Graphics Mode Refresh and Anti-Flickering
The Anti-Flikering coding methods for Graphics Mode Refresh (after clearing screen or viewport).
Preamble:
Refreshing (redrawing without precaution) a window in graphics mode can result in annoying flickering (this is due to the display of unwanted intermediate images picked up during the user refreshing phase).
The phenomenon is obviously intensified if the entire window is first erased before being completely updated, and this repeatedly.
This page presents the basic FreeBASIC coding techniques to fight against this phenomenon.
Main principles for anti-flickering
If one takes into account the only graphical user task (regardless of CPU resources needed for the OS), there are two main methods to avoid flickering:
- In first priority, use of a block '[Screenlock...Screenunlock]' to encapsulate the graphical instructions for refreshing.
But the documentation enlightens a warning to its use:
`It is strongly recommended that the lock on a page be held for as short a time as possible. Only screen drawing should occur while the screen is locked, input/output and waiting must be avoided. In Win32 and Linux the screen is locked by stopping the thread that processes also the OS' events. If the screen is kept locked for a long time the event queue could overflow and make the system unstable.`
- In a second priority, if the lock time is too long, use the principle of double video paging.
The instruction 'Screensync' is a reminiscence of the old QuickBASIC where there was only this type of instruction ('wait &h3DA, 8') to improve flickering.
It is empirical because it only allows to synchronize the drawing relating to the fixed dead time between two frames.
To be used occasionally with very little time drawing.
There is usually no interest to mix together these 2 (or 3) methods.
Then, in the display loop (if exists), user must provide enough CPU resources to OS (smoothed with 'Sleep' instruction at the end of the loop). Otherwise, it will take itself out of user control, resulting in a jerky display.
Note: Using 'Screensync' provides CPU resources to OS (dead time between end of graphic drawing and end of frame tracing), but by a non controllable way (because linked to the frame period).
Anti-flickering methods learning, through small example
A small program to enlighten (and compare the different efficiencies) the 4 following different methods from 2 to 5 (after the no method 1) to animate a drawing/printing to a graphic screen by a loop with clearing screen:
Method 1: Draw/Print loop to screen with raw coding (no method)
Algorithm:

    '    SCREEN 19, , 2 'to enable double-paging
    '    SCREENSET 0, 0 'to cancel double-paging
    '
    '  ┌─► CLS          'to clear the page
    '  │   Drawing      'to draw on the page
    '  │   Printing     'to print on the page
    '  └── Temporizing  'to avoid hogging the CPU

Method 2: Draw/Print loop to screen with synchronizing
Algorithm:

    '    SCREEN 19, , 2 'to enable double-paging
    '    SCREENSET 0, 0 'to cancel double-paging
    '
    '  ┌─► SCREENSYNC   'to synchronize between two frames
    '  │   CLS          'to clear the page
    '  │   Drawing      'to draw on the page
    '  │   Printing     'to print on the page
    '  └── Temporizing  'to avoid hogging the CPU

Method 3: Draw/Print loop to screen with locking
Algorithm:

    '    SCREEN 19, , 2 'to enable double-paging
    '    SCREENSET 0, 0 'to cancel double-paging
    '
    '  ┌─► SCREENLOCK   'to lock the page's frame buffer
    '  │   CLS          'to clear the page
    '  │   Drawing      'to draw on the page
    '  │   Printing     'to print on the page
    '  │   SCREENUNLOCK 'to unlock the page's frame buffer
    '  └── Temporizing  'to avoid hogging the CPU

Method 4: Draw/Print loop to screen with double buffering
Algorithm:

    '    SCREEN 19, , 2 'to enable double-paging
    '    SCREENSET 1, 0 'to activate double-paging
    '
    '  ┌─► CLS          'to clear the work page
    '  │   Drawing      'to draw on the work page
    '  │   Printing     'to print on the work page
    '  │   SCREENCOPY   'to copy the work page into the visible page
    '  └── Temporizing  'to avoid hogging the CPU

Note:
Double buffering and page flipping (below) are functionally equivalent (but not under the hood) if the work page is entirely refreshed at each iteration as here.
Method 5: Draw/Print loop to screen with page flipping
Algorithm:

    '    SCREEN 19, , 2     'to enable double-paging
    '    SCREENSET 1, 0     'to activate double-paging
    '    p0=0 : p1=1        'to initialize flipping
    '
    '  ┌─► CLS              'to clear the work page
    '  │   Drawing          'to draw on the work page
    '  │   Printing         'to print on the work page
    '  │   SCREENSET p0, p1 'to set the work page to the p0 value, and the visible page to the p1 value
    '  │   SWAP p0, p1      'to exchange the values of p0 and p1
    '  └── Temporizing      'to avoid hogging the CPU

Note:
Page flipping and double buffering (above) are functionally equivalent (but not under the hood) if the work page is entirely refreshed at each iteration as here.
Complete program listing
Code:
`
Declare Sub Draw_circle_recursion (ByVal x As Integer, ByVal y As Integer, ByVal r As Integer, ByVal rmin As Integer)
Dim I As Integer = 0
Dim Inc As Integer
Dim Key As String
Dim Code As Integer = 1
Dim Tempo As Integer = 3
Dim T As Single = Tempo
Dim p0 As Integer = 0
Dim p1 As Integer = 1
Screen 19, , 2
Do
If Code = 4 Or Code = 5 Then
ScreenSet 1, 0
Else
ScreenSet 0, 0
End If
Do
Select Case Code
Case 2
ScreenSync
Case 3
ScreenLock
End Select
Cls
Draw_circle_recursion(10 + I, 300, 9 + I * I / 29 / 29, 10)
Locate 1, 1
Select Case Code
Case 1
Print "1. Draw/Print loop to screen with raw coding:"
Print
Print " SCREEN 19, , 2 'to enable double-paging"
Print " SCREENSET 0, 0 'to cancel double-paging"
Print
Print " " & Chr(218) & Chr(196) & Chr(16) & " CLS"
Print " " & Chr(179) & " " & " " & " Drawing"
Print " " & Chr(179) & " " & " " & " Printing"
Print " " & Chr(192) & Chr(196) & Chr(196) & " Temporizing"; T; " ms";
Case 2
Print "2. Draw/Print loop to screen with synchronizing:"
Print
Print " SCREEN 19, , 2 'to enable double-paging"
Print " SCREENSET 0, 0 'to cancel double-paging"
Print
Print " " & Chr(218) & Chr(196) & Chr(16) & " SCREENSYNC"
Print " " & Chr(179) & " " & " " & " CLS"
Print " " & Chr(179) & " " & " " & " Drawing"
Print " " & Chr(179) & " " & " " & " Printing"
Print " " & Chr(192) & Chr(196) & Chr(196) & " Temporizing"; T; " ms";
Case 3
Print "3. Draw/Print loop to screen with locking:"
Print
Print " SCREEN 19, , 2 'to enable double-paging"
Print " SCREENSET 0, 0 'to cancel double-paging"
Print
Print " " & Chr(218) & Chr(196) & Chr(16) & " SCREENLOCK"
Print " " & Chr(179) & " " & " " & " CLS"
Print " " & Chr(179) & " " & " " & " Drawing"
Print " " & Chr(179) & " " & " " & " Printing"
Print " " & Chr(179) & " " & " " & " SCREENUNLOCK"
Print " " & Chr(192) & Chr(196) & Chr(196) & " Temporizing"; T; " ms";
Case 4
Print "4. Draw/Print loop to screen with double buffering:"
Print
Print " SCREEN 19, , 2 'to enable double-paging"
Print " SCREENSET 1, 0 'to activate double-paging"
Print
Print " " & Chr(218) & Chr(196) & Chr(16) & " CLS"
Print " " & Chr(179) & " " & " " & " Drawing"
Print " " & Chr(179) & " " & " " & " Printing"
Print " " & Chr(179) & " " & " " & " SCREENCOPY"
Print " " & Chr(192) & Chr(196) & Chr(196) & " Temporizing"; T; " ms";
Case 5
Print "5. Draw/Print loop to screen with page flipping:"
Print
Print " SCREEN 19, , 2 'to enable double-paging"
Print " SCREENSET 1, 0 'to activate double-paging"
Print " p0=0 : p1=1 'to initialize flipping"
Print
Print " " & Chr(218) & Chr(196) & Chr(16) & " CLS"
Print " " & Chr(179) & " " & " " & " Drawing"
Print " " & Chr(179) & " " & " " & " Printing"
Print " " & Chr(179) & " " & " " & " SCREENSET p0, p1"
Print " " & Chr(179) & " " & " " & " SWAP p0, p1"
Print " " & Chr(192) & Chr(196) & Chr(196) & " Temporizing"; T; " ms";
End Select
Locate 30, 1
Print "<1>: Draw/Print with raw coding"
Print "<2>: Draw/Print with synchronizing"
Print "<3>: Draw/Print with locking"
Print "<4>: Draw/Print with double buffering"
Print "<5>: Draw/Print with page flipping"
Print "<+/->: Tempo setting (+/-)"
Print
Print "<Escape> or click [X]: Quit";
Select Case Code
Case 3
ScreenUnlock
Case 4
ScreenCopy
Case 5
ScreenSet p0, p1
Swap p0, p1
End Select
If I = 0 Then
Inc = +1
ElseIf I = 480 Then
Inc = -1
End If
I = I + Inc
Key = Inkey
If Key = "+" And Tempo < 10 Then
Tempo = Tempo + 1
ElseIf Key = "-" And Tempo > 0 Then
Tempo = Tempo - 1
End If
If Tempo > 0 Then
T = Tempo
Else
T = 0.5
End If
Static As Integer K
K += 1
If K >= 25 / T Then
Sleep 25
K = 0
End If
Loop While Key <> "1" And Key <> "2" And Key <> "3" And Key <> "4" And Key <> "5" And Key <> Chr(27) And Key <> Chr(255) & "k"
Code = Val(Key)
Loop Until Key = Chr(27) Or Key = Chr(255) & "k"
Sub Draw_circle_recursion ( ByVal x As Integer, ByVal y As Integer, ByVal r As Integer, ByVal rmin As Integer )
Circle (x, y), r, r Shr 1
If r > rmin Then
Draw_circle_recursion(x + r Shr 1, y, r Shr 1, rmin)
Draw_circle_recursion(x - r Shr 1, y, r Shr 1, rmin)
Draw_circle_recursion(x, y + r Shr 1, r Shr 1, rmin)
Draw_circle_recursion(x, y - r Shr 1, r Shr 1, rmin)
Draw_circle_recursion(x + r Shr 1, y + r Shr 1, r Shr 2, rmin)
Draw_circle_recursion(x - r Shr 1, y + r Shr 1, r Shr 2, rmin)
Draw_circle_recursion(x + r Shr 1, y - r Shr 1, r Shr 2, rmin)
Draw_circle_recursion(x - r Shr 1, y - r Shr 1, r Shr 2, rmin)
End If
End Sub
`
Note on temporizing in the loop (for compatibility with any PC):
The true temporizing value applied is always 25 ms ('Sleep 25'), but only for one loop on N (with 'N = 25 / tempo', and 'tempo' the wanted value in ms between 0.5 and 10).
See also

- `ScreenSync`
- `ScreenLock`, `ScreenUnlock`
- `ScreenSet`, `ScreenCopy`
- Lite regulation function to be integrated into user loop for FPS control
- Fine-grain procedure for waiting and in-loop procedure for fine-regulating FPS