Commodore Amiga - Page 115

Sprite Control Word 1: SPRxPOS
This word contains the vertical (VSTART) and horizontal (HSTART) starting position
for the sprite. This is where the topmost line of the sprite will
be positioned.
Bits 15-8 contain the
low 8 bits of VSTART
Bits 7-0 contain the high 8 bits of HSTART
Sprite Control Word 2: SPRxCTL
This word contains the vertical stopping position of the sprite on the screen. It also con-
tains some
data having to do with sprite attachment, which is described later on.
Bits 15-8
Bit 7
Bits
6-3
Bit 2
Bit 1
Bit 0
SPRxCTL
The
low eight bits of VSTOP
(Used in attachment)
Unused (make zero)
The VSTART high bit
The VSTOP high bit
The HSTART low bit
The value (VSTOP - VSTART + 1) defines how many lines high the sprite will be.
Sprite Color Descriptor Words
It takes two color descriptor words to describe each horizontal line of a sprite; the high-
order word and the low-order word.
To calculate how many color descriptor words you
need, mUltiply the height of the sprite in lines by 2. The bits in the high-order color
descriptor word contribute the leftmost digit of the binary color selector number for each
pixel; the low-order word contributes the rightmost digit.
To form the color descriptor words, you first need to form a picture of the sprite, show-
ing the color of each pixel as a number from 0 -
3. Each number represents one of the
colors
in the sprite's color registers. For example, here is the spaceship sprite again:
102 Sprite Hardware