Fusion by HSV Transformation

  The most widely applied fusion procedure is the merging of panchromatic SPOT imagery (10 m) with three-color SPOT imagery (20 m) or multispectral LANDSAT TM imagery (30 m). The simplest, most wide-spread and probably most intuitive technique works as follows [Kraus (1990), Albertz (1991)]:

1. Take three spectral bands from the multispectral imagery;
2. register the low resolution color image to the high resolution panchromatic image (i.e. essentially to magnify the color image to the same pixel size as the panchromatic image);
3. transform the magnified color image from an RGB color system into the HSV color system (Hue, Saturation, Value; see [Foley et al. (1995)]);
4. replace the ``Value'' image by the high resolution panchromatic image;
5. transform back into the RGB color system.

This technique is known to work well for moderate resolution ratios (such as 1:3 for SPOT + LANDSAT TM). The results are still helpful but less reliable for resolution ratios such as 1:20, e.g. for fusion of SPOT color images with panchromatic aerial photography [Ersbøll et al. (1997)].

In Figure 3 the steps involved in simulating the panchromatic and color infrared image and the fusion by HSV transformation are shown as a flow chart.

 
Figure 3: Fusion by HSV transformation. 

It has to be noted, however, that fusion by HSV transformation can be applied only to multispectral imagery consisting of three bands, since the image has to be coded as an RGB image before fusion can take place.