Routines
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49061: Draw Object
Used by the routine at DrawGroupObjects.
Prepares an object of a group for drawing, and then passes it to the routine whose address is the operand of the JP at 49374 which is set by DrawLevelObjects. It works out the screen column and row of the object, and returns without doing anything if the object is not in view. Objects that can be entities are passed to AddObjectEntity first, and an object that has the flag for it in the row of flags of the group is also given an item record with AddItemRecord.
Input
IX The object, which is three bytes
Work out the screen column of the object. This is the depth of the cell plus the sum of the two halves of the first byte of the object, which are its offsets along the x and y axes, less the origin of the view.
DrawObject 49061 LD H,0 Set H to the depth of the cell, which is patched in.
49063 LD A,(IX+0) Fetch the first byte of the object.
49066 ADD A,A Multiply it by sixteen, to move the offset along the y axis to the top four bits...
49067 ADD A,A
49068 ADD A,A
49069 ADD A,A
49070 LD E,A ...and keep it in E.
49071 LD A,(IX+0) Fetch the first byte of the object again.
49074 AND %11110000 Keep the offset along the x axis in the top four bits.
49076 ADD A,E Add the two offsets to find the low byte of the screen column.
49077 JR NC,DrawObject_SetColumn Jump to DrawObject_SetColumn if there was no carry.
49079 INC H Increase the high byte of the screen column.
DrawObject_SetColumn 49080 LD L,A Put the low byte of the screen column in L.
49081 LD BC,0 Set BC to the origin of the view, which is patched in.
49084 ADD HL,BC Add the origin to the screen column.
Find out whether the object can be an entity. Objects of kind 129 and 143 are passed to AddObjectEntity, and any other kind is drawn straight away.
49085 LD A,(IX+1) Fetch the second byte of the object.
49088 AND %10001111 Keep bit 7 and the kind in bits 0-3.
49090 CP 129 Jump to DrawObject_Entity if it is kind 129.
49092 JR Z,DrawObject_Entity
49094 BIT 7,A Jump to DrawObject_Position if bit 7 is not set.
49096 JP Z,DrawObject_Position
49099 INC A Add one to the kind.
49100 AND %00001111 Keep the kind.
49102 JP NZ,DrawObject_Position Jump to DrawObject_Position unless the kind was 143.
DrawObject_Entity 49105 LD B,(IX+2) Fetch the third byte of the object into B...
49108 LD C,(IX+1) ...and the second byte into C.
49111 PUSH BC Save them.
49112 PUSH HL Save the screen column.
49113 LD A,C Copy the second byte into A.
49114 AND %10001111 Keep bit 7 and the kind.
49116 CP 143 Jump to DrawObject_Add if it is kind 143, so that the bytes are used as they are.
49118 JR Z,DrawObject_Add
Change an object of kind 129 into an object of kind 143 that has the size of the object in its third byte, so that it is recorded with that size.
49120 LD A,B Fetch the third byte of the object.
49121 RRA Shift the size in bits 4-5 to bits 2-3...
49122 RRA
49123 AND %00110000 ...and keep it.
49125 ADD A,16 Add 16.
49127 LD B,A Keep it in B.
49128 LD A,C Fetch the second byte of the object.
49129 AND %01110000 Keep the height in bits 4-6.
49131 ADD A,B Add the size to the height.
49132 LD (IX+2),1 Set the third byte of the object to 1.
49136 CP 128 Jump to DrawObject_SetKind if it is below 128.
49138 JR C,DrawObject_SetKind
49140 SUB 128 Take away 128.
49142 SET 6,(IX+2) Set bit 6 of the third byte of the object.
DrawObject_SetKind 49146 OR %10001111 Set bits 0-3 and bit 7, to make it kind 143.
49148 LD (IX+1),A Store it as the second byte of the object.
DrawObject_Add 49151 CALL AddObjectEntity Add an entity for the object, if it has one, and record it.
49154 POP HL Restore the screen column.
49155 POP BC Restore the second and third bytes of the object...
49156 LD (IX+1),C ...and put them back.
49159 LD (IX+2),B Put the third byte back.
49162 LD A,C Fetch the second byte of the object.
49163 AND %10001111 Keep bit 7 and the kind.
49165 CP 143 Compare it with 143.
49167 RET Z Return if it is kind 143 as it is an entity and is not drawn as a block.
49168 LD A,(IX+0) Fetch the first byte of the object.
49171 ADD A,A Multiply it by sixteen, to move the offset along the y axis to the top four bits...
49172 ADD A,A
49173 ADD A,A
49174 ADD A,A
49175 LD E,A ...and keep it in E.
Check that the object is not off the left of the view, or off the right of it, using the width of the object.
DrawObject_Position 49176 LD A,L Fetch the low byte of the screen column.
49177 AND %11111000 Round it down to a multiple of eight.
49179 LD L,A Put it back.
49180 PUSH HL Save the screen column.
49181 BIT 7,H Jump to DrawObject_Far if bit 7 of the high byte of the screen column is set.
49183 JR NZ,DrawObject_Far
49185 LD B,0 Clear B.
49187 LD C,(IX+2) Fetch the third byte of the object into C.
49190 SLA C Double it.
49192 LD A,C Copy it into A.
49193 AND %01110000 Keep the width in bits 4-6.
49195 LD D,A Keep it in D.
49196 LD A,C Fetch the doubled byte again.
49197 ADD A,A Shift it left three times, to move the depth to bits 4-6...
49198 ADD A,A
49199 ADD A,A
49200 AND %01110000 ...and keep it.
49202 ADD A,D Add the width.
49203 ADD A,31 Add 31 to make the distance to the left of the object.
49205 LD C,A Put it in C.
49206 ADD HL,BC Add it to the screen column.
49207 INC HL Move on by one more.
49208 RL H Rotate bit 7 of the high byte into the carry flag.
49210 POP HL Restore the screen column.
49211 RET NC Return if the object is off the left of the view.
49212 JR DrawObject_Visible Jump to DrawObject_Visible.
DrawObject_Far 49214 LD BC,32960 BC is 32960 the right edge of the view.
49217 SBC HL,BC Subtract it from the screen column.
49219 POP HL Restore the screen column.
49220 RET NC Return if the object is off the right of the view.
Work out the screen row of the object. This is half of the difference between its x and y coordinates, plus the offset to the view, less the height of the object.
DrawObject_Visible 49221 LD (48031),HL Store the screen column, as the one to draw at.
49224 LD A,(23738) Fetch the row of the cell into D.
49227 LD D,A Keep the y coordinate of the cell...
49228 LD HL,(23736) Fetch the column of the cell into H.
49231 LD A,(IX+0) Fetch the first byte of the object.
49234 AND %11110000 Keep the offset along the x axis.
49236 LD L,A Make it the low byte of the x coordinate.
49237 SRL D Halve the y coordinate...
49239 RR E
49241 SRL H ...and halve the x coordinate.
49243 RR L
49245 AND A Subtract half of the y coordinate from half of the x coordinate.
49246 SBC HL,DE
49248 LD DE,0 Set DE to the offset to the view, which is patched in.
49251 ADD HL,DE Add the offset.
49252 LD A,(IX+1) Fetch the second byte of the object.
49255 AND %01110000 Keep the height in bits 4-6.
49257 LD E,A Keep it in E.
49258 LD A,(IX+1) Fetch the second byte of the object again.
49261 AND %10001111 Keep bit 7 and the kind.
49263 CP 131 Jump to DrawObject_AddHeight if it is kind 131.
49265 JR Z,DrawObject_AddHeight
49267 CP 133 Jump to DrawObject_SubtractHeight unless it is kind 133.
49269 JR NZ,DrawObject_SubtractHeight
DrawObject_AddHeight 49271 LD A,16 A is 16 which is added to the height of these kinds.
49273 ADD A,E Add the height.
49274 LD E,A Keep it in E.
DrawObject_SubtractHeight 49275 LD D,0 Clear D.
49277 AND A Subtract the height from the screen row.
49278 SBC HL,DE
49280 PUSH HL Save the screen row.
Check that the object is not above the view.
49281 LD A,(IX+2) Fetch the third byte of the object.
49284 ADD A,A Shift it left three times, to move the depth to bits 3-5...
49285 ADD A,A
49286 ADD A,A
49287 AND %00111000 ...and keep it.
49289 ADD A,8 Add 8.
49291 LD E,A Keep it in E.
49292 ADD HL,DE Add it to the screen row.
49293 RL H Rotate bit 7 of the high byte into the carry flag.
49295 POP HL Restore the screen row.
49296 RET NC Return if the object is above the view.
Check that the object is not below the view, and work out the height of the object.
49297 PUSH HL Save the screen row.
49298 LD A,(IX+2) Fetch the third byte of the object and keep it in B.
49301 LD B,A
49302 AND %00111000 Keep the depth in bits 3-5.
49304 ADD A,8 Add 8.
49306 LD E,A Keep it in E.
49307 LD A,(IX+1) Fetch the second byte of the object.
49310 BIT 7,A Jump to DrawObject_SizeFromByte if bit 7 is set.
49312 JR NZ,DrawObject_SizeFromByte
49314 AND %00001111 Keep the kind.
49316 SUB 2 Take away 2.
49318 JR Z,DrawObject_StoreHeight Jump to DrawObject_StoreHeight if it is kind 2 with zero as the size.
DrawObject_SizeFromByte 49320 LD A,B Fetch the third byte of the object.
49321 RRA Shift the size in bits 4-5 to bits 2-3...
49322 RRA
49323 AND %00110000 ...and keep it.
49325 ADD A,16 Add 16.
DrawObject_StoreHeight 49327 LD (23797),A Store it as the height of the object, in 23797.
49330 ADD A,E Add the depth.
49331 LD E,A Keep it in E.
49332 SBC HL,DE Subtract it from the screen row.
49334 LD DE,32896 DE is 32896 the bottom of the view.
49337 SBC HL,DE Subtract it from the screen row.
49339 POP HL Restore the screen row.
49340 RET NC Return if the object is below the view.
49341 LD (48033),HL Store the screen row, as the one to draw at.
Work out the depth and the width of the object, and store them.
49344 LD A,(IX+2) Fetch the third byte of the object.
49347 ADD A,A Double it...
49348 LD B,A ...and keep it in B.
49349 AND %01110000 Keep the size in bits 4-6.
49351 ADD A,16 Add 16.
49353 LD (23796),A Store it as the depth of the object, in 23796.
49356 LD A,B Fetch the doubled byte again.
49357 ADD A,A Shift it left three times, to move the other size to bits 4-6...
49358 ADD A,A
49359 ADD A,A
49360 AND %01110000 ...and keep it.
49362 ADD A,16 Add 16.
49364 LD (23795),A Store it as the width of the object, in RecordDetails.
Give the object an item record if its flag in the row of flags of the group is set, and then pass it to the routine that handles it.
49367 LD A,(48015) Fetch the first byte of the row of flags of the group.
49370 RLA Rotate its top bit into the carry flag.
DrawObject_Record 49371 CALL C,AddItemRecord Add an item record for the object if the flag is set.
DrawObject_Handle 49374 JP DrawObject_Handle Jump to the routine that handles the object, which is patched in.
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