ScreenPtr
Returns a pointer to the current work page's frame buffer
Syntax
`
Declare Function ScreenPtr ( ) As Any Ptr
`
Usage
`
result = ScreenPtr
`
Return Value
a pointer to the current work page frame buffer memory, or NULL (`0`) if no graphics mode is set.
Description
`ScreenPtr` provides a way to directly read/write the working page's frame buffer. `ScreenLock` should be used before any read or writes are attempted. The pointer returned is valid up until any subsequent call to `Screen` or `ScreenRes`, which invalidates it.
`ScreenPtr` can also be used to test if a call to `Screen` or `ScreenRes` was successful, indicated by a non-NULL (`<> 0`) return value.
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 `ScreenInfo`.
Each row in the frame buffer is `pitch` bytes long. The frame buffer consists of `height` rows, stored in order of their position on the screen, running from top to bottom, left to right.
Because of the design of FreeBASIC graphics library, `ScreenPtr` (if non-NULL) will always point to the backbuffer, and never to actual video RAM.
Example
`
Const SCREEN_WIDTH = 640, SCREEN_HEIGHT = 480
Dim As Integer 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 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
`
`
Const SCREEN_WIDTH = 256, SCREEN_HEIGHT = 256
Dim As Integer 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 UInteger Ptr
Dim As UInteger 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
'' (UInteger 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
`
Dialect Differences

- Not available in the -lang qb dialect unless referenced with the alias `__Screenptr`.

Differences from QB

- New to FreeBASIC

See also

- `Screen (Graphics)`
- `ScreenRes`
- `ScreenInfo`
- `ScreenLock`
- `ScreenUnlock`