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; b = new colour for BG colour 0 (usually white) for 4 frames
ChangeBGPalColor0_4Frames:
call GetPredefRegisters
ld a, [rBGP]
or b
ld [rBGP], a
ld c, 4
call DelayFrames
ld a, [rBGP]
and %11111100
ld [rBGP], a
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
ld [$ff96], a
call .MutateWY
call .MutateWY
dec b
ld a, b
jr nz, .loop
xor a
ld [wDisableVBlankWYUpdate], a
ret
.MutateWY
ld a, [$ff96]
xor b
ld [$ff96], a
ld [rWY], a
ld c, 3
jp DelayFrames
PredefShakeScreenHorizontally:
; Moves the window right and then back in a sequence of progressively smaller
; numbers of pixels, starting at b.
call GetPredefRegisters
xor a
.loop
ld [$ff97], a
call .MutateWX
ld c, 1
call DelayFrames
call .MutateWX
dec b
ld a, b
jr nz, .loop
; restore normal WX
ld a, 7
ld [rWX], a
ret
.MutateWX
ld a, [$ff97]
xor b
ld [$ff97], a
bit 7, a
jr z, .skipZeroing
xor a ; zero a if it's negative
.skipZeroing
add 7
ld [rWX], a
ld c, 4
jp DelayFrames
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