Given an image URL or path, crop and resize it to be exactly a specific size. Crops centrally to force enforce the correct aspect ratio, and then resizes as per normal. Depends on the `when` and `gm` NPM modules. Returns a promise that resolves with an image buffer in a .PNG format.

crop-resize.js

/* 
* Given an image URL or path, crop and resize it to be exactly a specific size. 
* Crops centrally to force enforce the correct aspect ratio, and then resizes as per normal. 
* Depends on the `when` and `gm` NPM modules. 
* Returns a promise that resolves with an image buffer in a .PNG format.
*/
var when = require('when');
var gm   = require('gm');
var im   = gm.subClass({ imageMagick: true }); // use `im` in place of `gm` for heroku compatibility

/*
* Get the size of an image
*/
var getImageSize = function (file) {
  return when.promise(function (resolve, reject) {

    im(file).size(function (err, size) {
      if (err) {
        reject(err);
      } else {
        resolve(size);
      }
    });

  });
};


/*
* Crop and resize an image to precisely the correct dimensions.
* Crops first to get to the correct aspect ratio, and then resizes
* accordingly.
*
* Works with either a URL, or a path.
*/
module.exports = function (url, requiredWidth, requiredHeight) {
  return when.promise(function (resolve, reject) {

    // We need to resize and then crop. These dimensions
    // help us choose how to do this.
    var requiredRatio = requiredWidth / requiredHeight;

    // Get the size of the image
    getImageSize(url)

    // Crop to required dimensions and then resize
    .then(function (actualSize) {

      // If the actual width or height are too small, then reject
      if (actualSize.width < requiredWidth || actualSize.height < requiredHeight) {
        reject('image_too_small');
        return;
      }

      var actualRatio = actualSize.width / actualSize.height;

      // dimensions we'll crop to
      var cropWidth;
      var cropHeight;

      // crop co-ordinates (top left)
      var cropX;
      var cropY;

      // If actual ratio is too high, we need to crop the width
      if (actualRatio > requiredRatio) {
        cropWidth = actualSize.height * requiredRatio;
        cropHeight = actualSize.height; // no change
        cropX = (actualSize.width - cropWidth) / 2
        cropY = 0;
      }

      // If actual ratio is too low, we need to crop the height
      else if (actualRatio < requiredRatio) {
        cropWidth = actualSize.width
        cropHeight = actualSize.width / requiredRatio; // no change
        cropX = 0;
        cropY = (actualSize.height - cropHeight) / 2;
      }
      
      // we're already the correct ratio
      else {
        cropWidth = 0;
        cropHeight = 0;
        cropX = 0;
        cropY = 0;
      }

      // Crop and resize
      im(url)
      .crop(cropWidth, cropHeight, cropX, cropY)
      .resize(requiredWidth, requiredHeight)
      .toBuffer('PNG', function (err, buffer) {
        if (err) {
          reject(err);
        } else {
          resolve(buffer);
        }
      });

    });

  });
};