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Routines |
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Index 4 (13 turned round) of DEPTH_TABLE. With the boxes overlapping in U and in V, neither is simply behind the other, so each is placed by its nearest corner -- in the usual view the corner at the high side of U and the low side of V -- and how far back that corner is, which is how high up the screen it is: V less U. The candidate stays if its nearest corner is as far back as the other's, or further; otherwise the other becomes the candidate (DEPTH_SWAP). Turned round, the nearest corner is at the low side of U and the high side of V, and the test the other way about.
The sides are the half-unit bytes of SORT_AND_DRAW_THINGS; each difference has 256 added and is halved, to keep it positive, which does not change the order.
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| DEPTH_BY_CORNERS | D0D0 | LD A,(VIEW) | Turned round: the other corners | |||||||
| D0D3 | AND $01 | |||||||||
| D0D5 | JR NZ,DEPTH_BY_CORNERS_1 | |||||||||
| D0D7 | LD A,(IY+$01) | DE = how far back the other thing's nearest corner is: its low V side less its high U side | ||||||||
| D0DA | ADD A,(IY+$08) | |||||||||
| D0DD | RRA | |||||||||
| D0DE | ADD A,$40 | |||||||||
| D0E0 | LD C,A | |||||||||
| D0E1 | LD B,$00 | |||||||||
| D0E3 | LD A,(IY+$03) | |||||||||
| D0E6 | SUB (IY+$09) | |||||||||
| D0E9 | RRA | |||||||||
| D0EA | ADD A,$40 | |||||||||
| D0EC | LD L,A | |||||||||
| D0ED | LD H,$00 | |||||||||
| D0EF | AND A | |||||||||
| D0F0 | SBC HL,BC | |||||||||
| D0F2 | LD BC,$0100 | |||||||||
| D0F5 | ADD HL,BC | |||||||||
| D0F6 | SRL H | |||||||||
| D0F8 | RR L | |||||||||
| D0FA | EX DE,HL | |||||||||
| D0FB | LD A,(IX+$01) | The candidate's the same... | ||||||||
| D0FE | ADD A,(IX+$08) | |||||||||
| D101 | RRA | |||||||||
| D102 | ADD A,$40 | |||||||||
| D104 | LD C,A | |||||||||
| D105 | LD B,$00 | |||||||||
| D107 | LD A,(IX+$03) | |||||||||
| D10A | SUB (IX+$09) | |||||||||
| DEPTH_BY_CORNERS_0 | D10D | RRA | ...into HL | |||||||
| D10E | ADD A,$40 | |||||||||
| D110 | LD L,A | |||||||||
| D111 | LD H,$00 | |||||||||
| D113 | AND A | |||||||||
| D114 | SBC HL,BC | |||||||||
| D116 | LD BC,$0100 | |||||||||
| D119 | ADD HL,BC | |||||||||
| D11A | SRL H | |||||||||
| D11C | RR L | |||||||||
| D11E | AND A | The candidate's corner as far back or further: it stays | ||||||||
| D11F | SBC HL,DE | |||||||||
| D121 | RET NC | |||||||||
| D122 | JP DEPTH_SWAP | Otherwise the other takes its place | ||||||||
| DEPTH_BY_CORNERS_1 | D125 | LD A,(IX+$01) | Turned round: DE = the candidate's high V side less its low U side | |||||||
| D128 | SUB (IX+$08) | |||||||||
| D12B | RRA | |||||||||
| D12C | ADD A,$40 | |||||||||
| D12E | LD C,A | |||||||||
| D12F | LD B,$00 | |||||||||
| D131 | LD A,(IX+$03) | |||||||||
| D134 | ADD A,(IX+$09) | |||||||||
| D137 | RRA | |||||||||
| D138 | ADD A,$40 | |||||||||
| D13A | LD L,A | |||||||||
| D13B | LD H,$00 | |||||||||
| D13D | AND A | |||||||||
| D13E | SBC HL,BC | |||||||||
| D140 | LD BC,$0100 | |||||||||
| D143 | ADD HL,BC | |||||||||
| D144 | SRL H | |||||||||
| D146 | RR L | |||||||||
| D148 | EX DE,HL | |||||||||
| D149 | LD A,(IY+$01) | The other's the same, into HL; the candidate stays if the other's corner is as near as its own or nearer | ||||||||
| D14C | SUB (IY+$08) | |||||||||
| D14F | RRA | |||||||||
| D150 | ADD A,$40 | |||||||||
| D152 | LD C,A | |||||||||
| D153 | LD B,$00 | |||||||||
| D155 | LD A,(IY+$03) | |||||||||
| D158 | ADD A,(IY+$09) | |||||||||
| D15B | JR DEPTH_BY_CORNERS_0 | |||||||||
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