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Routines |
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Used by the routine at flip_sprite.
Sprites are flipped in place, and the header remembers which way round the data now is: bit 7 of its first byte for upside down, bit 6 for mirrored. Bits 7 and 6 of the object's byte 7 say which way it wants to be drawn. Where they disagree the data is turned and the header bit toggled. An object that keeps facing one way therefore costs nothing after the first frame; two objects that share a sprite but face opposite ways turn it back and forth every time each is drawn.
This part does the vertical flip, swapping whole rows end for end; hflip_sprite_data does the mirror. DE is kept.
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| vflip_sprite_data | D865 | PUSH DE | the header's bit 7 against the object's: the same means no vertical flip | |||||||
| D866 | LD A,(DE) | |||||||||
| D867 | XOR (IX+$07) | |||||||||
| D86A | AND $80 | |||||||||
| D86C | JR Z,hflip_sprite_data | |||||||||
| D86E | LD A,(DE) | toggle the header's bit | ||||||||
| D86F | XOR $80 | |||||||||
| D871 | LD (DE),A | |||||||||
| D872 | RLCA | B = bytes per row: the width in bits 0 to 3, times two for the mask and image bytes | ||||||||
| D873 | AND $1E | |||||||||
| D875 | LD B,A | |||||||||
| D876 | INC DE | C = the height in rows | ||||||||
| D877 | LD A,(DE) | |||||||||
| D878 | LD C,A | |||||||||
| D879 | INC DE | |||||||||
| D87A | PUSH DE | HL = the data's length; DE = one past the end of the data, and HL its start | ||||||||
| D87B | LD E,B | |||||||||
| D87C | LD D,$00 | |||||||||
| D87E | CALL HL_equals_DE_x_A | |||||||||
| D881 | POP DE | |||||||||
| D882 | ADD HL,DE | |||||||||
| D883 | EX DE,HL | |||||||||
| D884 | LD A,B | DE on the last byte of the last row, HL on the last byte of the first | ||||||||
| D885 | CALL add_HL_A | |||||||||
| D888 | DEC DE | |||||||||
| D889 | DEC HL | |||||||||
| D88A | SRL C | swap rows in pairs: half the height | ||||||||
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