You can scroll a playfield upward or downward in the window. Each time you display the playfield, the bitplane pointers start at a progressively higher or lower place in the big picture in memory. As the value of the pointer increases, more of the lower part of the picture is shown and the picture appears to scroll upward. As the value of the pointer decreases, more of the upper part is shown and the picture scrolls downward. On an NTSC system, with a display that has 200 vertical lines, each step can be as little as 1/200th of the screen. In interlaced mode each step could be 1/400th of the screen if clever manipulation of the pointers is used, but it is recommended that scrolling be done two lines at a time to maintain the odd/even field relationship. Using a PAL system with 256 lines on the display, the step can be 1/256th of a screen, or 1/512th of a screen in interlace. Figure 3-23: Vertical Scrolling To set up a playfield for vertical scrolling, you need to form bitplanes tall enough to allow for the amount of scrolling you want, write software to calculate the bitplane pointers for the scrolling you want, and allow for the Copper to use the resultant pointers. Assume you wish to scroll a playfield upward one line at a time. To accomplish this, before each field is displayed, the bitplane pointers have to increase by enough to ensure that the pointers begin one line lower each time. For a normal-sized, low resolution display in which the modulo is 0, the pointers would be incremented by 40 bytes each time.
[Back to Amiga Developer Docs]