code: function detectSubsampling( img ) {
                  var iw = img.naturalWidth,
                      ih = img.naturalHeight,
                      canvas, ctx;
  
                  // subsampling may happen overmegapixel image
                  if ( iw * ih > 1024 * 1024 ) {
                      canvas = document.createElement('canvas');
                      canvas.width = canvas.height = 1;
                      ctx = canvas.getContext('2d');
                      ctx.drawImage( img, -iw + 1, 0 );
  
                      // subsampled image becomes half smaller in rendering size.
                      // check alpha channel value to confirm image is covering
                      // edge pixel or not. if alpha value is 0
                      // image is not covering, hence subsampled.
                      return ctx.getImageData( 0, 0, 1, 1 ).data[ 3 ] === 0;
                  } else {
                      return false;
                  }
              }
docs: Detect subsampling in loaded image.
  In iOS, larger images than 2M pixels may be
  subsampled in rendering.