var fs = require('fs'),
    PNG = require('pngjs').PNG;

fs.createReadStream('s.png')
    .pipe(new PNG({
        filterType: 4
    }))
    .on('parsed', function() {

        for (var y = 0; y < this.height; y++) {
            for (var x = 0; x < this.width; x++) {
                var idx = (this.width * y + x) << 2;
                	if(this.data[idx] != 0 && this.data[idx] != this.data[idx+1]) {
					console.log(y, x)
           	  }
           }
        }

        this.pack().pipe(fs.createWriteStream('out.png'));
    });