On the fly resizing with Wordpress and NGINX

functions.php

<?php

add_filter('image_downsize', 'custom_image_downsize', 10, 3);
function custom_image_downsize($value = false, $post_id, $size = 'medium') {
	global $_wp_additional_image_sizes;

	if(is_array($size)) {
		$width = (int) @$size[0];
		$height = (int) @$size[1];
		$crop = (@$size[2]) ? 'c' : '';
	}
	elseif($img_size = @$_wp_additional_image_sizes[$size]) {
		$width = $img_size['width'];
		$height = $img_size['height'];
		$crop = ($img_size['crop']) ? 'c' : '';
	}
	elseif(is_string($size)) {
		$width = get_option($size.'_size_w');
		$height = get_option($size.'_size_h');
		$crop = get_option($size.'_crop') ? 'c' : '';

		if(!$crop) {
			$meta = wp_get_attachment_metadata($post_id);

			$full_w = absint(@$meta['width']);
			$full_h = absint(@$meta['height']);

			if($full_w && $full_w) {
				$height = floor($width*($full_h/$full_w));
			}
		}
	}

	if(@$width && @$height && $img_url = wp_get_attachment_url($post_id)) {
		$parts = pathinfo($img_url);
		$img_url = sprintf('%s/%s-%dx%d%s.%s', $parts['dirname'], $parts['filename'], $width, $height, $crop, $parts['extension']);
		return array($img_url, $width, $height, false);
	}

	return false;
}

?>

nginx.conf

set $upload_path path/to/uploads;

location ~ "/image/crop/(.*)\.(jpg|jpeg|png|gif)" {
  alias $upload_path$1.$2;
  image_filter crop $arg_width $arg_height;
  image_filter_jpeg_quality 75;
}

location ~ "/image/resize/(.*)\.(jpg|jpeg|png|gif)" {
  alias $upload_path$1.$2;
  image_filter resize $arg_width $arg_height;
  image_filter_jpeg_quality 75;
}

location ~ "/uploads/(.*)-([0-9]+)x([0-9]+)(c)?\.(jpg|jpeg|png|gif)" {
  set $crop $4;
  set $action resize;
  if ($crop = c) { set $action crop; }
  
  set $image_path $1-$2x$3$4.$5;
  set $image_uri image/$action/$1.$5?width=$2&height=$3&action=$action;
  
  if (!-f $request_filename) {
    proxy_pass http://127.0.0.1:80/$image_uri;
    break;
  }
  
  proxy_store $upload_root$image_path;
  proxy_store_access useR:rw group:rw all:r;
  proxy_temp_path /tmp/images;
  proxy_set_header Host $host;
}