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  1. #1

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    Tutorial - Accurate Game Loop

    This tutorial will show how to make an accurate game loop
    using QueryPerformanceCounter API.


    The first thing we need to do is to declare the API functions:
    VB Code:
    1. Private Declare Function QueryPerformanceCounter Lib "kernel32" (lpPerformanceCount As Currency) As Long
    2. Private Declare Function QueryPerformanceFrequency Lib "kernel32" (lpFrequency As Currency) As Long
    This counter can differ from machine to machine, that's why we
    have to get the frequency of it using QueryPerformanceFrequency.


    Now we need a variable telling if the loop is on or off.
    We also need to know what to lock the FPS at.
    VB Code:
    1. Dim Running As Boolean
    2. Cost FPS As Double = 61.3
    I've set the FPS to 61.3 just so we can see the accuracy.


    Now head on to the loop.
    We will nead a few variables to keep track on time and interval in this loop:
    VB Code:
    1. Dim cTimer As Currency
    2. Dim cTimer2 As Currency
    3. Dim Freq As Currency
    4. Dim Interval As Double
    cTimer is used to get the time when running the game code.
    cTimer2 is used to get the time just before the game code.
    Freq is used to store the frequency value from the API.
    Interval will store the time to wait for next tick based on the FPS and frequency.

    The loop will start off by getting the frequency and calculating the interval.
    Since this will be constant during the looping, we only need to do this once.
    VB Code:
    1. QueryPerformanceFrequency Freq
    2. Interval = Freq / FPS
    Now the loop will start.
    It will start by checking if the Running boolean is set to True.
    Then it will check if it's time to enter the game code part.
    Last it will handle windows messages (DoEvents), or else the program will lock.
    VB Code:
    1. While Running
    2.     QueryPerformanceCounter cTimer2 'get current time
    3.     If cTimer2 >= cTimer + Interval Then 'compare current time with previous time and see if enough time has passed
    4.         Me.Caption = "FPS: " & Format(Round(Freq / (cTimer2 - cTimer), 1), "0.0") 'display fps on screen
    5.         QueryPerformanceCounter cTimer 'get current time
    6.        
    7.         'Game code goes here.
    8.     End If
    9.     DoEvents
    10. Wend
    That's it!
    To start the loop set Running to True and call the loop sub.
    To end it, set Running to False.


    Here's a finished example:

    VB Code:
    1. Option Explicit
    2.  
    3. Private Declare Function QueryPerformanceCounter Lib "kernel32" (lpPerformanceCount As Currency) As Long
    4. Private Declare Function QueryPerformanceFrequency Lib "kernel32" (lpFrequency As Currency) As Long
    5.  
    6. Dim Running As Boolean                                                                  'To know if the game is running
    7. Const FPS As Double = 61.3                                                              'Frames Per Second
    8.  
    9. Private Sub Form_Load()
    10.     Me.Show                                                                             'Display main window
    11.     Running = True
    12.     GameLoop                                                                            'Start the loop
    13. End Sub
    14.  
    15. Private Sub GameLoop()
    16.     Dim cTimer As Currency                                                              'Time of last tick
    17.     Dim cTimer2 As Currency                                                             'Current time
    18.     Dim Freq As Currency                                                                'Frequency of Counter
    19.     Dim Interval As Double                                                              'Time of each frame
    20.    
    21.     QueryPerformanceFrequency Freq                                                      'Get Frequency
    22.     Interval = Freq / FPS                                                               'Calculate interval based on FPS and Frequency
    23.    
    24.     While Running                                                                       'Start loop
    25.         QueryPerformanceCounter cTimer2                                                 'Get current time
    26.         If cTimer2 >= cTimer + Interval Then                                            'Check if it's time to run code
    27.             Me.Caption = "FPS: " & Format(Round(Freq / (cTimer2 - cTimer), 1), "0.0")   'Display FPS on screen
    28.             QueryPerformanceCounter cTimer                                              'Get time for later use
    29.            
    30.             'Enter code here
    31.         End If
    32.         DoEvents
    33.     Wend                                                                                'End loop
    34. End Sub
    35.  
    36. Private Sub Form_Unload(Cancel As Integer)
    37.     Running = False                                                                     'Exit loop
    38. End Sub
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    My tutorials: Anti-Alias Pixels, Accurate Game Loop, Resource File

  2. #2
    New Member
    Join Date
    Jul 2010
    Posts
    8

    Re: Tutorial - Accurate Game Loop

    Wow this is old. But anyway...

    I get an error when I try to run the code - "Must have start up form or Sub Main()" Followed by another "Out of Memory".

    Any help would be appreciated, I'm new to VB and programming.

    EDIT: Nevermind I got it. >.>
    Last edited by diocassius; Jul 29th, 2010 at 06:36 PM.

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