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Sideways stairs with diagonal movement

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Rangi edited this page Feb 10, 2019 · 11 revisions

All the stairs in Gen 1 and 2 only go upwards. Gen 3 had a few that go downwards, like the ones descending towards Trainer Tower in FRLG, but it took until Gen 4 with its true 3D maps for sideways stairs to be introduced. ROM hacks have already implemented sideways stairs in Gen 3; this tutorial shows how to implement them in pokecrystal.

(The code for this feature was adapted from Pokémon Brass.)

Here are all the changes needed to implement diagonal stairs, applied to a copy of pokecrystal. You can clone Rangi42/pokecrystal and checkout the diagonal-stairs branch to test it for yourself.

Contents

  1. Define collision types for sideways stairs
  2. Add a step type for sideways stairs
  3. Add movement commands for diagonal steps
  4. Use the diagonal step movement for the sideways stair collision type
  5. Add sideways stairs to a map

1. Define collision types for sideways stairs

Edit constants/collision_constants.asm:

 ; collision data types (see data/tilesets/*_collision.asm)
 ; TileCollisionTable indexes (see data/collision_permissions.asm)
 COLL_FLOOR             EQU $00
 ...
 COLL_DOWN_WALL         EQU $b3 ; unused
+COLL_DIAGONAL_STAIRS_RIGHT EQU $d0
+COLL_DIAGONAL_STAIRS_LEFT  EQU $d1
 COLL_FF                EQU $ff ; garbage

; collision data type nybbles
 LO_NYBBLE_GRASS      EQU $07
 ...
 HI_NYBBLE_UNUSED_C0  EQU $c0
+HI_NYBBLE_DIAGONAL_STAIRS EQU $d0

And edit data/collision_permissions.asm:

 TileCollisionTable::
 ; entries correspond to COLL_* constants
 	...
-	NONTALKABLE  LANDTILE ; d0
-	NONTALKABLE  LANDTILE ; d1
+	NONTALKABLE  LANDTILE ; COLL_DIAGONAL_STAIRS_RIGHT
+	NONTALKABLE  LANDTILE ; COLL_DIAGONAL_STAIRS_LEFT
 	...

2. Add a step type for sideways stairs

Edit constants/map_object_constants.asm:

 ; StepTypesJumptable indexes (see engine/overworld/map_objects.asm)
 	const_def
 	const STEP_TYPE_00              ; 00
 	...
 	const STEP_TYPE_SKYFALL_TOP     ; 19
+	const STEP_TYPE_NPC_DIAGONAL_STAIRS
+	const STEP_TYPE_PLAYER_DIAGONAL_STAIRS

Then edit engine/overworld/map_objects.asm:

 StepTypesJumptable:
 ; entries correspond to STEP_TYPE_* constants
 	dw ObjectMovementReset ; 00
 	...
 	dw SkyfallTop ; 19
+	dw NPCDiagonalStairs
+	dw PlayerDiagonalStairs
+NPCDiagonalStairs:
+	ret
+
+PlayerDiagonalStairs:
+	call Field1cAnonymousJumptable
+; anonymous dw
+	dw .InitHorizontal1
+	dw .StepHorizontal
+	dw .InitHorizontal2
+	dw .StepHorizontal
+	dw .InitVertical
+	dw .StepVertical
+
+.InitHorizontal2:
+	call GetNextTile
+.InitHorizontal1:
+	ld hl, wPlayerStepFlags
+	set 7, [hl]
+	call IncrementObjectStructField1c
+.StepHorizontal:
+	call UpdateDiagonalStairsPosition
+	call UpdatePlayerStep
+	ld hl, OBJECT_STEP_DURATION
+	add hl, bc
+	dec [hl]
+	ret nz
+	call CopyNextCoordsTileToStandingCoordsTile
+	ld hl, OBJECT_FLAGS2
+	add hl, bc
+	res 3, [hl]
+	ld hl, wPlayerStepFlags
+	set 6, [hl]
+	set 4, [hl]
+	jp IncrementObjectStructField1c
+
+.InitVertical:
+ 	ld hl, OBJECT_ACTION
+	add hl, bc
+	ld [hl], OBJECT_ACTION_STAND
+
+; If you start on the bottom half of a block, you go up;
+; if you start on the top half, you go down.
+	ld a, [wMetatileStandingY]
+	and a
+	ld a, DOWN
+	jr z, .got_dir
+	ld a, UP
+.got_dir
+	ld hl, OBJECT_DIRECTION_WALKING
+	add hl, bc
+	ld [hl], a
+
+	call GetNextTile
+	ld hl, wPlayerStepFlags
+	set 7, [hl]
+	call IncrementObjectStructField1c
+.StepVertical:
+	call UpdateDiagonalStairsPosition
+	call UpdatePlayerStep
+	ld hl, OBJECT_STEP_DURATION
+	add hl, bc
+	dec [hl]
+	ret nz
+	ld hl, wPlayerStepFlags
+	set 6, [hl]
+	call CopyNextCoordsTileToStandingCoordsTile
+	ld hl, OBJECT_STEP_TYPE
+	add hl, bc
+	ld [hl], STEP_TYPE_SLEEP
+	ret
+
+UpdateDiagonalStairsPosition:
+	ld a, [wMetatileStandingY]
+	and a
+	ld e, 1
+	jr z, .goingdown
+	ld e, -1
+.goingdown
+	ld hl, OBJECT_SPRITE_Y_OFFSET
+	add hl, bc
+	ld a, [hl]
+	add e
+	ld [hl], a
+	ret

TODO: Explain changes.

3. Add movement commands for diagonal steps

Edit macros/scripts/movement.asm:

 ; MovementPointers indexes (see engine/overworld/movement.asm)
 	enum_start 0, +4

 ; Directional movements

 	enum movement_turn_head ; $00
 turn_head: MACRO
 	db movement_turn_head | \1
 ENDM

 ...

 	enum movement_fast_jump_step ; $34
 fast_jump_step: MACRO
 	db movement_fast_jump_step | \1
 ENDM
+
+	enum movement_stairs_step
+stairs_step: MACRO
+	db movement_stairs_step | \1
+ENDM

 __enumdir__ = +1

Then edit engine/overworld/movement.asm:

 MovementPointers:
 ; entries correspond to macros/scripts/movement.asm enumeration
 	dw Movement_turn_head_down        ; 00
 	...
 	dw Movement_fast_jump_step_right  ; 37
+	dw Movement_stairs_step_down
+	dw Movement_stairs_step_up
+	dw Movement_stairs_step_left
+	dw Movement_stairs_step_right
 	dw Movement_remove_sliding        ; 38
 	...
+Movement_stairs_step_down:
+	ld a, STEP_WALK << 2 | DOWN
+	jp DiagonalStairsStep
+
+Movement_stairs_step_up:
+	ld a, STEP_WALK << 2 | UP
+	jp DiagonalStairsStep
+
+Movement_stairs_step_left:
+	ld a, STEP_WALK << 2 | LEFT
+	jp DiagonalStairsStep
+
+Movement_stairs_step_right:
+	ld a, STEP_WALK << 2 | RIGHT
+	jp DiagonalStairsStep
+
+DiagonalStairsStep:
+	call InitStep
+	ld hl, OBJECT_1F
+	add hl, bc
+	ld [hl], $0
+
+	ld hl, OBJECT_ACTION
+	add hl, bc
+	ld [hl], OBJECT_ACTION_STEP
+
+	ld hl, wCenteredObject
+	ldh a, [hMapObjectIndexBuffer]
+	cp [hl]
+	jr z, .player
+
+	ld hl, OBJECT_STEP_TYPE
+	add hl, bc
+	ld [hl], STEP_TYPE_NPC_DIAGONAL_STAIRS
+	ret
+
+.player
+	ld hl, OBJECT_STEP_TYPE
+	add hl, bc
+	ld [hl], STEP_TYPE_PLAYER_DIAGONAL_STAIRS
+	ret

TODO: Explain changes.

4. Use the diagonal step movement for the sideways stair collision type

Edit constants/map_object_constants.asm again:

 ; DoPlayerMovement.DoStep arguments (see engine/overworld/player_movement.asm)
 	const_def
 	const STEP_SLOW          ; 0
 	...
 	const STEP_WALK_IN_PLACE ; 7
+	const STEP_DIAGONAL_STAIRS

Then edit engine/overworld/player_movement.asm:

 DoPlayerMovement::

 	...

 .Normal:
 	call .CheckForced
 	call .GetAction
 	call .CheckTile
 	ret c
 	call .CheckTurning
 	ret c
 	call .TryStep
 	ret c
 	call .TryJump
 	ret c
+	call .TryDiagonalStairs
+	ret c
 	call .CheckWarp
 	ret c
 	jr .NotMoving

 	...

 .Ice:
 	call .CheckForced
 	call .GetAction
 	call .CheckTile
 	ret c
 	call .CheckTurning
 	ret c
 	call .TryStep
 	ret c
 	call .TryJump
 	ret c
+	call .TryDiagonalStairs
+	ret c
 	call .CheckWarp
 	ret c
 	ld a, [wWalkingDirection]
 	cp STANDING
 	jr z, .HitWall
 	call .BumpSound
 .HitWall:
 	call .StandInPlace
 	xor a
 	ret
 .TryJump:
 	ld a, [wPlayerStandingTile]
 	ld e, a
 	and $f0
 	cp HI_NYBBLE_LEDGES
 	jr nz, .DontJump

 	ld a, e
 	and 7
 	ld e, a
 	ld d, 0
 	ld hl, .data_8021e
 	add hl, de
 	ld a, [wFacingDirection]
 	and [hl]
 	jr z, .DontJump

 	ld de, SFX_JUMP_OVER_LEDGE
 	call PlaySFX
 	ld a, STEP_LEDGE
 	call .DoStep
 	ld a, 7
 	scf
 	ret

 .DontJump:
 	xor a
 	ret

 .data_8021e
 	db FACE_RIGHT             ; COLL_HOP_RIGHT
 	db FACE_LEFT              ; COLL_HOP_LEFT
 	db FACE_UP                ; COLL_HOP_UP
 	db FACE_DOWN              ; COLL_HOP_DOWN
 	db FACE_RIGHT | FACE_DOWN ; COLL_HOP_DOWN_RIGHT
 	db FACE_DOWN | FACE_LEFT  ; COLL_HOP_DOWN_LEFT
 	db FACE_UP | FACE_RIGHT   ; COLL_HOP_UP_RIGHT
 	db FACE_UP | FACE_LEFT    ; COLL_HOP_UP_LEFT
+
+.TryDiagonalStairs:
+	ld a, [wPlayerStandingTile]
+	ld e, a
+	and $f0
+	cp HI_NYBBLE_DIAGONAL_STAIRS
+	jr nz, .DontDiagonalStairs
+
+	ld a, e
+	and 7
+	ld e, a
+	ld d, 0
+	ld hl, .FacingStairsTable
+	add hl, de
+	ld a, [wFacingDirection]
+	and [hl]
+	jr z, .DontDiagonalStairs
+
+	ld a, STEP_DIAGONAL_STAIRS
+	call .DoStep
+	ld a, 7
+	scf
+	ret
+
+.FacingStairsTable:
+	db FACE_RIGHT
+	db FACE_LEFT
+
+.DontDiagonalStairs:
+	xor a
+	ret
 .Steps:
 ; entries correspond to STEP_* constants
 	dw .SlowStep
 	dw .NormalStep
 	dw .FastStep
 	dw .JumpStep
 	dw .SlideStep
 	dw .TurningStep
 	dw .BackJumpStep
 	dw .FinishFacing
+	dw .DiagonalStairsStep

 .SlowStep:
 	slow_step DOWN
 	slow_step UP
 	slow_step LEFT
 	slow_step RIGHT
 	...
 .FinishFacing:
 	db $80 | DOWN
 	db $80 | UP
 	db $80 | LEFT
 	db $80 | RIGHT
+.DiagonalStairsStep:
+	stairs_step DOWN
+	stairs_step UP
+	stairs_step LEFT
+	stairs_step RIGHT

TODO: Explain changes.

5. Add sideways stairs to a map

Here are some sideways stair tiles:

Screenshot

Let's say you want to add some sideways stairs to the cliffs on Route 45. Since maps/Route45.blk uses the johto tileset, here's what that would involve:

  1. Add the stair tiles to gfx/tilesets/johto.png
  2. Assign the GRAY color to those tiles in gfx/tilesets/johto_palette_map.asm
  3. Design sideways stair blocks in data/tilesets/johto_metatiles.bin
  4. Assign the DIAGONAL_STAIRS_LEFT and DIAGONAL_STAIRS_RIGHT collision types to those blocks in data/tilesets/johto_collision.asm
  5. Redesign maps/Route45.blk to use the sideways stair blocks

You can use Polished Map to edit maps and tilesets; refer to the new map and new tileset tutorials for more information.

Now test it out!

Screenshot

TODO: Explain how to use COLL_DIAGONAL_STAIRS_RIGHT and COLL_DIAGONAL_STAIRS_LEFT.

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