jQuery('.modal-footer .capture').click(function(){
    var target = jQuery('#manipulated-img')
    html2canvas(target, {
      onrendered: function(canvas) {
        var img = canvas.toDataURL()
        jQuery.ajax({
            data: {'action': 'store-img', 'img' : img},
            type: 'POST',
            url: ajaxurl,
            success:function(ret){
                jQuery('.product-addon-manipulated-image > p > input').val(ret)
                jQuery('#myModal').fadeOut()
            }
        })
      }
    });
})

public function store_img() {
    //just a random name for the image file
    $random = md5( uniqid( mt_rand(), true ) );

    //convert the binary to image using file_put_contents
    $dir = wp_upload_dir()['basedir'] . '/add-on';
    if ( ! file_exists( $dir ) ) {
        wp_mkdir_p( $dir );
    }
    $savefile = @file_put_contents( $dir . "/$random.png", base64_decode( explode( ",", $_POST['img'] )[1] ) );
    //if the file saved properly, print the file name
    if( $savefile ){
        echo wp_upload_dir()['baseurl'] . "/add-on/$random.png";
    } else{
        echo "failed";
    }
    die();
}