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