BUG: SetPixel() Doesn't Work Properly on a PS Printer

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SYMPTOMS

In a Windows 3.1 printing application, calling the SetPixel() command on a device context created for the Windows PostScript printer driver version 3.56 or earlier causes the pixel that is drawn to be larger than expected if a line with a thickness greater than 1 was drawn previously.

CAUSE

The PostScript driver's implementation of SetPixel() draws a circle with the thickness set to the thickness of the last line drawn with the LineTo() command if the thickness of the pen used is greater than 1.

STATUS

Microsoft has confirmed this to be a problem with the Windows PostScript printer driver shipped with Windows 3.1. This problem has been corrected in the PostScript driver for Windows NT.

RESOLUTION

Instead of using the SetPixel() command, use the PatBlt() command to draw the pixel. This is needed only if a pen of width greater than 1 was used to draw lines with the Moveto() and LineTo() commands.

For example, if you were calling SetPixel() using the following line of code to draw a black pixel at coordinates (10,10)

   SetPixel(hPrnDC, 10, 10, RGB(0,0,0));

   PatBlt(hPrnDC, 10, 10, 1, 1, BLACKNESS);

MORE INFORMATION

Once a pen of thickness greater than 1 is created and selected into the printer's device context and a LineTo() command is called, every call to SetPixel() will result in a circle the width of the pen. If a pen of width 0 (zero) or 1 is created and used by the LineTo() command, the lines drawn will be the correct thickness, but the SetPixel() command will still draw with the thickness of the most recently used pen with a width greater than 1.

For example, the following code will draw the first two pixels correctly. After the pen of width 50 is created and selected into the printer's device context, the first call to SetPixel works correctly because LineTo() has not been called yet. The call to SetPixel after the line is drawn results in a circle the thickness of the line. After that, a pen of thickness 1 is created and selected into the device context. The line drawn afterwards is drawn correctly, but both calls to SetPixel() result in thick circles.

Sample Code

      if (StartDoc(hPrnDC,&di) > 0)
      {
         HPEN hPen, hPen2, hOldPen;

         StartPage(hPrnDC);
         hPen = CreatePen(PS_SOLID,1,RGB(0,0,0));
         hOldPen = SelectObject(hPrnDC,hPen);
         SetPixel(hPrnDC,100,10,RGB(0,0,0));
         MoveTo(hPrnDC,220,10);
         LineTo(hPrnDC,700,10);
         SetPixel(hPrnDC,800,10,RGB(0,0,0));

         hPen2 = CreatePen(PS_SOLID,50,RGB(0,0,0));
         SelectObject(hPrnDC,hPen2);
         SetPixel(hPrnDC,100,110,RGB(0,0,0));
         MoveTo(hPrnDC,220,110);
         LineTo(hPrnDC,700,110);
         SetPixel(hPrnDC,800,110,RGB(0,0,0));

         SelectObject(hPrnDC,hPen);
         SetPixel(hPrnDC,100,210,RGB(0,0,0));
         MoveTo(hPrnDC,220,210);
         LineTo(hPrnDC,700,210);
         SetPixel(hPrnDC,800,210,RGB(0,0,0));

         DeleteObject(hPen2);
         DeleteObject(SelectObject(hPrnDC,hOldPen));
         EndPage(hPrnDC);
         EndDoc(hPrnDC);
      }