FIX: Overflow Error When CurrentX Or CurrentY Greater Than 32K

The information in this article applies to:

- Microsoft Visual Basic programming system for Windows,

  versions 2.0 and 3.0

SYMPTOMS

An Overflow error results if you attempt to set CurrentX or CurrentY to a value greater than 32,767 while the current ScaleMode is set to Twips. When using another ScaleMode such as pixels, the same problem occurs if the conversion of the CurrentX or CurrentY value to twips is greater than 32,767.

However, when you use the Print method (or other graphics method) you can correctly cause the value of CurrentX or CurrentY to exceed 32,767 when the ScaleMode is set to twips.

CAUSE

When CurrentX or CurrentY is set explicitly, Visual Basic incorrectly converts the value using the current scale mode to twips. If the result of the conversion to twips is greater the 32,767, an Overflow error occurs. For example, if the ScaleMode is set to Pixels, CurrentX and CurrentY cannot exceed approximately 2731 pixels if the twips per pixel ratio is 12 because 12 times 2731 is 32,772 which is greater than 32767.

When setting CurrentX or CurrentY, Visual Basic should convert the value using the current ScaleMode to pixels rather than twips before comparing the result to 32,767. As a result of this bug, CurrentX and CurrentY are each restricted to a limit 12-14 times smaller (depending on TwipsPerPixelX or TwipsPerPixelY) than they should be.

WORKAROUND

To work around the problem, call the Windows API functions:

• Call TextOut to control the position of text in a picture box or a form.
• Call MoveTo and LineTo to control the position of a line.
• Call other appropriate Windows API functions to position the output for other graphics methods such as the circle method.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem has been fixed in Visual Basic version 4.0.

MORE INFORMATION

Because the ratio of twips per pixel varies from one device (or screen resolution) to another, you will need to calculate the limit for the device you are using. To calculate the exact pixel limit of CurrentX, divide 32768 by Screen.TwipsPerPixelX. To calculate the limit of CurrentY, divide 32768 by Screen.TwipsPerPixelY. To find the limit of CurrentX and CurrentY for your printer, use the Printer object in place of the Screen object in the calculations above.

Example for Using API Calls as Workaround

The following example shows how to use the three API calls TextOut, MoveTo, and LineTo to work around the problem. Note that when you call Windows API functions to print or draw, all X and Y coordinates are measured in pixels regardless of the current ScaleMode setting.

1. Start Visual Basic or from the File menu, choose New Project (ALT, F, N) if Visual Basic is already running. Form1 is created by default.

2. Add the following declarations to the General section of Form1:

      ' Enter the following Declare statement on one, single line:
      Declare Function TextOut Lib "GDI" (ByVal hDC As Integer,
         ByVal X As Integer, ByVal Y As Integer, ByVal lpString As String,
         ByVal nCount As Integer) As Integer
   
      ' Enter the following Declare statement on one, single line:
      Declare Function MoveTo Lib "GDI" (ByVal hDC As Integer,
         ByVal X As Integer, ByVal Y As Integer) As Long
   
      ' Enter the following Declare statement on one, single line:
      Declare Function LineTo Lib "GDI" (ByVal hDC As Integer,
         ByVal X As Integer, ByVal Y As Integer) As Integer
   
   

3. Add the following code to the Form_Click event:

      Sub Form_Click ()
         X1% = 100
         Y1% = 100
         X2 %= 200
         Y2 %= 200
   
         retvaL& = TextOut(FORM1.hDC, 100, 100, "ONE LINE", 8)
   
         retvaL& = MoveTo(FORM1.hDC, X1%, Y1%)
   
         retvaL& = LineTo(FORM1.hDC, X2%, Y2%)
   
      End Sub
   
   

4. From the Run menu, choose start (ALT, R, S), or press the F5 key to run the program.

5. Click the form, and you will see the words "ONE LINE" on the form and a diagonal line from the upper left to the lower right. The line starts at the X1 and Y1 coordinates given in the MoveTo API call and ends at the X2 and Y2 coordinates given in the LineTo API call. The words "ONE LINE" should appear 100 pixels from the top and 100 pixels from the left. Note that TextOut may be used without MoveTo because TextOut gives its own coordinates. However using LineTo without using MoveTo results in a line stating from the current output position.