10.2 Gray Scale Conversion

10.2 Gray Scale Conversion

Gray scale conversion is one of the simplest image enhancement techniques. Gray scale conversion can be performed using the following function.

y = f (x)

where x : original input data
y : converted output data

In this section, the following five typical types are introduced, though there are many more functions that could be used. ( see Figure 10.2.1)

a. Linear conversion

y = ax + b
a : gain , b : offset

contrast stretch is one of linear conversion as follows.

Statistical procedures can be also applied in two ways as follows.

(1) Conversion of average and standard deviation

where xm : average of input image
Sx : standard deviation of input image
ym : average of output image
Sy : standard deviation of output image

(2) Regression
In such cases as multi-date images for producing a mosaic or radiometric adjustment, a selected image can be related to other images using regression technique. Line noise due to different detectors, for example Landsat MSS, can be eliminated by using the regression technique between different detectors.

Figure 10.2.2 shows various examples of gray scale conversion.

b. Fold conversion
Multiple linear curves are applied in order to enhance only a part of the gray scale.
c. Saw conversion
Where a discontinuous gray scale, occurs, drastic contrast stretch can be made.
d. Continuous function
Function such as exponential, logarithm, polynomials etc. may be applied.
e. Local gray scale conversion
Instead of the conversion being applied to the whole scene by a single formula, parameters are changed with respect to small local areas.