node gm - keep Aspect Ratio on resize and fill with blur background (with imagemagick)

image_blur.js

var GM = require('gm').subClass({ imageMagick: true }),
	Q = require('q'), // promisses support
	fs = require('fs'),
	path = require('path');

function create_thumb(img, dest_img, width, height) {
	var deferred = Q.defer();
	
	Q.when(get_image_size(img)).then(function (size) {
		var thumb_width = width,
			thumb_height = height,
			blurred_img_width = 0,
			blurred_img_height = 0,
			tmp_file;
		
		if (size.height > size.width) {
			thumb_width = null;
			thumb_height = height;
		} else {
			thumb_width = width;
			thumb_height = null;
		}
		
		blurred_img_width = width;
		blurred_img_height = Math.round(width * (size.height / size.width));

		if (blurred_img_height < height) {
			blurred_img_height = height;
			blurred_img_width = Math.round(thumb_height * (size.width / size.height));
		}
		
		tmp_file = img.replace(new RegExp('(' + path.extname(img) + ')$', 'i'), '_$1'); // temporary file. Eg: file.jpg => file_.jpg
		
		GM(img)
			.resize(thumb_width, thumb_height)
			.quality(90)
			.strip()
			.interlace('plane')
			.write(tmp_file, function (err) {
				if (err)
					return deferred.reject(err);

				GM(file).gravity('Center')
					.resize(blurred_img_width, blurred_img_height, '!')
					.crop(dimensions.w, dimensions.h)
					.blur('0xsigma')
					.quality(90)
					.strip()
					.interlace('plane')
					.stream('jpeg', function (err, stream_img_blur) {
						if (err)
							return deferred.reject(err);

						GM(stream_img_blur).compose('over')
							.gravity('Center')
							.composite(tmp_file)
							.quality(90)
							.strip()
							.interlace('plane')
							.write(dest_img, function (err) {
								if (err)
									deferred.reject(err);

								try {
									fs.unlinkSync(tmp_file);
								} catch (err) {
									return deferred.reject(err);
								}

								deferred.resolve(dest_img);
							});
					});
				});
	}, function (err) {
		deferred.reject();
	});
}

function get_image_size(img) {
	var deferred = Q.defer();
	
	try {
		GM(img).size((err, size) => {
			if (err)
				return deferred.reject(err);

			deferred.resolve(size);
		});
	} catch (err) {
		deferred.reject(err);
	}

	return deferred.promise;
}

This method allows to create a resized image with blur background, filling the width x height dimensions.

I took the ideia from http://wizards-toolkit.org/discourse-server/viewtopic.php?t=28035

Needs refactoring, I renamed some variables to fit right in this gist. Contributions are welcome.