By Darrell Aldrich
While written for Applesoft II, this article is also applicable to Integer Basic HIRES graphics mode. The High Resolution Graphics screen is composed of 280 vertical bars (X coordinates), by 160 units high (Y coordinates). The even-numbered bars are violet in color, while the odd-numbered bars are green. The color white is produced by plotting adjacent green and violet bars. (Green+Violet=White. )
By plotting only on even bars, (even X coordinates), violet plots can be made. However, since we are plotting only 50% of the points on the screen, horizontal resolution decreases to 140 points. Plotting green is exactly the same as plotting violet, except plotting is done on the odd-numbered bars (odd X coordinates ).
The HIRES routines produce the four available colors, (green, violet, white and black) by allowing us to mask off either the green or violet bars. Remember, when you set the HCOLOR variable (or location 81210) for green, that a point may not be plotted on a violet bar, and the inverse is true for plotting violet on a green bar.