SCREENPTR

• 来源: https://www.freebasic.net/wiki/wikka.php?wakka=KeyPgScreenptr
• 最后更新: 2023-07-09

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返回当前工作页面帧缓冲区的指针

语法

declare function Screenptr ( ) as any ptr

用法

result = Screenptr

返回值

指向当前工作页面帧缓冲区内存的指针，如果未设置图形模式则返回 NULL（0）。

说明

Screenptr 提供了一种直接读/写工作页面帧缓冲区的方法。在尝试任何读写之前应使用 Screenlock。返回的指针有效期到下一次对 Screen (Graphics) 或 Screenres 的调用为止，那时它将失效。

Screenptr 也可用于测试对 Screen (Graphics) 或 Screenres 的调用是否成功，非 NULL（<> 0）的返回值表示成功。

要访问屏幕缓冲区中的像素，需要知道屏幕的每像素字节数和行间距（每行字节数），以及宽度和高度以避免越界。这些信息可以使用 Screeninfo 获取。

帧缓冲区中的每一行长 pitch 字节。帧缓冲区由 height 行组成，按照它们在屏幕上的位置顺序存储，从上到下、从左到右。

由于 FreeBASIC 图形库的设计，Screenptr（如果非 NULL）将始终指向后备缓冲区，而不是实际的视频 RAM。

示例

start GeSHi

Const SCREEN_WIDTH = 640, SCREEN_HEIGHT = 480
Dim As Long w, h, bypp, pitch

'' Make 8-bit screen.
ScreenRes SCREEN_WIDTH, SCREEN_HEIGHT, 8

'' Get screen info (w and h should match the constants above, bypp should be 1)
ScreenInfo w, h, , bypp, pitch

'' Get the address of the frame buffer. An Any Ptr
'' is used here to allow simple pointer arithmetic
Dim buffer As Any Ptr = ScreenPtr()
If (buffer = 0) Then
    Print "Error: graphics screen not initialized."
    Sleep
    End -1
End If

'' Lock the screen to allow direct frame buffer access
ScreenLock()
   
    '' Find the address of the pixel in the centre of the screen
    '' It's an 8-bit pixel, so use a UByte Ptr.
    Dim As Integer x = w \ 2, y = h \ 2
    Dim As UByte Ptr pixel = buffer + (y * pitch) + (x * bypp)
   
   
    '' Set the center pixel color to 10 (light green).
    *pixel = 10

'' Unlock the screen.
ScreenUnlock()

'' Wait for the user to press a key before closing the program
Sleep

end GeSHi

start GeSHi

Const SCREEN_WIDTH = 256, SCREEN_HEIGHT = 256
Dim As Long w, h, bypp, pitch

'' Make 32-bit screen.
ScreenRes SCREEN_WIDTH, SCREEN_HEIGHT, 32

'' Get screen info (w and h should match the constants above, bypp should be 4)
ScreenInfo w, h, , bypp, pitch

'' Get the address of the frame buffer. An Any Ptr
'' is used here to allow simple pointer arithmetic
Dim buffer As Any Ptr = ScreenPtr()
If (buffer = 0) Then
    Print "Error: graphics screen not initialized."
    Sleep
    End -1
End If

'' Lock the screen to allow direct frame buffer access
ScreenLock()
   
    '' Set row address to the start of the buffer
    Dim As Any Ptr row = buffer
   
    '' Iterate over all the pixels in the screen:
   
    For y As Integer = 0 To h - 1
       
        '' Set pixel address to the start of the row
        '' It's a 32-bit pixel, so use a ULong Ptr
        Dim As ULong Ptr pixel = row
       
        For x As Integer = 0 To w - 1
           
            '' Set the pixel value
            *pixel = RGB(x, x Xor y, y)
           
            '' Get the next pixel address
            '' (ULong Ptr will increment by 4 bytes)
            pixel += 1
           
        Next x
       
        '' Go to the next row
        row += pitch
       
    Next y

'' Unlock the screen.
ScreenUnlock()

'' Wait for the user to press a key before closing the program
Sleep

end GeSHi

方言差异

• 在 -lang qb 方言中不可用，除非使用别名 __Screenptr 引用。

与 QB 的区别

• FreeBASIC 新增

另请参阅

• Screen (Graphics)
• Screenres
• ScreenInfo
• Screenlock
• Screenunlock

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