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Routines |
| Prev: 49850 | Up: Map | Next: 50002 |
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Works out which of two objects which overlap should be drawn first. For each object it finds a number from the difference between a vertical and a horizontal edge, and it swaps the objects if the numbers are the wrong way round. The edges which it uses are different when the view is rotated.
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| CompareObjectDepths | 49861 | LD A,(41576) | Fetch the rotation of the view. | |||||||
| 49864 | AND %00000001 | Test the lowest bit. | ||||||||
| 49866 | JR NZ,CompareObjectDepths_Rotated | Jump if the view is rotated. | ||||||||
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Find the number for the second object, in DE.
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| 49868 | LD A,(IY+1) | Fetch the horizontal position, add the horizontal size, halve it and add 64. | ||||||||
| 49871 | ADD A,(IY+8) | |||||||||
| 49874 | RRA | |||||||||
| 49875 | ADD A,64 | |||||||||
| 49877 | LD C,A | Keep it in BC. | ||||||||
| 49878 | LD B,0 | |||||||||
| 49880 | LD A,(IY+3) | Fetch the vertical position, subtract the vertical size, halve it and add 64. | ||||||||
| 49883 | SUB (IY+9) | |||||||||
| 49886 | RRA | |||||||||
| 49887 | ADD A,64 | |||||||||
| 49889 | LD L,A | Make the top edge a sixteen bit number in HL. | ||||||||
| 49890 | LD H,0 | |||||||||
| 49892 | AND A | Subtract the horizontal edge. | ||||||||
| 49893 | SBC HL,BC | |||||||||
| 49895 | LD BC,256 | Add 256. | ||||||||
| 49898 | ADD HL,BC | |||||||||
| 49899 | SRL H | Divide the result by two... | ||||||||
| 49901 | RR L | |||||||||
| 49903 | EX DE,HL | ...and keep it in DE. | ||||||||
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Find the number for the first object, in HL, in the same way.
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| 49904 | LD A,(IX+1) | Fetch the horizontal position, add the horizontal size, halve it and add 64. | ||||||||
| 49907 | ADD A,(IX+8) | |||||||||
| 49910 | RRA | |||||||||
| 49911 | ADD A,64 | |||||||||
| 49913 | LD C,A | Keep it in BC. | ||||||||
| 49914 | LD B,0 | |||||||||
| 49916 | LD A,(IX+3) | Fetch the vertical position and subtract the vertical size. | ||||||||
| 49919 | SUB (IX+9) | |||||||||
| CompareObjectDepths_Finish | 49922 | RRA | Halve it and add 64. | |||||||
| 49923 | ADD A,64 | |||||||||
| 49925 | LD L,A | Make the top edge a sixteen bit number in HL. | ||||||||
| 49926 | LD H,0 | |||||||||
| 49928 | AND A | Subtract the horizontal edge. | ||||||||
| 49929 | SBC HL,BC | |||||||||
| 49931 | LD BC,256 | Add 256. | ||||||||
| 49934 | ADD HL,BC | |||||||||
| 49935 | SRL H | Divide the result by two. | ||||||||
| 49937 | RR L | |||||||||
| 49939 | AND A | Subtract the number for the other object. | ||||||||
| 49940 | SBC HL,DE | |||||||||
| 49942 | RET NC | Return if the first number is not the lower. | ||||||||
| 49943 | JP ChooseOtherObject | Otherwise swap the objects. | ||||||||
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Find the numbers when the view is rotated. The number for the first object goes in DE...
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| CompareObjectDepths_Rotated | 49946 | LD A,(IX+1) | Fetch the horizontal position, subtract the horizontal size, halve it and add 64. | |||||||
| 49949 | SUB (IX+8) | |||||||||
| 49952 | RRA | |||||||||
| 49953 | ADD A,64 | |||||||||
| 49955 | LD C,A | Keep it in BC. | ||||||||
| 49956 | LD B,0 | |||||||||
| 49958 | LD A,(IX+3) | Fetch the vertical position, add the vertical size, halve it and add 64. | ||||||||
| 49961 | ADD A,(IX+9) | |||||||||
| 49964 | RRA | |||||||||
| 49965 | ADD A,64 | |||||||||
| 49967 | LD L,A | Make the lower edge a sixteen bit number in HL. | ||||||||
| 49968 | LD H,0 | |||||||||
| 49970 | AND A | Subtract the horizontal edge. | ||||||||
| 49971 | SBC HL,BC | |||||||||
| 49973 | LD BC,256 | Add 256. | ||||||||
| 49976 | ADD HL,BC | |||||||||
| 49977 | SRL H | Divide the result by two... | ||||||||
| 49979 | RR L | |||||||||
| 49981 | EX DE,HL | ...and keep it in DE. | ||||||||
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...and the number for the second object is found in HL by the end of the routine above.
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| 49982 | LD A,(IY+1) | Fetch the horizontal position, subtract the horizontal size, halve it and add 64. | ||||||||
| 49985 | SUB (IY+8) | |||||||||
| 49988 | RRA | |||||||||
| 49989 | ADD A,64 | |||||||||
| 49991 | LD C,A | Keep it in BC. | ||||||||
| 49992 | LD B,0 | |||||||||
| 49994 | LD A,(IY+3) | Fetch the vertical position and add the vertical size. | ||||||||
| 49997 | ADD A,(IY+9) | |||||||||
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Go to find the number.
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| 50000 | JR CompareObjectDepths_Finish | Jump to CompareObjectDepths_Finish. | ||||||||
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View the equivalent code in;
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| Prev: 49850 | Up: Map | Next: 50002 |