mirror of
https://github.com/lscharen/iigs-game-engine.git
synced 2024-06-01 16:41:32 +00:00
45 lines
2.0 KiB
JavaScript
45 lines
2.0 KiB
JavaScript
/**
|
|
* Basic sprite compiler
|
|
*
|
|
* GTE has some specific needs that makes existing tools (like MrSprite) inappropriate. GTE
|
|
* sprites need to reference some internal data structures and have slightly difference code
|
|
* in order to handle clipping to the playfield bounds.
|
|
*
|
|
* The core sprite drawing approach is the same (set up Bank 1 direct page and stack), but
|
|
* the setup and dispatch are a bit different.
|
|
*
|
|
* A Note on Clipping
|
|
*
|
|
* GTE supports two clipping buffers for sprites to use. The first one is a static buffer
|
|
* that is aligned with the playfield and is used to clip the sprite when crossing the
|
|
* left and right boundaries, but since it's a static image, mask data can be put anywhere
|
|
* that the sprites should not show through, so irregular borders and sprite punch-outs
|
|
* on the playfield are possible.
|
|
*
|
|
* The second buffer matches the current tiles in the playfield and can be used as a
|
|
* dynamic mask of the playfield. Since the sprite code itself must use this data,
|
|
* different variations of the same sprite can be created to stand in "front" and "behind"
|
|
* different screen elements.
|
|
*
|
|
* The sprite requires the X and Y registers for this. The most general code that
|
|
* should be used for each sprite word is this:
|
|
*
|
|
* Example: DATA = $5670, MASK = $000F, screen_mask = $FF00, field_mask = $F0FF
|
|
* lda DP ; A = $1234
|
|
* eor #DATA ; A = $4444
|
|
* and #~MASK ; A = $4440
|
|
* and screen_mask,y ; A = $4400
|
|
* and >field_mask,x ; A = $4000
|
|
* eor DP ; A = $5234 <-- Only the high nibble is set to the sprite data
|
|
* sta DP
|
|
*
|
|
* It is not *required* that sprites use this approach, any compiled sprites code can be used,
|
|
* so if a sprite does not need to be masked, than any of the fast sprite approaches can be
|
|
* used.
|
|
*
|
|
* For clipping vertically, we pass in the starting and finishing lines in a register and
|
|
* a sprite record is set up to allow the sprite to be entered in the middle and exited
|
|
* before the last line of the sprite.
|
|
*/
|
|
|