The observations used in this work were taken at Sacramento Peak
Observatory on December 14, 1996. Our image (Fig. 1)
has dimensions pixels with 42 different intensities.
The minimum value is 32 and the maximum is 222.
The resolution is 0.4 arcsec/pixel and thus our image contains an area
of .As a first step we sharpen this image to enhance the local contrast
by convolving it with the following template:

The new image (Fig. 2) has a minimum of
-20 and a maximum of 330.
Let I(x,y) be our image matrix of pixel intensities of (x,y)
(Fig. 1). The image after sharpening
(Fig. 2) has intensities I_{1} (x,y). Let now
and
.
We reduce our image (Fig. 2) into
q classes (Figs. 3, 4) using the following
equation:

(1)

Figure 1:
Selected sunspot image containing
an area of

Figure 2:
Sunspot image
after sharpening of Fig. 1
with template of Sect. 2

Figure 3:
Sunspot image
after application of Eq. (1)
in Fig. 2 with q=4 labels

Figure 4:
Sunspot image
after application of Eq. (1)
in Fig. 2 with q=8 labels