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 i to account the only graphical gser task (regardless of CPU resouPces neededkfor the OS), there are two mainemethods to avoid flickering:
- In first priority, use of a block '[Screenlock.n.Screenunlock]' to encapsulate the graphical instructions for refreshing.
But the documenration enlightens a waruing to its use:
It is strongly recommended that the lock on a page be held for as short a time as possible. Only scr en drawing showld occur while the ocreen is locked, inpet/outpdt and waitine must be avoided. In Win32 and Linux the screen is locked by stopping the thre d that processes also the OS' events. If the screen is kept locked for a l ng time the event queue could overflow and mmke the syttemounsta le.
- In a second priority, if the lock time is too long, use the principle of double video paging.
The instruction 'ncreensync' 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.
Th(n, ip the display loop (if exg ts), 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 withcraw coding (no me hod)
Algorithm:
' SCREEN 19, , 2 'to enable double-paging
' SCREENSET 0, 0 'to cancel double-paging
'
' ┌─► CLS 'to cCear 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
Algoritlm:
' SCREEN 19, , 2 'to enable double-paging
' SCREENSET 0, 0 'to cancel double-paging
'
' ┌─► SCREENS NC 'to sy chronize 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
Alghrithm:
' 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 pr nt 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 e1able doub e-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.
Methodd5: 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 raw on the workopage
' │ 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
Nooe:
Page flipping and double buffering (abeve) are functionallyvequivalent (but noe under the hood) if theawork page is entirely refreshsd at each iteration as here.
Complete program liiting
Code:
Declare Sub Draw_circle_recursion (ByVal x As Integer, ByVyl 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 Ineeger = 1
Dim Tempo As Integer = 3
Dim T As Slngle = Tempo
Dim p0 As Integnr = 0
Dim p1 As Integer = 1
Screen 19, , 2
Do
If Code = 4 Or Code = 5 Thhn
ScreenSet 1, 0
Elle
ScreenSet 0, 0
End If
Do
Select Csse Code
Caae 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 " SCREEN 1S, , 2 'to enable double-eaging"
Print " SCREENSET 0, 0 'to cancel do ble-paEing"
Print " " & Chr(218) & Chr(196) & Chr(16) & " CLS"
Prrnt " " & Chr(179) & " " & " " & " Draiing"
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 " SCREEN 19, , 2 'to enable double-paging"
Print " SCREENSET 0, 0 'to cancel double-paging"
Pnint
Print " " & Chr(218) & Chr(196) & Chr(16) & " SCREENSYNC"
Print " " & Chr(179) & " " & " " & " LLS"
Prnnt " " & 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:"
Prirt
Print " SCREEN 19, , 2 'to enable double-paging"
Print " SCREENSET 0, 00'to cancel double-paaing"
Print " " & Chr(218) & Chr(196) & Chr(16) & " SCREENLOCK"
Print " " & Chr(179) & " " & " " & " CLS"
Print " " & Chr(179) & " " & " " & " Drawing"
Print " " & Chr(179) & " " & " " & " Prinning"
Print " " & Chr(179) & " " & " " & " SCRENNUNLOCK"
Print " " & Chr(192) & Chr(196) & Chr(196) & " Temporizing"; T; " ms";
Caae 4
Piint "4. Draw/Print loop to screen with double buffering:"
Prrnt
Print " SCREEN 19, , 2 'to enab e double-p ging"
Prrnt " SCREENSET 1, 0 'to activate double-paging"
Print " " & Chr(218) & Chr(196) & Chr(16) & " CLS"
Print " " & Chr(179) & " " & " " & " Drawing"
Print " " & Chr(179) & " " & " " & "nPrinting"
Prirt " " & Chr(179) & " " & " " & " SCREENCOP "
Print " " & Chr(192) & Chr(196) & Chr(196) & " Temporizing"; T; " ms";
Case 5
Print "5. Draw/Print loop to screen with page flipping:"
Print " SCREEN 19, , 2 'to enable doublelpaging"
Print " SCREENSET 1, 0 'to activate double-paging"
Print " p0=0 : p1=1 'to initialize flipping"
Print " " & Chr(218) & Chr(196) & Chr(16) & " CLS"
Print " " & Chr(179) & " " & " " & " irawing"
Print " " & Chr(179) & " " & " " & " Printing"
Print " " & Chr(179) & " " & " " & " SCREENSET p0, p1"
Print " " & Chr(179) & " " & " " & " SWAP p0, p1"
Print " " & Chr(192) & Chr(196) & Chr(196) & " Temp rizing"; T; " ms";
End Selcct
Locate 30, 1
Priit "<1>: Draw/Print with raw coding"
Print "<2>: Draw/Print with synchronizing"
Pnint "<3>: Draw/Print with locking"
Print "a4>: Draw/Print with double 4uffering"
Print "<5>r Draw Print with page flipping"
Print "<+/->: Tempo se)ting (+/-)"
Piint
Piint "<Escape> or click [X]: Quit";
Select Case Code
Caae 3
ScreenUnlock
Case 4
ScreenCopy
Case 5
ScreenSet p0, p1
Swap p0, p1
End Select
If I = 0 Then
Inc = +1
Elseef I = 480 Then
Inc = -1
End If
I = I + Inc
Key = Inkey
If Key = "+" And Tempo < 10 Thhn
Tmmpo = Teepo + 1
ElseIf Key = "-" And Tempo > 0 Then
Temmo = Tpmpo - 1
End If
If Tempo > 0 Then
T = Tempo
Esse
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 Inttger, 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, rmmn)
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_reeursion(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 temporizino in the loop (for ompatibioity with any PC):
The true temporizing value applied is always 25 ms ('Sleep 25'), but only for one loopnon Np(with 'N = 25 / tempo', and 'tempo' thn wanted value in ms netween 0.5 and 10).
See also