图形模式刷新与防闪烁

• 来源: https://www.freebasic.net/wiki/wikka.php?wakka=ProPgAntiFlickering
• 最后更新: 2024-02-18

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图形模式刷新（清屏或视口后）的防闪烁编码方法。

前言：

在图形模式下刷新（不加预防措施地重绘）窗口可能会导致烦人的闪烁（这是由于在用户刷新阶段显示了不需要的中间图像所致）。

如果在每次完全更新之前先清除整个窗口，并且反复执行，这种现象显然会加剧。

本页介绍对抗这种现象的基本 FreeBASIC 编码技术。

防闪烁的主要原则

如果只考虑图形用户任务（不考虑操作系统所需的 CPU 资源），有两种主要方法可以避免闪烁：

• 首选方法，使用 '[Screenlock...Screenunlock]' 块来封装用于刷新的图形指令。

但文档中对其使用有如下警告：

强烈建议页面上的锁定保持时间尽可能短。在屏幕锁定期间只应进行屏幕绘图，必须避免输入/输出和等待操作。在 Win32 和 Linux 中，锁定屏幕会通过停止同时处理操作系统事件的线程来实现。如果屏幕长时间保持锁定，事件队列可能溢出，使系统不稳定。

• 其次，如果锁定时间过长，使用双视频分页原则。

'Screensync' 指令是旧 QuickBASIC 的遗留物，在那里只有这种类型的指令（'wait &h3DA, 8'）来改善闪烁。

它是经验性的，因为它只允许同步与两帧之间固定死时间相关的绘图。

适用于绘图时间非常短的场合偶尔使用。

通常不建议将这 2（或 3）种方法混合使用。

然后，在显示循环中（如果存在），用户必须为操作系统提供足够的 CPU 资源（通过在循环末尾使用 'Sleep' 指令平滑控制）。否则，它将失去用户控制，导致显示卡顿。

注意：使用 'Screensync' 为操作系统提供 CPU 资源（图形绘制结束和帧跟踪结束之间的死时间），但以不可控制的方式（因为与帧周期相关联）。

通过小示例学习防闪烁方法

一个小程序，用于说明（并比较不同效率）以下 4 种不同方法（方法 2 到 5，在"无方法"的方法 1 之后），通过带清屏循环对图形屏幕进行绘图/打印动画：

方法 1：使用原始编码（无方法）进行绘图/打印循环到屏幕

算法：

'    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

方法 2：使用同步进行绘图/打印循环到屏幕

算法：

'    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

方法 3：使用锁定进行绘图/打印循环到屏幕

算法：

'    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

方法 4：使用双缓冲进行绘图/打印循环到屏幕

算法：

'    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

注意：

如果工作页面在每次迭代时都完全刷新（如此处），双缓冲和翻页（见下文）在功能上是等效的（但内部实现不同）。

方法 5：使用翻页进行绘图/打印循环到屏幕

算法：

'    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

注意：

如果工作页面在每次迭代时都完全刷新（如此处），翻页和双缓冲（见上文）在功能上是等效的（但内部实现不同）。

完整程序清单

代码：

start GeSHi

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

end GeSHi

关于循环中定时的说明（为了与任何 PC 兼容）：

实际应用的定时值始终为 25 ms（'Sleep 25'），但仅在 N 次循环中执行一次（其中 'N = 25 / tempo'，'tempo' 是以 ms 为单位的所需值，范围在 0.5 到 10 之间）。

参见

• Screensync
• Screenlock, Screenunlock
• Screenset, Screencopy
• 集成到用户循环中用于 FPS 控制的简单调节函数
• 用于等待和循环内精细调节 FPS 的细粒度过程

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