var block_col_amt = 15;
var block_row_amt = 10;
var blocks = [];
for(var r = 0; r < block_row_amt; r++) {
    blocks[r] = [];
    for(var c = 0; c < block_col_amt; c++) {
        var rand = Math.random();
        blocks[r][c] = 0;
        if(rand > 0.5) {
            blocks[r][c] = 1;
        }
    }
}

var draw = function() {
    if (!this.loadPixels) { return; }
    this.loadPixels();
    var pix=this.imageData.data;
    
    // change blocks
    for(var r = 0; r < block_row_amt; r++) {
        blocks[r] = [];
        for(var c = 0; c < block_col_amt; c++) {
            var rand = Math.random();
            blocks[r][c] = 0;
            if(rand > 0.5) {
                blocks[r][c] = 1;
            }
        }
    }
    
    for(var i=0; i<pix.length; i+=4) {
        var x = (i>>2) % width;
        var y = Math.floor(((i>>2) - x)/width);
        var row = Math.floor(y / height * block_row_amt);
        var col = Math.floor(x / width * block_col_amt);
        
        if(blocks[row][col] === 0) {
            pix[i] = 0;
            pix[i+1] = 0;
            pix[i+2] = 0;
        } else {
            pix[i] = 255;
            pix[i+1] = 255;
            pix[i+2] = 255;
        }
    }
    
    this.updatePixels();
};