.DrawObjectStack AND #%00111111 \ Because the tile has an object on it, the tile data STA topObjectOnStack \ contains the number of the top object on the tile in \ bits 0 to 5, so extract the object number into \ topObjectOnStack LDX #0 \ We now count the number of objects that are stacked on \ the tile, so initialise a counter in X .stak1 INX \ Increment the object counter in X to record that we \ have found an object in the stack AND #%00111111 \ Because the tile has an object on it, the tile data TAY \ contains the number of the top object on the tile in \ bits 0 to 5, so extract the object number into Y (so \ object #Y is on the tile, though it might be part of \ an object stack) LDA objectFlags,Y \ Set A to the object flags for the object on the tile CMP #%01000000 \ If bit 6 of the object flags for object #Y is set BCS stak1 \ then object #Y is stacked on top of another object, \ and that object number is in bits 0 to 5 of the object \ flags, so jump to stak1 to increase the object count, \ extract that object number from A and check the flags \ again (so this works down through the stack of objects \ until we reach the object at the bottom of the stack) DEX \ Set objectStackCounter = X - 1 so we can use it as a STX objectStackCounter \ counter as we work through all the objects in the \ stack BEQ stak7 \ If there is only one object on the tile then the stack \ size will be zero, so jump to stak7 to draw the object \ stack from object #Y down (which in this case will \ just draw object #Y) \ If we get here then there is more than one object in \ the stack, so we now work out in which direction to \ draw the stack: top to bottom or bottom to top \ \ If the player is at a lower altitude than the object \ at the bottom of the stack, then the whole stack is \ above the player and the object on the top of the \ stack is the furthest away, so we need to draw the \ stack from top to bottom \ \ If it's the other way around and the player is at the \ same or higher altitude than the bottom of the stack, \ then we need to draw the stack from bottom to top, as \ the bottom object is the furthest away \ \ There is one exception: if the object stack is the \ Sentinel's tower and we are at the exact same height \ as the tower, then we draw the stack from top to \ bottom to ensure the Sentinel's feet are correctly \ hidden by the platform \ \ At this point, object #Y is the bottom object in the \ stack and object #topObjectOnStack is the top object .stak2 LDX playerObject \ Set X to the object number of the player LDA yObjectLo,X \ Set (A T) to the vertical distance between the player SEC \ and the object we are drawing, as follows: SBC yObjectLo,Y \ STA T \ (A T) = yObject,X - yObject,Y \ \ starting with the low bytes LDA yObjectHi,X \ And then the high bytes SBC yObjectHi,Y BMI stak6 \ If the result is negative then the player (object #X) \ is at a lower altitude than the object we are drawing \ (object #Y), so jump to stak6 to draw the entire \ object stack from top to bottom, starting with the top \ object and working down to the bottom \ If we get here then the player is level with or higher \ than the object we are drawing (object #Y) ORA T \ If at least one of A and T is non-zero then (A T) > 0, BNE stak3 \ then the player is at a higher altitude than the \ object we are drawing, so jump to stak3 to skip the \ following check \ If we get here then the player is level with the \ object we are drawing, so we need to check whether \ we are thinking of drawing the Sentinel's tower, \ because if we are, we need to draw the Sentinel before \ the tower to ensure the Sentinel's feet are correctly \ hidden by the platform LDA objectTypes,Y \ If the object we are drawing is the Sentinel's tower CMP #6 \ (an object of type 6), then jump to stak6 to draw the BEQ stak6 \ entire object stack from top to bottom, starting with \ the top object (the Sentinel) and working down to the \ bottom object (the tower) .stak3 \ If we get here then the player is higher than the \ object we are drawing (object #Y), so we need to draw \ the object stack upwards, from the bottom to the top \ \ This process is a bit convoluted, as the object flags \ only store object numbers that are below other objects \ rather than above, so we have to repeatedly count down \ the stack to work out which object to draw next \ \ The first time we enter this loop, object #Y is the \ object at the bottom of the stack, so this loop draws \ the stack from the very bottom to the top \ \ The second time we enter this loop, object #Y is the \ object above the original object #Y, so it's the next \ object up \ \ We loop around until we have drawn the whole stack \ from bottom to top \ \ This loop therefore draws object #Y and then works out \ which object is above object #Y in the stack and loops \ back to repeat the process, until we reach the top of \ the stack JSR DrawObject \ Draw object #Y onto the screen or into the screen \ buffer \ We now work out which object is above object #Y, which \ we do by counting down the stack by the right amount, \ from the top object down \ \ This count is stored in objectStackCounter, which \ contains the number of objects we still have to draw, \ which is the same as the number of objects above \ object #Y LDY topObjectOnStack \ Set Y to the number of the object on top of the stack, \ so we can start counting down from the top DEC objectStackCounter \ Decrement the object stack counter as we have just \ drawn object #Y BMI stak8 \ If we have drawn all the objects in the stack then the \ counter will now be negative, so jump to stak8 to \ return from the subroutine as we have drawn the whole \ stack LDX objectStackCounter \ Otherwise set X to the updated object stack counter, \ which is the number of objects above the object that \ we just drew BPL stak5 \ Jump to stak5 to start counting down the stack for X \ objects from the top, to give us the object that's \ above object #Y in the stack (this BPL is effectively \ a JMP as we know objectStackCounter is positive, so X \ must also be positive) .stak4 \ If we get here then we need to move down the object \ stack by one place from object #Y, updating Y to the \ number of the next object down LDA objectFlags,Y \ Set A to the object flags for object #Y AND #%00111111 \ Object #Y is stacked on top of another object, so the TAY \ number of the object below is in bits 0 to 5 of the \ object flags, so extract the next object number down \ the stack into Y DEX \ Decrement the counter in X as we have just moved down \ the stack by one place .stak5 \ This is the entry point for the loop that counts X \ objects down the stack, from the top object down BNE stak4 \ If X <> 0 then loop back to stak4 to move down the \ stack BEQ stak2 \ Otherwise jump to stak2 to draw object #Y, as that is \ the next object down the stack from the object we just \ drew (this BEQ is effectively a JMP as we just passed \ through a BNE) .stak6 LDY topObjectOnStack \ Set Y = topObjectOnStack so the following loop draws \ the entire object stack from top to bottom, starting \ with the top object and working down to the bottom .stak7 \ This loop draws all the objects in the stack in turn, \ from object #Y at the top, all the way down to the \ object at the bottom of the stack JSR DrawObject \ Draw object #Y onto the screen or into the screen \ buffer LDA objectFlags,Y \ Set A to the object flags for the object we just drew AND #%00111111 \ If object #Y is stacked on top of another object, then TAY \ the number of the object below is in bits 0 to 5 of \ the object flags, so extract the next object number \ down the stack into Y, so we can draw that object next DEC objectStackCounter \ Decrement the object stack counter as we have just \ drawn an object BPL stak7 \ Loop back until we have drawn all the objects in the \ stack, from object #Y down to the bottom of the stack .stak8 RTS \ Return from the subroutineName: DrawObjectStack [Show more] Type: Subroutine Category: Drawing objects Summary: Draw an entire stack of objectsContext: See this subroutine in context in the source code References: This subroutine is called as follows: * DrawTileAndObjects calls DrawObjectStack
Arguments: A The tile data for the tile containing the stack
[X]
Subroutine DrawObject (category: Drawing objects)
Draw a 3D object
[X]
Variable objectFlags in workspace Stack variables
Object flags for up to 64 objects
[X]
Variable objectStackCounter in workspace Main variable workspace
A counter for the objects in an object stack, for use when drawing the stack
[X]
Variable objectTypes (category: 3D objects)
The object types table for up to 64 objects
[X]
Variable playerObject in workspace Zero page
The number of the player object
[X]
Label stak1 is local to this routine
[X]
Label stak2 is local to this routine
[X]
Label stak3 is local to this routine
[X]
Label stak4 is local to this routine
[X]
Label stak5 is local to this routine
[X]
Label stak6 is local to this routine
[X]
Label stak7 is local to this routine
[X]
Label stak8 is local to this routine
[X]
Variable topObjectOnStack in workspace Main variable workspace
The number of the object on the top of an object stack
[X]
Variable yObjectHi (category: 3D objects)
The y-coordinates in 3D space for the 3D objects (high byte)
[X]
Variable yObjectLo (category: 3D objects)
The y-coordinates in 3D space for the 3D objects (low byte)