PRB: _getpixel() Return Value Unchanged After _setbkcolor()

MS-DOS                      | WINDOWS

The information in this article applies to:

• The C Run-time (CRT), included with:

      - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, and 6.0ax
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

SYMPTOMS

In an MS-DOS application, the _getpixel() function always returns the value zero (0) when the specified pixel is drawn in the background color. Using the _setbkcolor() function to change the color of the background does not affect the return value from _getpixel().

CAUSE

The _getpixel() function returns the color index for the specified pixel, not the value for the color mapped to a particular color index. The background color in graphics mode is always mapped to palette entry 0.

RESOLUTION

If the application changes the color index for a particular point on the screen, the _getpixel() function returns a different value. Specify the desired color index with the _setcolor() function and call one of the graphics drawing functions, such as _floodfill(), _setpixel(), and so on.

MORE INFORMATION

The following code example demonstrates this situation.

Sample Code

/* Compile options needed: none
 * Link options needed: GRAPHICS.LIB
 */

// Note: C 5.1 does not support _MAXRESMODE

#include <graph.h>
#include <stdio.h>

void main(void)

   short xvar = 100, yvar = 100, result1 = 0, result2 = 0;
   _setvideomode(_MAXRESMODE);
   result1 = _getpixel(xvar, yvar);
   _setbkcolor(_BLUE);
   result2 = _getpixel(xvar, yvar);
   _setvideomode(_DEFAULTMODE);
   printf("Before _setbkcolor - %d\n", result1);
   printf("After _setbkcolor  - %d\n", result2);