Routines
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6212: Locate Radar Blip
Used by the routines at UpdateRadar and CollisionCup.
Works out where a target is shown on the radar. The radar is centred on the player and one radar pixel covers 20 map units. The column (00-18) and row (00-1C) are stored in the third and fourth bytes of the blip record, measured from the top left of the radar. If the target is more than 0C pixels to the left or right of the player, or more than 0E pixels above or below, then both are set to 00 and the blip is hidden.
Input
BC Target X position (doubled)
DE Target Y position (doubled)
IX Radar blip record
Work out the horizontal distance between the player and the target.
LocateRadarBlip 6212 LD HL,($5E05) HL=player X map position (PlayerMapPosition).
6215 SLA L Double the value so it matches the scale of the target position.
6217 RL H
6219 AND A Reset the carry flag.
621A SBC HL,BC Work out the horizontal distance from the target to the player.
621C JR NC,OffsetBlipLeft If the target is not to the right of the player then jump to OffsetBlipLeft.
The target is to the right of the player, so make the distance positive.
621E CALL NegateHL Call NegateHL.
6221 CALL MultiplyHLByFour Call MultiplyHLByFour to multiply the distance by 04 which leaves the distance in radar pixels in H.
6224 LD A,$0C Start from the centre column of the radar (0C).
6226 CP H Compare the distance with 0C.
6227 JR C,HideRadarBlip If the target is more than 0C pixels to the right then jump to HideRadarBlip.
6229 ADD A,H Add the distance to the centre column to give the column to the right of the player.
This entry point is used by the routine at OffsetBlipLeft.
Store the column and do the same for the vertical distance.
LocateRadarBlip_StoreColumn 622A LD (IX+$02),A Store the column in the blip record.
622D LD HL,($5E08) HL=player Y map position (PlayerMapPositionY).
6230 SLA L Double the value so it matches the scale of the target position.
6232 RL H
6234 AND A Reset the carry flag.
6235 SBC HL,DE Work out the vertical distance from the target to the player.
6237 JR NC,OffsetBlipDown If the target is not further up the map than the player then jump to OffsetBlipDown.
The target is further up the map than the player, so make the distance positive.
6239 CALL NegateHL Call NegateHL.
623C CALL MultiplyHLByFour Call MultiplyHLByFour to multiply the distance by 04 which leaves the distance in radar pixels in H.
623F LD A,$0E Start from the centre row of the radar (0E).
6241 CP H Compare the distance with 0E.
6242 JR C,HideRadarBlip If the target is more than 0E pixels away then jump to HideRadarBlip.
6244 ADD A,H Add the distance to the centre row.
This entry point is used by the routine at OffsetBlipDown.
Rows are counted down the radar, so flip the value to give the row above the centre.
LocateRadarBlip_FlipRow 6245 NEG Negate A.
6247 ADD A,$1C Add 1C so the row counts down from the top of the radar.
This entry point is used by the routine at HideRadarBlip.
Store the row.
LocateRadarBlip_StoreRow 6249 LD (IX+$03),A Store the row in the blip record.
624C RET Return.
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