Sonic game programming, how does it work?

[ultimar235]

A while ago I made a Sonic game in very basic C++ using OpenGL. It lacked curves, etc and featured a new combat system for Sonic which turned out to be pretty awesome. Sadly I cannot get this code to work anymore as I wrote it 6 years ago in VS 97.

I was thinking of starting again but I cannot figure out how they did the following:
- made Sonic run up curves and down
- made it so it was harder to run up hills at lower speed
- loop the loops

I'm wondering if anyone can help to explain generally how each of these things was achieved. I'm not asking for code or anything, I would just like to know the techniques. Even if you don't really know the exact techniques they used, a guess as to how they did it is still good!

[Arcilux]

Utilizing physics. Basically determine movement location by using velocity. Very weak and unthought out pseudo-code:

locX = 0
--GameLoop--
if(isGoingUpHill)
velocity = 5
else if(isGoingDownHill)
velocity = 15
else if(isFalling)
velocity = 12
else //on regular ground
velocity = 10
locX += velocity
--End GameLoop--

For looping and going up down, you would just add in some form of bounding boxes to control what is solid and modify the code to have a locY and velocityY

Here is the basic movement code I used for creating a clone of the original arcade Donkey Kong. It takes into consideration the location of the ladders to determine if Plumberman is climbing or jumping, and checks to see if the ladder he is climbing is broken or complete. This code is ~3 years old, and I was a novice when I wrote it, but it might give some ideas. Note this is VB.NET, not C++

Public Class Character : Inherits ControlSprite

    Private _upKey As Keys
    Private _downKey As Keys
    Private _leftKey As Keys
    Private _rightKey As Keys
    Private _isUpKeyPressed As Boolean
    Private _isDownKeyPressed As Boolean
    Private _isLeftKeyPressed As Boolean
    Private _isRightKeyPressed As Boolean

    Private ladders(18) As Ladder
    Private platforms(140) As Platform
    Private row = 7

    Private _isJumping As Boolean
    Private _initY As Single = 0

    Public MaxPositionX As Single
    Public MinPositionX As Single
    Public MaxPositionY As Single
    Public MinPositionY As Single

    Public Const Width As Integer = 10
    Public Const Height As Integer = 10
    Private Shared Color As Color = Color.DarkSlateBlue
    Private Const _speed As Double = 40

    Public Sub New(ByVal control As Control, ByVal maxPosX As Single, ByVal maxPosY As Single)
        MyBase.New(control)
        control.BackgroundImage = My.Resources.Mario
        MaxPositionX = maxPosX
        MaxPositionY = maxPosY
        MinPositionX = 0
        MinPositionY = 0

        Location.X = 10
        Location.Y = maxPosY

        control.BackColor = Color
        control.Width = Width
        control.Height = Height

        setHeightWidth()

        _upKey = Keys.Up
        _downKey = Keys.Down
        _leftKey = Keys.Left
        _rightKey = Keys.Right

    End Sub

    Public Overrides Sub Update(ByVal gameTime As Double, ByVal elapsedTime As Double)
        processMove()

        MyBase.Update(gameTime, elapsedTime)

        If Location.Y < MinPositionY Then
            Location.Y = MinPositionY
        End If
        If Location.Y > MaxPositionY - Height Then
            Location.Y = MaxPositionY - Height
        End If
        If Location.X < MinPositionX Then
            Location.X = MinPositionX
        End If
        If Location.X > MaxPositionX - Width Then
            Location.X = MaxPositionX - Width
        End If
    End Sub

    Public Sub setLadders(ByRef ladders() As Ladder)
        Me.ladders = ladders
    End Sub

    Public Function hasWon() As Boolean
        If Location.Y < 30 And Location.X < MaxPositionX / 2 Then
            Return True
        End If
        Return False
    End Function

    Private Sub processMove()
        Dim workingVelocityY As Double = 0.0
        Dim workingVelocityX As Double = 0.0

        If Not collision() And Not _isJumping And Not Location.Y < onPlatform() Then
            workingVelocityY += _speed / 2
        End If

        If _isJumping Then
            'jumpDown()
            If Location.Y >= _initY Then
                _isJumping = False
            Else
                workingVelocityY += _speed / 4
            End If
        End If

        If _isUpKeyPressed Then
            If collision() Then
                workingVelocityY -= _speed
            ElseIf Not _isJumping Then
                jumpUp()
            End If
        End If
        If _isDownKeyPressed Then
            If aboveLadder() Then
                workingVelocityY += _speed
            End If
        End If
        If _isLeftKeyPressed Then
            workingVelocityX += -_speed
        End If
        If _isRightKeyPressed Then
            workingVelocityX += _speed
        End If
        Velocity.Y = CSng(workingVelocityY)
        Velocity.X = CSng(workingVelocityX)
    End Sub

    Private Sub jumpUp()
        _initY = CSng(Location.Y)
        Location.Y = Location.Y - 8
        _isJumping = True
    End Sub

    Private Sub jumpDown()
        If Location.Y >= _initY Then
            _isJumping = False
        Else
            Velocity.Y = CSng(_speed)
        End If
    End Sub

    Private Function collision() As Boolean
        For i = 0 To 17
            If Not (Location.X > ladders(i).Location.X + ladders(i).Size.Width OrElse Location.X + Size.Width < ladders(i).Location.X OrElse Location.Y > ladders(i).Location.Y + ladders(i).Size.Height OrElse Location.Y + Size.Height < ladders(i).Location.Y) Then
                ladders(i).Draw()
                If Not (ladders(i).isBroken) Then
                    _isJumping = False
                    Return True
                End If
            End If
        Next
        Return False
    End Function

    Private Function aboveLadder() As Boolean
        For i = 0 To 17
            If Not (Location.X > ladders(i).Location.X + ladders(i).Size.Width OrElse Location.X + Size.Width < ladders(i).Location.X) Then
                If Not (Location.Y + Height + 2 > ladders(i).Location.Y + ladders(i).Size.Height OrElse Location.Y + Height + 2 < ladders(i).Location.Y) Then
                    ladders(i).Draw()
                    Return True
                End If
            End If
        Next
        Return False
    End Function

    Public Sub KeyDown(ByVal keys As Keys)

        If keys = _upKey Then
            _isUpKeyPressed = True
        End If
        If keys = _downKey Then
            _isDownKeyPressed = True
        End If
        If keys = _leftKey Then
            _isLeftKeyPressed = True
        End If
        If keys = _rightKey Then
            _isRightKeyPressed = True
        End If
    End Sub

    Public Sub KeyUp(ByVal keys As Keys)

        If keys = _upKey Then
            _isUpKeyPressed = False
        End If
        If keys = _downKey Then
            _isDownKeyPressed = False
        End If
        If keys = _leftKey Then
            _isLeftKeyPressed = False
        End If
        If keys = _rightKey Then
            _isRightKeyPressed = False
        End If
    End Sub
End Class

[ultimar235]

Thanks for the insight and code was interesting.

I've never really thought about trying to implement physics. I guess most of the equations are fairly simple especially for a 2D world. Think i'll give it a shot.