McIDAS Learning Guide
Version 2006

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Enhancements - Basic Concepts

There are two types of enhancements: color enhancements and grayscale enhancements. A color enhancement changes grayshades to colors and a grayscale enhancement changes a grayshade to a different grayshade value.

Color Enhancements

A color enhancement is a table of colors that corresponds to brightness values. Color enhancements are useful for tracking cloud features. For example, to track the tops of thunderstorms overshooting the tropopause, you can color all brightness values between 180 and 250 red.

Color enhancements are created with the EU MAKE command. You can create a color enhancement by assigning a color to a brightness value or a brightness range. For example, you could create an enhancement table where the color green corresponds to the brightness range 50 to 79, the color blue corresponds to the brightness range 80 to 99, and the color red corresponds to brightness value 100.

In addition, you can create a color enhancement by specifying color intensities. The values within the brightness range are interpolated within the color intensity range. For example, if the brightness range 0 to 71 is assigned to a blue color intensity of 203 to 255, a green color intensity of 173 to 200, and a red color intensity of 3 to 100, as shown below, the pixels with a low brightness value (near 0) will have corresponding low red, green, and blue intensities, and the pixels with high brightness values (near 71) will have corresponding high red, green, and blue intensities.

 Brightness  Blue      Green      Red    
  min max   min max   min max   min max  
  --- ---   --- ---   --- ---   --- ---  
    0  71   203 255   173 200     3 100
Once you create an enhancement table, you can save it using EU SAVE and then restore it using the command EU REST. You can apply the same or different enhancement tables to each frame on the workstation.

Grayscale Enhancements

Normally, a pixel's digital value, stored in an area, correlates to a brightness value. Each brightness value appears as a different shade of gray when the image is displayed. When a grayscale enhancement is applied, the correlation between the digital values and the displayed grayshades changes. You can change the grayscale contrast of an image two ways: using image contrast stretching or using image data stretching.

Image Contrast Stretching

Image contrast stretching changes the grayscale of the displayed image; it does not change the area data values. You can change the grayscale contrast of an image using the EB command. You can run the EB command two ways: using the command line and using the mouse. Using the command line, you specify the lower and upper brightness values to be enhanced. All pixels with brightness values below the lower input values and above the upper input value remain unchanged. The brightness values between the range are linearly interpolated. Using the mouse controlled version, you move the mouse to increase or decrease the brightness of the image. You can save grayscale enhancements and apply them to other images using the EU SAVE and EU REST commands. The example below shows the original contrast of an image and the contrast of an image after contrast stretching.

Original Image Contrast stretched image

Image Data Stretching

Image data stretching changes the grayscale of an image by stretching area data values to brightness values. To stretch the image data values, you must create a table that defines the values to stretch, as shown below. 
SU TABLE MB
BREAKPOINTS STORED IN TABLE : MB.ST         
INPUT       OUTPUT                          
-----       ------                          
 162.8         250                          
 192.3         250                          
 192.4         250                          
 209.3         10                          
 209.4         10                           
 213.3         10                           
 213.4         75                           
 219.3         75                           
 219.4         156                          
 230.3         156                          
 230.4         117                          
 241.3         117                          
 241.4         167                          
 279.8         102                          
 279.9         102                          
 301.9         0                            
 302           0                          
 330           0                            
CALIBRATION TYPE  : AAA                     
CALIBRATION UNITS : TEMP                    
BAND NUMBER       : -1                      
INTERPOLATION TYPE: LIN

The SU command defines tables to stretch raw, radiance, temperature, albedo, or brightness values (depending on the calibration type) to a user-defined brightness value. Stretch tables are used with the IMGDISP command to emphasize weather features in an image. The example below shows an image before and after an MB data stretch table was applied.

Original Image Data Stretched image

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