3.1 Java1489-plotting 3d surfaces using java  (Page 20/24)

The paint method for this class begins by setting up a pair of nested for loops that will be used to process each elevation point on the surface, and by initializing the color values for red, green, andblue to 0 in the innermost loop.

Set white and black for max and min values

If the elevation value is equal to 255, color values are set to cause that elevation to be painted white. If the elevation value is equal to 0, color values are set to cause that elevation to be painted black.

Listing 23 sets the color values to cause the extreme values of 0 and 255 to be painted black and white. Note that the code in Listing 23 is the beginning of a series of if-else constructs.

Elevations other than the extreme ends

If the elevation is not one of the extreme values of 0 or 255, control passes to code that subdivides the total elevation range from 1 to 254 into thefollowing four ranges and then sets the color values for each range separately:

• 0 less than elevation less than or equal 63
• 63 less than elevation less than or equal 127
• 127 less than elevation less than or equal 191
• 191 less than elevation less than or equal 254

Processing the lowest range

Listing 24 shows the code that is used to process the lowest range of elevations between 1 and 63.

What we are shooting for here is to produce color values that will result in a smooth gradient of color from blue at the low end to aqua at the high end ofthe range.

(See the leftmost one-fourth of the calibration scale for the middle image in Figure 1 .)

Scale the elevation values

Listing 24 begins by multiplying the elevation value by a factor of 4 to put it into the range from 4 to 252. This makes the elevation values compatible withallowable color values that range from 0 to 255.

The color aqua

The color aqua is produced by mixing equal amounts of blue and green. Listing 24 holds the value of blue constant at 255 and increases the value of green in proportion to the elevation value. Thus, at the lower end of the range, blue hasa value of 255 and green has a value of 4. (This is almost pure blue.) At the upper end of the range, blue still has a value of 255 and green has avalue of 252. (This is almost the pure secondary color aqua.)

In all cases, the value of red is 0 within this range. These color values (blue and temp) will be used later to instantiate a Color object, which will be used to control the plotting color for that portion of thedisplay.

Processing the other three ranges

Now that you know the basic scheme, you shouldn't have any difficulty understanding the code for processing the other three ranges shown in Listing 25 .