#Include "fbgfx.bi"
Using FB

ScreenRes 640, 480, 32
Const PI = 3.14159

Type Vec3D
    As Single x, y, z
End Type

' === Basic 3D rotation ===
Function RotateY(v As Vec3D, angle As Single) As Vec3D
    Dim As Vec3D o
    o.x = v.x * Cos(angle) + v.z * Sin(angle)
    o.z = -v.x * Sin(angle) + v.z * Cos(angle)
    o.y = v.y
    Return o
End Function

Function RotateX(v As Vec3D, angle As Single) As Vec3D
    Dim As Vec3D o
    o.y = v.y * Cos(angle) - v.z * Sin(angle)
    o.z = v.y * Sin(angle) + v.z * Cos(angle)
    o.x = v.x
    Return o
End Function

Function Project(v As Vec3D) As Vec3D
    Dim As Single scale = 400 / (v.z + 4)
    Dim As Vec3D o
    o.x = v.x * scale + 320
    o.y = v.y * scale + 240
    Return o
End Function

' Cube vertices
Dim Shared As Vec3D cube(0 To 7) = { _
    (-1, -1, -1), (1, -1, -1), (1, 1, -1), (-1, 1, -1), _
    (-1, -1, 1), (1, -1, 1), (1, 1, 1), (-1, 1, 1) }

Dim As Integer face(0 To 5, 0 To 3) = { _
    {0,1,2,3}, {4,5,6,7}, {0,1,5,4}, _
    {2,3,7,6}, {0,3,7,4}, {1,2,6,5} }

' Create striped background
For y As Integer = 0 To 479
    For x As Integer = 0 To 639
        If (y \ 10) Mod 2 = 0 Then
            PSet (x, y), RGB(50, 50, 50)
        Else
            PSet (x, y), RGB(120, 120, 120)
        End If
    Next
Next

' Store background
Dim As Any Ptr bg = ImageCreate(640, 480)
Get (0,0)-(639,479), bg

Dim As Single angle = 0

Do
    ScreenLock
    Put (0,0), bg, PSet

    ' Rotate + project cube
    Dim As Vec3D p(7)
    For i As Integer = 0 To 7
        Dim As Vec3D r = RotateX(cube(i), angle)
        r = RotateY(r, angle * 1.3)
        p(i) = Project(r)
    Next

    ' Draw cube faces with stealth effect
    For f As Integer = 0 To 5
        ' Backface culling
        Dim As Single ax = p(face(f,1)).x - p(face(f,0)).x
        Dim As Single ay = p(face(f,1)).y - p(face(f,0)).y
        Dim As Single bx = p(face(f,2)).x - p(face(f,1)).x
        Dim As Single by = p(face(f,2)).y - p(face(f,1)).y
        If (ax*by - ay*bx) < 0 Then Continue For

        ' Bounding box
        Dim As Integer minX = 640, minY = 480, maxX = 0, maxY = 0
        For i As Integer = 0 To 3
            Dim px as integer = CInt(p(face(f,i)).x)
            Dim py as integer = CInt(p(face(f,i)).y)
            If px < minX Then minX = px
            If px > maxX Then maxX = px
            If py < minY Then minY = py
            If py > maxY Then maxY = py
        Next

        ' Clip
        minX = IIf(minX < 0, 0, minX)
        minY = IIf(minY < 0, 0, minY)
        maxX = IIf(maxX > 639, 639, maxX)
        maxY = IIf(maxY > 479, 479, maxY)

        ' Draw distorted face
        For y As Integer = minY To maxY
            For x As Integer = minX To maxX
                ' Simple point-in-quad via winding number or barycentric test
                ' We cheat here for demo: just draw inside bbox with distortion
                Dim col as long = Point(x, y)
                Dim r as ubyte = (col Shr 16 And 255) + 30 : If r > 255 Then r = 255
                Dim g as ubyte = (col Shr 8 And 255) + 30 : If g > 255 Then g = 255
                Dim b as ubyte = (col And 255) + 30 : If b > 255 Then b = 255
                PSet (x, y), RGB(r, g, b)
            Next
        Next
    Next

    ScreenUnLock

    angle += 0.01
    Sleep 16
Loop Until InKey = Chr(27)
ImageDestroy bg