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' A pixel-perfect collision demo using bitmasks in QB64

' This program requires two PNG files: "player.png" and "target.png"

' The transparent background of the PNGs is crucial for this to work.



' === Setup constants and types ===

CONST SCREEN_WIDTH = 800

CONST SCREEN_HEIGHT = 600

CONST BORDER_COLOR = _RGB32(255, 255, 255)

CONST COLLISION_COLOR = _RGB32(255, 0, 0)

CONST NO_COLLISION_COLOR = _RGB32(0, 255, 0)

CONST PLAYER_SPEED = 2



TYPE Sprite

    image AS LONG

    mask AS _BIT

    width AS INTEGER

    height AS INTEGER

    x AS INTEGER

    y AS INTEGER

END TYPE



' === Function to check for pixel collision ===

FUNCTION IsPixelCollision (a AS Sprite, b AS Sprite) AS _BIT

    ' Step 1: Bounding box check (broad phase)

    IF a.x + a.width <= b.x OR a.x >= b.x + b.width THEN IsPixelCollision = 0: EXIT FUNCTION

    IF a.y + a.height <= b.y OR a.y >= b.y + b.height THEN IsPixelCollision = 0: EXIT FUNCTION



    ' Step 2: Pixel-perfect check (narrow phase)

    ' Determine the overlapping rectangle

    startx = _MAX(a.x, b.x)

    starty = _MAX(a.y, b.y)

    endx = _MIN(a.x + a.width, b.x + b.width)

    endy = _MIN(a.y + a.height, b.y + b.height)



    ' Iterate through the overlapping area

    FOR y = starty TO endy

        FOR x = startx TO endx

            ' Get the corresponding pixel coordinates in each sprite's mask

            mask_a_x = x - a.x

            mask_a_y = y - a.y

            mask_b_x = x - b.x

            mask_b_y = y - b.y



            ' Check if both masks have a solid pixel at this position

            IF _BITGET(a.mask, mask_a_x, mask_a_y) AND _BITGET(b.mask, mask_b_x, mask_b_y) THEN

                IsPixelCollision = -1

                EXIT FUNCTION

            END IF

        NEXT x

    NEXT y



    IsPixelCollision = 0

END FUNCTION



' === Function to generate a bitmask from an image handle ===

FUNCTION CreateBitmask (image_handle AS LONG) AS _BIT

    mask = _BITNEW(_WIDTH(image_handle), _HEIGHT(image_handle))

    _DEST mask



    _SOURCE image_handle

    FOR y = 0 TO _HEIGHT(image_handle) - 1

        FOR x = 0 TO _WIDTH(image_handle) - 1

            ' Check if pixel is not transparent (assuming _CLEARCOLOR is used for transparency)

            IF _POINT(x, y) <> _CLEARCOLOR THEN

                _BITSET mask, x, y, -1 ' Set the bit

            END IF

        NEXT x

    NEXT y

    _SOURCE 0

    _DEST 0

    CreateBitmask = mask

END FUNCTION



' === Main program starts here ===

DIM player AS Sprite

DIM target AS Sprite



SCREEN _NEWIMAGE(SCREEN_WIDTH, SCREEN_HEIGHT, 32)

_TITLE "QB64 Bitmask Collision Demo"

_CLEARCOLOR _RGB32(0, 0, 0) ' The transparent color for the PNGs

_FULLSCREEN _KEEP ' Prevents the screen from being resized



' Load images and create sprites

player.image = _LOADIMAGE("player.png")

IF player.image = -1 THEN PRINT "Error loading player.png": SYSTEM

player.width = _WIDTH(player.image)

player.height = _HEIGHT(player.image)

player.mask = CreateBitmask(player.image)

player.x = 100

player.y = 100



target.image = _LOADIMAGE("target.png")

IF target.image = -1 THEN PRINT "Error loading target.png": SYSTEM

target.width = _WIDTH(target.image)

target.height = _HEIGHT(target.image)

target.mask = CreateBitmask(target.image)

target.x = 400

target.y = 250



' Main game loop

DO

    ' Handle input for player movement

    IF _KEYHIT(57416) THEN player.y = player.y - PLAYER_SPEED ' Up arrow

    IF _KEYHIT(57424) THEN player.y = player.y + PLAYER_SPEED ' Down arrow

    IF _KEYHIT(57419) THEN player.x = player.x - PLAYER_SPEED ' Left arrow

    IF _KEYHIT(57421) THEN player.x = player.x + PLAYER_SPEED ' Right arrow



    ' Check for collision and update border color

    IF IsPixelCollision(player, target) THEN

        border_color = COLLISION_COLOR

    ELSE

        border_color = NO_COLLISION_COLOR

    END IF



    ' Double buffering for smooth animation

    _LIMIT 60

    _NEWIMAGE SCREEN_WIDTH, SCREEN_HEIGHT, 32 ' Create a new blank buffer

    _DISPLAY ' Swap buffers



    ' Draw the sprites

    _PUTIMAGE (player.x, player.y), player.image

    _PUTIMAGE (target.x, target.y), target.image



    ' Draw a bounding box and collision status

    LINE (player.x, player.y)-(player.x + player.width, player.y + player.height), border_color, B

    LINE (target.x, target.y)-(target.x + target.width, target.y + target.height), NO_COLLISION_COLOR, B



    _DISPLAY ' Show the new frame

LOOP UNTIL _KEYHIT(1) ' Loop until the escape key is pressed



SYSTEM

Dim As _Unsigned Long BSC, BJC, SJC, BSJC, CP 'bob+ sam collision, bob + joe collision, ect

$Console



Randomize Timer



Display = _NewImage(800, 600, 32)

Screen Display

_Console On

Bob = _NewImage(64, 64, 32)

Sam = _NewImage(64, 64, 32)

Joe = _NewImage(64, 64, 32)

Cls , _RGBA(255, 0, 0, 128), Bob

Cls , _RGBA(0, 255, 0, 128), Sam

Cls , _RGBA(0, 0, 255, 128), Joe



CollisionScreen = _NewImage(64, 64, 32)

_Source CollisionScreen 'to read the blended colors on that screen

_Dest CollisionScreen

_PutImage , Bob 'put bob on the screen

_PutImage , Sam 'put sam on the screen

BSC = Point(0, 0) 'we now know the blended color for a bob + sam collision

_PutImage , Joe 'put joe on the screen

BSJC = Point(0, 0) 'we now know the blended color for a bob + sam + joe collision

Cls , 0 'clear the screen we use for blending

_PutImage , Bob 'bob back on the screen

_PutImage , Joe 'joe on the screen

BJC = Point(0, 0) 'and now we know the blended color for bob + joe collison

Cls , 0 'clear the screen one last time

_PutImage , Sam 'put Sam on the screen

_PutImage , Joe 'put Joe on the screen

SJC = Point(0, 0) 'and now we know the blended color for a sam + joe collision

_Source Display 'restore the source to original screen

_Dest Display

_FreeImage CollisionScreen 'free that screen as we're done getting our blended colors



Dim As Long x(2), y(2)

For i = 0 To 2

    x(i) = Rnd * 700: y(i) = Rnd * 500 'random staring point of a character on the screen

Next

For i = 0 To 2

    Do Until xmove(i) <> 0: xmove(i) = Rnd * 10 - 5: Loop

    Do Until ymove(i) <> 0: ymove(i) = Rnd * 10 - 5: Loop

Next



Do

    Cls , 0 'clear the screen

    For i = 0 To 2 'generate movement

        If x(i) + xmove(i) >= 800 - 64 Then xmove(i) = -xmove(i)

        If x(i) + xmove(i) <= 0 Then xmove(i) = -xmove(i)

        If y(i) + ymove(i) >= 600 - 64 Then ymove(i) = -ymove(i)

        If y(i) + ymove(i) <= 0 Then ymove(i) = -ymove(i)

        x(i) = x(i) + xmove(i)

        y(i) = y(i) + ymove(i)

    Next

    _PutImage (x(0), y(0)), Bob

    _PutImage (x(1), y(1)), Sam

    _PutImage (x(2), y(2)), Joe



    'now do a quick collision detection check

    BSCE = 0: BSJCE = 0: SJCE = 0: BJCE = 0

    For x = 0 To 799: For y = 0 To 679 'detect collisions

            Select Case Point(x, y)

                Case BSC: BSCE = -1

                Case BSJC: BSJCE = -1

                Case SJC: SJCE = -1

                Case BJC: BJCE = -1

            End Select

    Next y, x

    If BSCE Then _Echo "Bob and Sam Collided!" 'report collisions

    If BSJCE Then _Echo "All three collided!"

    If SJCE Then _Echo "Sam and Joe Collided!"

    If BJCE Then _Echo "Bob and Joe Collided!"



    _Limit 120

    _Display

Loop Until _KeyHit

System

Quote:' Steve's code appears to search whole screen pixel by pixel plus some??? 679?

For x = 0 To 799: For y = 0 To 679 'detect collisions