Custom

Parameter to the Put graphics statement which selects a custom method

Syntax
   Put [ target, ] [ STEP ] ( x,y ), source [ ,( x1,y1 )-( x2,y2 ) ], 
   Custom, custom_function_ptr [, parameter]

Parameters
   Custom
      Required.
   custom_function_ptr
      name of the custom user defined function.
   parameter
      optional Pointer to be passed to the custom function; if omitted, the 
      default value is zero (0).

Description
   Custom selects a custom user defined function as the method for blitting 
   an image buffer.

   The Custom method uses a user-defined function to calculate the final 
   pixel values to be drawn to the destination buffer. This function will 
   be called once for every pixel of the source image, and will receive the 
   source and destination pixel values, and a data pointer passed by the Put
   function. The pixel value returned will be the value used to draw to the 
   destination buffer. The function has the form:

   Declare Function identifier ( _
      ByVal source_pixel As ULong, _
      ByVal destination_pixel As ULong, _
      ByVal parameter As Any Ptr _
      ) As ULong

   identifier is the name of the function.
   source_pixel is the current pixel value of the source image.
   destination_pixel is the current pixel value of the destination image.
   parameter is the parameter that is passed by the Put command.  If it was 
   omitted, its value will be zero.

Example
   Function dither ( ByVal source_pixel As ULong, ByVal destination_pixel As ULong, ByVal parameter As Any Ptr ) As ULong
      
      ''either returns the source pixel or the destination pixel, depending on the outcome of rnd
      
      Dim threshold As Single = 0.5
      If parameter <> 0 Then threshold = *CPtr(Single Ptr, parameter)
      
      If Rnd() < threshold Then
         Return source_pixel
      Else
         Return destination_pixel
      End If
      
   End Function

   Dim img As Any Ptr, threshold As Single

   '' set up a screen
   ScreenRes 320, 200, 16, 2
   ScreenSet 0, 1

   '' create an image
   img = ImageCreate(32, 32)
   Line img, ( 0,  0)-(15,  15), RGB(255,   0,   0), bf
   Line img, (16,  0)-(31,  15), RGB(  0,   0, 255), bf
   Line img, ( 0, 16)-(15,  31), RGB(  0, 255,   0), bf
   Line img, (16, 16)-(31,  31), RGB(255,   0, 255), bf

   '' dither the image with varying thresholds
   Do Until Len(Inkey)
      
      Cls
      
      threshold = 0.2
      Put ( 80 - 16, 100 - 16), img, Custom, @dither, @threshold
      
      '' default threshold = 0.5
      Put (160 - 16, 100 - 16), img, Custom, @dither
      
      threshold = 0.8
      Put (240 - 16, 100 - 16), img, Custom, @dither, @threshold
      
      ScreenCopy
      Sleep 25
      
   Loop

   '' free the image memory
   ImageDestroy img

Dialect Differences
   * Not available in the -lang qb dialect.

Differences from QB
   * New to FreeBASIC

See also
   * Put (Graphics)

