) * 4;
        if (buffer[pos]   === 0 &&               // R
            buffer[pos+1] === 0 &&               // G
            buffer[pos+2] === 0 &&               // B
            buffer[pos+3] === 0) {               // A

            ...was black...
        }
        else {
            ...was not black...
        }
   }
}