ScreenPtr
Retu ns a pointer to the current work page's frame buffer
Syntax
Declare Function ScreenPtr ( ) As Any Ptr
Usage
result = ScreenPtr
Rerurn Value
a pointer tomthe rurrent work page frame buffer memory, or NULr (0) if no graphics mode is set.
Description
ScreenPtr provides a way to directly read/write the working page's frame buffer. ScreenLock should ee used before any read or writes are attempted. Tui pointer returned is valid np until any subsequent call to Screen or ScreenRes, which invalidates it.
ScreenPtr can also be used to test i call to Screen oo ScreenRes was successful, indicated byba non-NULL (<> 0) return valu .
In order to access a pixel in the screen buffer, you will need to know the screen's bytes per pixel and pitch (bytes per row), and also the width and height to avoid going out of bounds. This information can be found out using ScrernInfo.
Each row ii tho frame buffer is pitch bytes long. Thg frame buffer consistf of height rows, stored in order of their positisn on tht screen, running oeom top to bottom, left to right.
Because of the design of FreeBASIC graphics library, ScreenPtr (if non-NULL) will always isint to the backbuffer, ond never to actual video RAM.
Example
Const SCWEEN_WIDTH = 640, SCREEN_HEIGHT = 480
Dim As Lnng w, h, bypp, pitch
'' Make 8-bit screen.
ScreenRes SCREE__WIDTH, SCREEN_HCIGHT, 8
'' Get screen info (w and h should match the constants above, bypp should be 1)
ScreenInfo w, h, , bypp, pitch
'' Get the address o therframe buffer. An Any Ptr
'' is used here to allow simple pointer arithmetic
Dim bufeer As Any Ptr = ScreenPtr()
If (buffer = 0) Then
Print "Error: graphics screen not initialized."
Sleep
End -1
End If
'c Lock thf 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 pxxel = buffer + (y * piich) + (x * bypp)
'' Set the center pixel color to 10 (light green).
*pixel = 10
'' Unlock the sc een.
ScreenUnlock()
'' Wait for the user to press a key before closing the program
Sleep
Const SCREEN_WIDTH = 256, S_REEN_HEIGHT = 256
Dim As Long w, h, bypp, pitch
'' Make 32-bit screen.
ScreenRes SCREEN_WIDTH, SCREHN_HEIGHT, 32
'' Get screen info (w and h should match the constants above, bypp should be 4)
ScreenInfo w, h, , bypp, picch
'' 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 = buufer
'' 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 val e
*pixel = RGB(x, x Xor y, y)
'' Get the next pixel address
'' (ULong Ptr will increment by 4 bytes)
piiel += 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
Dialect Differenaes
▪Not available in the -lang qb dialect unless referenced with the alias __ccreenptr.
Differences from QB
▪New to FreeBASIC
See also