limby-resize

npm install limby-resize

Resize with canvas or imagemagick node modules.

Wrapper for both.

Better resizing with canvas

Normally, resizing with canvas produces some not so great images. This module implements it's own resizing algorithm.

Math behind the algorithm

lets say we have 3 pixels being resized into 2 pixels.

normally, each pixel will have 4 numbers: red, green, blue, alpha. Lets just look at a simplified version where pixels are just 1 number.

Lets say the original image is (these represent 3 different pixels, separated by |):

0 | 100 | 255

The regular canvas drawImage resize will grab nearest neighbor and produce

0 | 100 or 0 | 255

This sometimes is fine, but it loses details and it can be a very ugly and jagged image. If you think about it, the sum of all the color in the original is 355 (0 + 100 + 255), leaving the average pixel 118.33. The resized average pixel would be 50 or 127.5, which could look okay or very different!

The image algorithm implemented in limby-resize will produce a similar image to imagemagick, keeping all the pixel data, so the average pixel will be the same.

Our algorithm would produce the following image:

33 | 201.3

(0 * .66 + 100 * .33) | (100 * .33 + 255 * .66)

The total in ours is 234.3, leaving the average of 117.15, which is going to equal the first image ( if we weren't rounding to 2 decimals for this example ).

tl;dr

This image resizer is better than the default canvas drawImage, but it is slower and slightly processor intensive.

Overall, this library wraps both imagemagic and canvas so you can switch out either one at a whim

Usage

Canvas

    var resizer = require('limby-resize')({
      canvas: require('canvas'),
    });
    
    resizer.resize('/tmp/image01.jpg', {
      width: 300,
      height: 500,
      destination: '/uploads/myimage.jpg',
    });

Image Magick

    var resizer = require('limby-resize')({
      imagemagick: require('imagemagick'),
    });
    
    resizer.resize('/tmp/myanimation.gif', {
      width: 300,
      height: 500,
      coalesce: true, // animated gif support ( if your image magick supports )
      destination: '/uploads/myanimation.gif',
    });
    
    // [0] takes first frame for previews, etc
    resizer.resize('/tmp/myanimation.gif[0]', {
      width: 300,
      height: 500,
      destination: '/uploads/myanimation_preview.gif',
    });

Note: If you scale up ( make a bigger image ), it will bypass the algorithm and use default drawImage

• Gif support only for image magick at the moment

canvas will just take the first frame, similar to using [0] with image magick

Browser support

lib/canvas_resize.js should be able to be included on the frontend for better resizing client side.

    var img, canvas, resized;
    img = new Image;
    img.onload = function(){
      canvas = document.createElement('canvas');
      canvas.width = img.width;
      canvas.height = img.height;
      canvas.getContext('2d').drawImage(img, 0, 0, img.width, img.height);
      resized = document.createElement('canvas');
      resized.width = 300;
      resized.height = 500;
      // see lib/canvas_resize for window.canvasResize = function(){...}
      canvasResize(canvas, resized);
      // resized will now be a properly resized version of canvas
    }
    
    img.src = '/path/to/img.jpg';