Figure 4-9 Typical Example Of Sprite Reuse - Commodore Amiga Hardware Reference Manual

Part
ofa
screen
display
Each image of this sprite
may be placed at any
desired spot, horizontally
or vertically. However,
at least one video line
must separate the bottom
of one usage of a sprite .
from the starting point
of the next usage.
Figure 4-9: Typical Example of Sprite Reuse
In
single-sprite usage, two all-zero words are placed at the end of the data structure to
stop the DMA channel from retrieving any more data for that particular sprite during
that display field. To reuse a DMA channel, you replace this pair of zero words with
another complete sprite data structure, which describes the reuse of the DMA channel at
a position lower on the screen than the first use. You place the two all-zero words at the
end of the data structure that contains the information for all usages of the DMA chan-
nel. For example, figure 4-10 shows the data structure that describes the picture above.
The only restrictions on the re.use of sprites during a single display field is that the bot-
tom line of one usage of a sprite must be separated from the top line of the next usage
by at least one horizontal scan line. This restriction is necessary because only two DMA
cycles per horizontal scan line are allotted to each of the eight channels. The sprite
channel needs the time during the blank line to fetch the control word describing the
next usage of the sprite.
The following example displays the spaceship sprite and then redisplays it as a different
object. Only the sprite data list is affected, so only the data list is shown here. How-
ever, the sprite looks best with the color registers set as shown in the example.
112 Sprite Hardware