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Part 7: collision detection against platforms, snowball AI using them
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@ped7g
ped7g committed Feb 8, 2020
1 parent fe6ce5e commit fe8b226
Showing 1 changed file with 131 additions and 7 deletions.
138 changes: 131 additions & 7 deletions SpecBong.asm
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Original file line number Diff line number Diff line change
Expand Up @@ -204,6 +204,11 @@ GameLoop:
call Player1MoveByControls
call SnowballvsPlayerCollision

IF 0 ; DEBUG wait for fire key after frame
.waitForFire: call ReadInputDevices : ld a,(Player1Controls) : bit JOY_BIT_FIRE,a : jr z,.waitForFire
.waitForRelease: call ReadInputDevices : ld a,(Player1Controls) : bit JOY_BIT_FIRE,a : jr nz,.waitForRelease
ENDIF

; do the GameLoop infinitely
jr GameLoop

Expand Down Expand Up @@ -238,6 +243,8 @@ SnowballvsPlayerCollision:
; the collision detection will use circle formula (x*x+y*y=r*r), but we will first reject
; the fine-calculation by box-check, player must be +-15px (cetre vs centre) near ball
; to do the fine centres distance test (16*16=256 => overflow in the simplified MUL logic)
bit 7,(ix+S_SPRITE_4B_ATTR.vpat)
jr z,.skipCollisionCheck ; ball is invisible, skip the test
; read and normalize snowball pos X
ld e,(ix+S_SPRITE_4B_ATTR.x)
ld d,(ix+S_SPRITE_4B_ATTR.mrx8)
Expand Down Expand Up @@ -315,6 +322,60 @@ SnowballvsPlayerCollision:
ld (ix+S_SPRITE_4B_ATTR.mrx8),a
ret

;-------------------------------------------------------------------------------------
; Part 7 - platforms collisions
; These don't check the image pixels, but instead there are few columns accross
; the screen, and for each column there can be 8 platforms defined. These data are
; hand-tuned for the background image. Each column is 16px wide, so there are 16 columns
; per PAPER area. But the background is actually only 192x192 (12 columns), and I will
; define +1 column extra on each side in case some sprite is partially out of screen.
; Single column data is 16 bytes: 8x[height of platform, extras] where extras will be
; 8bit flags for things like ladders/etc.

GetPlatformPosUnder:
; In: IX = sprite pointer (centre x-coordinate is used for collision, i.e. +8)
; Out: A = platform coordinate (in sprite coordinates 0..255), C = extras byte
; for X coordinate outside of range, or very low Y the [A:255, C:0] is always returned
push hl
call .implementation
; returns through here only when outside of range or no platform found
ld a,255
ld c,0
pop hl
ret
.implementation:
bit 0,(ix+S_SPRITE_4B_ATTR.mrx8)
ret nz ; 256..511 is invalid X range (no column data)
ld a,(ix+S_SPRITE_4B_ATTR.x)
sub 16-8 ; -16 to skip 16px, +8 to test centre of sprite (not left edge)
ret c ; 0..7 is invalid X range (no column data)
; each column is 8 platforms x2 bytes = 16 bytes -> the X coordinate top 4 bits
; are indentical to address of particular column! (no need to multiply/divide)
cp low PlatformsCollisionDataEnd
ret nc ; 224 <= (X-16) -> invalid X range (224 = 14*16) - 14 columns are defined
and -16 ; clear the bottom four bits of X -> becomes low-byte of address
ld l,a
ld h,high PlatformsCollisionData ; HL = address of column data
ld a,(ix+S_SPRITE_4B_ATTR.y) ; raw sprite Y (top edge)
cp 255-13
ret nc ; already too low to even check (after +13 only 13..254 are valid for check)
add a,13 ; the base-line coordinate, the sprite can be 2px deep into platform to "hit" it
; now we are ready to compare against the data in column table
jr .columnDataLoopEntry
.columnDataLoop:
inc l
inc l
.columnDataLoopEntry:
cp (hl)
jr nc,.columnDataLoop ; platformY <= spriteY_base_line -> will not catch this one
; this platform is below baseline, report it as hit
ld a,(hl)
inc l
ld c,(hl)
pop hl ; discard return address from .implementation
pop hl ; restore HL
ret ; return directly to caller with results in A and C

;-------------------------------------------------------------------------------------
; "AI" subroutines

Expand Down Expand Up @@ -407,15 +468,40 @@ SnowballsAI:
ld de,S_SPRITE_4B_ATTR
ld b,SNOWBALLS_CNT-1 ; move all of them except last
.loop:
bit 7,(ix+S_SPRITE_4B_ATTR.vpat)
jr z,.doNextSnowball ; if the sprite is not visible, don't process it
; check the Y coordinate against platform's collisions
call GetPlatformPosUnder
sub 16 ; snowball wants platform at +16 (bottom of ball is bottom of sprite)
cp (ix+S_SPRITE_4B_ATTR.y)
jr z,.isInOrAtPlatform ; at plaform
jr c,.isInOrAtPlatform ; in platform (will reset Y to be *at*)
ld l,a ; keep the platformY for compare
; falling, keep the previous direction (extracted from mirroX flag into C=0/1)
ld c,0
bit 3,(ix+S_SPRITE_4B_ATTR.mrx8) ; mirroX bit
jr z,.fallingRight
inc c ; falling left
.fallingRight
ld a,(ix+S_SPRITE_4B_ATTR.y)
inc a ; Y += 1
cp l ; did it land at platform now?
adc a,0 ; if not, do another Y += 1 (falling at +2 speed)
; continue with "in/at platform" code, but keeps direction, sets new fall Y
.isInOrAtPlatform:
ld (ix+S_SPRITE_4B_ATTR.y),a
; check if the ball is not under the screen, make it invisible then (+simpler AI)
rl (ix+S_SPRITE_4B_ATTR.vpat) ; throw away visibility flag
cp 192+32 ; Fc=1 if (Y < 192+32)
rr (ix+S_SPRITE_4B_ATTR.vpat) ; set new visibility from carry
; if at platform, choose direction by platform extras
rr c ; extras bit 0 to carry
sbc a,a ; 0 = right, $FF = left
; HL = current X coordinate (9 bit)
ld l,(ix+S_SPRITE_4B_ATTR.x)
ld h,(ix+S_SPRITE_4B_ATTR.mrx8)
; adjust it by some +- value deducted from B (32..1 index)
ld c,0 ; mirrorX flag = 0
ld a,b
and 3 ; A = 0,1,2,3
sli a ; A = 1, 3, 5, 7
sub 4 ; A = -3, -1, +1, +3
sli a ; Right: A=+1 Fc=0 || Left: A=-1 Fc=1
; do: HL += signed(A) (the "add hl,a" is "unsigned", so extra jump+adjust needed)
jr nc,.moveRight
dec h
Expand All @@ -434,11 +520,11 @@ SnowballsAI:
ld a,(TotalFrames)
xor b
and 7
jr nz,.notEightFrameYet
jr nz,.doNextSnowball
ld a,(ix+S_SPRITE_4B_ATTR.vpat)
xor 1
ld (ix+S_SPRITE_4B_ATTR.vpat),a
.notEightFrameYet:
.doNextSnowball:
add ix,de ; next snowball
djnz .loop ; do all of them
ret
Expand Down Expand Up @@ -547,6 +633,44 @@ SprSnowballs: EQU Sprites + 0*S_SPRITE_4B_ATTR ; first snowball sprite
SprPlayer: EQU Sprites + SNOWBALLS_CNT*S_SPRITE_4B_ATTR ; player sprite is here
SprCollisionFx: EQU SprPlayer + S_SPRITE_4B_ATTR

;-------------------------------------------------------------------------------------
; Part 7 - platforms collisions - data of platforms and their heights + extras

ALIGN 256 ; align for simpler calculation of address
PlatformsCollisionData: ; the Y-coord of platforms are in sprite coordinates! (+32: 0..255)
; extras:
; bit 0: 0=slope to right, 1=slope to left (to affect snowballs movement)
; bit 1: there's ladder near the platform
; column 0 (sprite coordinates 16..31)
DB 75, 0, 127, 0, 185, 0, 222, 1, 255, 0, 255, 0, 255, 0, 255, 0
; column 1 (sprite coordinates 32..47)
DB 75, 0, 128, 0, 186, 0, 222, 1, 255, 0, 255, 0, 255, 0, 255, 0
; column 2 (sprite coordinates 48..)
DB 75, 0, 108, 1, 129, 0, 167, 1, 187, 0, 222, 1, 255, 0, 255, 0
; column 3 (sprite coordinates 64..)
DB 75, 0, 107, 3, 130, 2, 166, 3, 188, 2, 222, 1, 255, 0, 255, 0
; column 4 (sprite coordinates 80..)
DB 51, 1, 75, 0, 106, 1, 131, 0, 165, 1, 189, 0, 222, 1, 255, 0
; column 5 (sprite coordinates 96..)
DB 51, 1, 75, 0, 105, 1, 132, 0, 164, 1, 190, 0, 222, 1, 255, 0
; column 6 (sprite coordinates 112..)
DB 51, 3, 75, 2, 104, 1, 133, 0, 163, 1, 191, 0, 221, 1, 255, 0
; column 7 (sprite coordinates 128..)
DB 51, 3, 75, 2, 103, 1, 134, 0, 162, 1, 192, 0, 220, 1, 255, 0
; column 8 (sprite coordinates 144..)
DB 76, 0, 102, 1, 135, 0, 161, 1, 193, 0, 219, 1, 255, 0, 255, 0
; column 9 (sprite coordinates 160..)
DB 77, 0, 101, 1, 136, 0, 160, 1, 194, 0, 218, 1, 255, 0, 255, 0
; column 10 (sprite coordinates 176..)
DB 78, 2, 100, 3, 137, 2, 159, 3, 195, 2, 217, 3, 255, 0, 255, 0
; column 11 (sprite coordinates 192..)
DB 79, 0, 99, 1, 138, 0, 158, 1, 196, 0, 216, 1, 255, 0, 255, 0
; column 12 (sprite coordinates 208..)
DB 98, 1, 157, 1, 215, 1, 255, 0, 255, 0, 255, 0, 255, 0, 255, 0
; column 13 (sprite coordinates 224..)
DB 97, 1, 156, 1, 214, 1, 255, 0, 255, 0, 255, 0, 255, 0, 255, 0
PlatformsCollisionDataEnd:

;-------------------------------------------------------------------------------------
; reserve area for stack at $B800..$BFFF region
ORG $B800
Expand Down

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