Use the Synthetic Color Image transform to transform a gray scale image into a synthetic color image. ENVI changes the gray scale image into a color image by applying high pass and low pass filters to the image to separate high and low frequency information. Low frequency information is assigned to the hue, and high frequency information is assigned to the value, and a fixed saturation level is used. These hue, saturation, and value (HSV) data are transformed into red, green, and blue (RGB) space, producing a color image.

This transform is often used with radar data to improve the display of subtle large-scale features while retaining fine detail. It works best for areas with low to moderate topographical relief. In radar images, it is often difficult to see low frequency variations because of high frequency features from small scale topography. Low frequency information is usually due to surface scattering variations from lithology or vegetation.

Reference

Daily, M., 1983, Hue-saturation-intensity split-spectrum processing of Seasat radar imagery, Photogrammetric Engineering and Remote Sensing, Vol. 49, No. 3, pp. 349-355.

  1. From the Toolbox, select Transform > Synthetic Color Image.

    2. The Input File dialog appears.

  2. Select an input band and perform any optional spatial subsetting, then click OK. The Synthetic Color Parameters dialog appears.
  3. Set the High Pass Kernel Size and Low Pass Kernel Size for the filters. The high pass kernel size should be the number of pixels corresponding to the size of the high frequency slope-dominated scattering. The low pass kernel size should be the number of pixels corresponding to the size of the low frequency diffuse scattering.
  4. Enter a saturation value from 0 to 1. Higher saturation values produce more saturated or pure colors.
  5. Select output to File or Memory.
  6. Click OK. ENVI adds the resulting output to the Layer Manager.