4 / Forming a Sprite / Building the Data Structure

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After defining the sprite, you need to build its data structure, which is
a series of 16-bit words in a contiguous memory area. Some of the words
contain position and control information and some contain color
descriptions. To create a sprite's data structure, you need to:
 * Write the horizontal and vertical position of the sprite into
 the first control word.
 * Write the vertical stopping position into the second control word.
 * Translate the decimal color numbers 0 - 3 in your sprite grid
 picture into binary color numbers. Use the binary values to build
 color descriptor (data) words and write these words into the data
 structure.
 * Write the control words that indicate the end of the sprite data
 structure.
 Warning:
 --------
 Sprite data, like all other data accessed by the custom chips, must
 be loaded into Chip RAM. Be sure all of your sprite data structures
 are word aligned in Chip Memory.
Table 4-1 shows a sprite data structure with the memory location and
function of each word:
 Memory
 Location 16-bit Word Function
 -------- ----------- --------
 N Sprite control word 1 Vertical and horizontal start position
 N+1 Sprite control word 2 Vertical stop position
 N+2 Color descriptor low word Color bits for line 1
 N+3 Color descriptor high word Color bits for line 1
 N+4 Color descriptor low word Color bits for line 2
 N+5 Color descriptor high word Color bits for line 2
 -
 -
 -
 End-of-data words Two words indicating
 the next usage of this sprite
 Table 4-1: Sprite Data Structure

All memory addresses for sprites are word addresses. You will need enough
contiguous memory to provide room for two words for the control
information, two words for each horizontal line in the sprite, and two
end-of-data words.
Because this data structure must be accessible by the special-purpose
chips, you must ensure that this data is located within chip memory.
Figure 4-7 shows how the data structure relates to the sprite.
 <- - - - 16 bits - - - - ->
 ___________________________ __
 | | | | Each group of words
 | VSTART, HSTART | | defines one vertical
 | |---------------------------| | usage of a sprite.
 | VSTOP, control bits | |--- Each one contains the
 | |___________________________| | starting location and
 | physical appearance
 | ___________________________ - | - of this sprite image.
 | | | |
 | | low word of data, line 1 | |
 |---------------------------| | |
 | | high word of data, line 1 | |
 |___________________________| | |
 | |
 ------ | |
 increasing ------ data describing | Pairs of words
 addresses ------ central lines | |- - containing color
 ------ if this sprite | information for
 | ------ | | pixel lines.
 ___________________________ |
 | | | | |
 | low word of data, line 1 | |
 | |---------------------------| | |
 | high word of data, line 1 | |
 | |___________________________| __| |
 - - -
 | ___________________________
 | | Last word pair contains
 | | 0000000000000000 | all zeros if this sprite
 |---------------------------|- - - - processor is to used
 | | 0000000000000000 | only once vertically in
 \|/ |___________________________| the display frame.
 Part of a screen display
 _______________________
 | |
 | |
 | |
 | |
 | |
 | HSTART |
 | |
 | | | Each word pair
 | | ___________________
 | VSTART _ _| _ | | |
 | _/_\_ _ _|_ _ | low word of pair |
 | |_____| _ _|_ _ <----|-------------------|
 | VSTOP _ _ \_/ | | high word of pair |
 | | | | |___________________|
 | |
 | |<--->| | describes one video
 |_______________________| line of the sprite
 Figure 4-7: Data Structure Layout
 Sprite Control Word 1 : SPRxPOS Sprite Color Descriptor Words 
 Sprite Control Word 2 : SPRxCTL End-of-data Words 

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