1 files changed, 73 insertions, 0 deletions
diff --git a/engine/gfx/screen_effects.asm b/engine/gfx/screen_effects.asm
new file mode 100644
index 00000000..973a951a
--- /dev/null
+++ b/engine/gfx/screen_effects.asm
@@ -0,0 +1,73 @@
+; inverts the BGP for 4 (6 on CGB due to lag) frames
+ChangeBGPalColor0_4Frames:
+	call GetPredefRegisters ; leftover of red/blue, has no use here
+	ldh a, [rBGP]
+	xor $ff
+	ldh [rBGP], a
+	call UpdateGBCPal_BGP
+	ld c, 4
+	call DelayFrames
+	ldh a, [rBGP]
+	xor $ff
+	ldh [rBGP], a
+	call UpdateGBCPal_BGP
+	ret
+
+PredefShakeScreenVertically:
+; Moves the window down and then back in a sequence of progressively smaller
+; numbers of pixels, starting at b.
+	call GetPredefRegisters
+	ld a, 1
+	ld [wDisableVBlankWYUpdate], a
+	xor a
+.loop
+	ldh [hMutateWY], a
+	call .MutateWY
+	call .MutateWY
+	dec b
+	ld a, b
+	jr nz, .loop
+	xor a
+	ld [wDisableVBlankWYUpdate], a
+	ret
+
+.MutateWY
+	ldh a, [hMutateWY]
+	xor b
+	ldh [hMutateWY], a
+	ldh [rWY], a
+	ld c, 3
+	jp DelayFrames
+
+PredefShakeScreenHorizontally:
+; Moves the window right and then back in a sequence of progressively smaller
+; numbers of pixels, starting at b.
+	call GetPredefRegisters
+	xor a
+.loop
+	ldh [hMutateWX], a
+	call .MutateWX
+	ld c, 1
+	call DelayFrames
+	call .MutateWX
+	dec b
+	ld a, b
+	jr nz, .loop
+
+; restore normal WX
+	ld a, 7
+	ldh [rWX], a
+	ret
+
+.MutateWX
+	ldh a, [hMutateWX]
+	xor b
+	ldh [hMutateWX], a
+	bit 7, a
+	jr z, .skipZeroing
+	xor a ; zero a if it's negative
+.skipZeroing
+	add 7
+	ldh [rWX], a
+	ld c, 4
+	jp DelayFrames