Usage

Using ssim is simple. You can pass two images (paths, buffers or urls) to ssim and it will return a promise with the resulting mean ssim (mssim). For instance:

import ssim from 'ssim.js';

ssim('./img1.jpg', './img2.jpg')
	.then(out => console.log(`SSIM: ${out.mssim} (generated in: ${out.performance}ms)`))
	.catch(err => console.error('Error generating SSIM', err));

See it in action here.

You can specify any additional options as a 3rd parameter. Something like:

import ssim from 'ssim.js';

ssim('./img1.jpg', './img2.jpg', { downsample: 'fast' })

The parameters section details all options available.

Parameters

You can pass a 3rd parameter containing a plain object and any of the following options:

Parameter	Default	Description
windowSize	11	window size for the SSIM map
k1	0.01	The first stability constant
k2	0.03	The second stability constant
bitDepth	8	The number of bits used to encode each pixel
downsample	'original'	false / 'original' / 'fast'
ssim	'fast'	'original' / 'fast'

• Setting k1 or k2 to 0 will use UQI (Universal Quality Index) since it's a subset of SSIM (when C1 or C2 are 0)
• When using canvas images will be retrieved as 8 bits/channel. You can use the bitDepth parameter to modify how SSIM computes the value but it will have no effect on how the image is read
• There are several downsampling options:
  • false: disables downsizing
  • 'original': (default) implements the same downsizing than the original Matlab scripts
  • fast: relies on the canvas to do the downsizing (may lead to different results)
• There are several 'ssim' options:
  • 'original': Uses a line-by-line port of the Matlab scripts and attempts to match them as closely as possible
  • 'fast': (default) Uses a mathematically equivalent approach to 'original' but it's faster. By not focusing on a line-by-line port, it uses two 1-dimensional filters instead of a 2-D gaussian which results on a 2-3x performance improvement.

Output

Parameter	Description
mssim	Mean SSIM, the average of all ssim_map values
ssim_map	An array of arrays containing the ssim value at each window
performance	The total time to compute SSIM (in milliseconds)

To view the steps taken to validate the results, check the wiki page here.