Routines
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47219: Test Object Overlap
Used by the routine at SlideActorPastObjects.
Tests whether an actor at a position would overlap with any object in a list. The list is made up of four byte entries. The first two bytes are the address of the graphic of the object, then the X and Y positions of the object. The list ends with an address of zero. The first two bytes of the graphic are the height in pixels and the width in bytes.
Input
B Height of the actor
C Width of the actor
D Y position of the actor
E X position of the actor
IX Pointer to the list of objects
Output
F Carry flag set if the actor overlaps with an object
TestObjectOverlap 47219 LD L,(IX+0) Fetch the low byte of the address of the object.
47222 LD H,(IX+1) Fetch the high byte of the address of the object.
47225 LD A,H Combine the bytes to check for zero.
47226 OR L
47227 RET Z Return if the address is zero (the end of the list), with the carry flag reset.
47228 LD A,(HL) Fetch the height of the object in pixels.
47229 INC HL Move to the width of the object.
47230 LD H,(HL) Fetch the width of the object in bytes.
47231 LD L,A Set the height of the object in L.
47232 LD A,H Fetch the width of the object in bytes.
47233 RLCA Multiply by eight.
47234 RLCA
47235 RLCA
47236 AND %11111000 Keep only bits 3-7.
47238 LD H,A H=the width of the object in pixels.
Compare the X positions.
47239 LD A,E Fetch the X position of the actor.
47240 SUB (IX+2) Subtract the X position of the object.
47243 JP P,TestObjectOverlap_Right Jump to TestObjectOverlap_Right if the actor is not to the left of the object.
47246 CPL Flip the bits to get the distance from the object, less one.
47247 CP C Compare with the width of the actor.
TestObjectOverlap_Horizontal 47248 JR NC,TestObjectOverlap_Next Jump to TestObjectOverlap_Next if the actor is too far away.
Compare the Y positions.
47250 LD A,D Fetch the Y position of the actor.
47251 SUB L Subtract the height of the object.
47252 SUB (IX+3) Subtract the Y position of the object.
47255 JP P,TestObjectOverlap_Below Jump to TestObjectOverlap_Below if the feet of the actor are below the object.
47258 CPL Flip the bits to get the distance from the object, less one.
47259 CP L Compare with the height of the object.
TestObjectOverlap_Return 47260 RET C Return if the actor overlaps with the object.
Move on to the next object.
TestObjectOverlap_Next 47261 INC IX Move IX on four bytes.
47263 INC IX
47265 INC IX
47267 INC IX
47269 JR TestObjectOverlap Jump to TestObjectOverlap.
The actor is to the right of the object so compare with the width of the object.
TestObjectOverlap_Right 47271 CP H Compare with the width of the object.
47272 JR TestObjectOverlap_Horizontal Jump to TestObjectOverlap_Horizontal.
The feet of the actor are below the object, so compare with the height of the actor.
TestObjectOverlap_Below 47274 CP B Compare with the height of the actor.
47275 JR TestObjectOverlap_Return Jump to TestObjectOverlap_Return.
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