Collision check when Jumping a Ledge

Walkable tiles explanation

In Pokémon Green/Red/Blue/Yellow, which determines if a 16x16 overworld block is walkable is his lower-left tile!

Subtitles:
1. Walkable
2. Not walkable
3. Walkable
4. No walkable
5. Player comparison

Check in data\tilesets\collision_tile_ids.asm for walkable tiles in the overworld tileset (gfx\tilesets\overworld.png):

data\tilesets\collision_tile_ids.asm

MACRO coll_tiles
	IF _NARG
		db \# ; all args
	ENDC
	db -1 ; end
ENDM

Underground_Coll::
	coll_tiles $0b, $0c, $13, $15, $18

-Overworld_Coll::
-	coll_tiles $00, $10, $1b, $20, $21, $23, $2c, $2d, $2e, $30, $31, $33, $39, $3c, $3e, $52, $54, $58, $5b

RedsHouse1_Coll::
RedsHouse2_Coll::
	coll_tiles $01, $02, $03, $11, $12, $13, $14, $1c, $1a
...

Subtitles:
Blue around tiles: walkable

Knowning this, this code add a check when jumping down a ledge.

HandleLedges in engine\overworld\ledges.asm

HandleLedges::
...
	inc hl
	jr .loop
.foundMatch
+	ld a, [wSpritePlayerStateData1FacingDirection]
+	cp SPRITE_FACING_DOWN
+	lda_coord  8, 13
+	jr z, .checkCollision
+	ld a, [wSpritePlayerStateData1FacingDirection]
+	cp SPRITE_FACING_LEFT
+	lda_coord  4,  9
+	jr z, .checkCollision
+	ld a, [wSpritePlayerStateData1FacingDirection]
+	cp SPRITE_FACING_RIGHT
+	lda_coord 12,  9
+	jr z, .checkCollision
+.noCollisionTile
	ldh a, [hJoyHeld]
	and e
	ret z
...
...
	ld a, SFX_LEDGE
	call PlaySound
	ret
+.checkCollision
+	cp $00
+	jr z, .noCollisionTile
+	cp $10
+	jr z, .noCollisionTile
+	cp $1b
+	jr z, .noCollisionTile
+	cp $20
+	jr z, .noCollisionTile
+	cp $21
+	jr z, .noCollisionTile
+	cp $23
+	jr z, .noCollisionTile
+	cp $2c
+	jr z, .noCollisionTile
+	cp $2d
+	jr z, .noCollisionTile
+	cp $2e
+	jr z, .noCollisionTile
+	cp $30
+	jr z, .noCollisionTile
+	cp $31
+	jr z, .noCollisionTile
+	cp $33
+	jr z, .noCollisionTile
+	cp $39
+	jr z, .noCollisionTile
+	cp $3c
+	jr z, .noCollisionTile
+	cp $3e
+	jr z, .noCollisionTile
+	cp $52
+	jr z, .noCollisionTile
+	cp $54
+	jr z, .noCollisionTile
+	cp $58
+	jr z, .noCollisionTile
+	cp $5b
+	jr z, .noCollisionTile
+	ret

INCLUDE "data/tilesets/ledge_tiles.asm"

...

Optimization

Remove non-sense collision checks, like the houses door ($1b), cave door ($58), cut tree ($2d and $3e), right shore ($54), up shore ($33) and water pillar ($31).

HandleLegdes::
...
	ld a, SFX_LEDGE
	call PlaySound
	ret
.checkCollision
	cp $00
	jr z, .noCollisionTile
	cp $10
	jr z, .noCollisionTile
-	cp $1b
-	jr z, .noCollisionTile
	cp $20
	jr z, .noCollisionTile
	cp $21
	jr z, .noCollisionTile
	cp $23
	jr z, .noCollisionTile
	cp $2c
	jr z, .noCollisionTile
-	cp $2d
-	jr z, .noCollisionTile
	cp $2e
	jr z, .noCollisionTile
	cp $30
	jr z, .noCollisionTile
-	cp $31
-	jr z, .noCollisionTile
-	cp $33
-	jr z, .noCollisionTile
	cp $39
	jr z, .noCollisionTile
        cp $3c
	jr z, .noCollisionTile
-	cp $3e
-	jr z, .noCollisionTile
	cp $52
	jr z, .noCollisionTile
-	cp $54
-	jr z, .noCollisionTile
-	cp $58
-	jr z, .noCollisionTile
	cp $5b
	jr z, .noCollisionTile
	ret

INCLUDE "data/tilesets/ledge_tiles.asm"

...