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Q67759: _getpixel() Return Value is Not Changed by _setbkcolor() Call

Article: Q67759
Version(s): 5.10 6.00 6.00a
Operating System: MS-DOS
Flags: ENDUSER | s_quickc
Last Modified: 6-FEB-1991

In the sample program below, _getpixel() will return a 0 (zero).
Setting _setbkcolor() to any other color has no effect on the return
value from _getpixel().

The _getpixel() function is documented as returning a color index for
a specific pixel. In the example program, there are no colors on the
screen except the background color (color index 0). When the call to
_setbkcolor() is made, the color mapped to index 0 is changed;
however, the color index for the pixel is not changed. Therefore,
_getpixel() still returns 0. This same behavior is exhibited when
using the _remappallette() function on index 0 and the
_remapallpallette() function.

To change the color index at a particular point on the screen, first
call the _setcolor() function with the desired color index, then use
one of the graphics drawing functions, for example, _floodfill(),
_setpixel(), etc.

Sample Code
-----------

// Note: C 5.10 doesn't 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);
}