PRB: Picture.Circle Works Incorrectly When the Radius is Very Large and the Angle is Very Small

ID: Q215336

The information in this article applies to:

Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0

SYMPTOMS

The Circle method of the PictureBox works incorrectly while drawing an arc with a very large radius and a very small angle. On NT you may get a distorted arc. On Windows 95 you may get nothing.

CAUSE

In Visual Basic, the Circle method of the PictureBox calls the Arc API. The problem occurs because the Arc API works incorrectly when the radius is very large and the angle is very small.

RESOLUTION

To work around this problem, you need to implement your own Arc function in Visual Basic or C. Usually the C function provides better performance.

MORE INFORMATION

Steps to Reproduce Behavior

Create a new Standard EXE project (Project1). Form1 is created by default.

Add a PictureBox to Form1 and change its Name to picTest.

Add the following code to the code module of Form1:


Option Explicit

Private Const CirConst As Double = 3.14159265358979
Private Const PISTEP As Double = CirConst / 180#

Private Sub Form_Load()
   picTest.AutoRedraw = True
   picTest.BackColor = vbBlack
   picTest.ForeColor = vbWhite
   Me.WindowState = 2
End Sub

Private Sub Form_Resize()
   Dim retval As Long
   Dim x As Integer, y As Integer
   Dim r As Long
   Dim a1 As Double, a2 As Double
        
   picTest.Move 0, 0
   picTest.Height = Me.ScaleHeight * 0.98
   picTest.Width = picTest.Height
    
   Const ZZ As Double = 0.15
   picTest.Scale (-ZZ, ZZ + ZZ)-(ZZ, 0)
   picTest.Cls
   picTest.Circle (-10, 0), 10, , 0 * PISTEP , 1 * PISTEP 
End Sub

Run the application in NT and you see a distorted arc. Run it in Windows 95 and you see nothing.

Steps to Implement Your Own Arc Function in C

Start Microsoft Visual C++ and create a new Win32 Dynamic-Link Library project.

Type "MyGDI" in the Project name text box and click OK.

In the next dialog box, choose "a simple dll project" and click the Finish button.

Click OK in the next dialog box.

In MyGDI.cpp file, add the following code after the line #include "stdafx.h":


#include "math.h"

extern "C" {
BOOL _stdcall MyArc(HDC hdc, int X, int Y, DWORD dwRadius, 
               float fStartDegrees, float fSweepDegrees);
}

BOOL _stdcall MyArc(HDC hdc, int X, int Y, DWORD dwRadius, 
               float fStartDegrees, float fSweepDegrees)
{
   float fCurrentAngle;         // Current angle in radians
   float fStepAngle = 0.003f;   // The sweep increment value in radians
   float fStartRadians;         // Start angle in radians
   float fEndRadians;           // End angle in radians
   int ix, iy;                  // Current point on arc
   float fTwoPi = 2.0f * 3.1415926535f;
   int iMM;
   POINT pt, pt1;
   SIZE szVP, szW;
   POINT szOrg, szVPOrg;
    
   // save the old mapping mode
   GetWindowExtEx(hdc, &szW);
   GetWindowOrgEx(hdc, &szOrg);
   GetViewportExtEx(hdc, &szVP);
   GetViewportOrgEx(hdc, &szVPOrg);

   pt.x = X;
   pt.y = Y;
   
   LPtoDP(hdc, &pt, 1);
   X = pt.x;
   Y = pt.y;
    
   // Calculate the new radius
   pt.x = dwRadius;
   pt.y = 0;
   LPtoDP(hdc, &pt, 1);

   pt1.x = 0;
   pt1.y = 0;
   LPtoDP(hdc, &pt1, 1);

   pt.x -= pt1.x;
   pt.y -= pt1.y;
   dwRadius = (DWORD)sqrt(pt.x * pt.x + pt.y * pt.y);

   //set the mapping mode to be something that can be used 
   //to draw arc directly
   iMM = SetMapMode(hdc, MM_ANISOTROPIC);
   SetViewportExtEx(hdc, 1, 1, NULL);
   SetViewportOrgEx(hdc, 0, 0, NULL);
   SetWindowExtEx(hdc, 1, 1, NULL);
   SetWindowOrgEx(hdc, 0, 0, NULL);
    
   // Get the starting and ending angle in radians 
   if (fSweepDegrees > 0.0f) {
      fStartRadians = ((fStartDegrees / 360.0f) * fTwoPi); 
      fEndRadians = (((fStartDegrees + fSweepDegrees) / 360.0f) * fTwoPi);
    }   else {
      fStartRadians = (((fStartDegrees + fSweepDegrees)  / 360.0f) * fTwoPi); 
      fEndRadians =  ((fStartDegrees / 360.0f) * fTwoPi);
    }
    
    // Calculate the starting point for the sweep via 
    // polar -> cartesian conversion 
    ix = X + (int)((float)dwRadius * (float)cos(fStartRadians));
    iy = Y - (int)((float)dwRadius * (float)sin(fStartRadians));
    
    // Draw a line to the starting point 
    MoveToEx(hdc, ix, iy, NULL); 
    
    // Calculate and draw the sweep 
    for (fCurrentAngle = fStartRadians; 
    fCurrentAngle <= fEndRadians; 
    fCurrentAngle += fStepAngle) { 
        
       // Calculate the current point in the sweep via 
       // polar -> cartesian conversion 
       ix = X + (int)((float)dwRadius * (float)cos(fCurrentAngle));
       iy = Y - (int)((float)dwRadius * (float)sin(fCurrentAngle));
        
       // Draw a line segment to current point 
       LineTo(hdc, ix, iy); 
   }              
    
   //restore the mapping mode
   SetMapMode(hdc, iMM);
   SetViewportExtEx(hdc, szVP.cx, szVP.cy, NULL);
   SetViewportOrgEx(hdc, szVPOrg.x, szVPOrg.y, NULL);
   SetWindowExtEx(hdc, szW.cx, szW.cy, NULL);
   SetWindowOrgEx(hdc, szOrg.x, szOrg.y, NULL);
    
   return TRUE;
}

Under the File menu, select New. Choose Text File from the list on the Files tab. Type "MyGDI.def" (without the quotes) in the File Name text box. Click OK.

Add the following code to the MyGDI.def file.
```
LIBRARY MyGDI

EXPORTS
	MyArc	@1 
```

Build the project and move the MyGDI.dll to the windows system directory or the same directory as the Visual Basic project.

Steps To Use Your Arc Function in Visual Basic

Create a new Standard EXE project. Form1 is created by default.

Add a picturebox to Form1. Use the default name, Picture1.

Add the following to the code window of Form1:


Option Explicit
Private Type POINTAPI
        x As Long
        y As Long
End Type

Private Type size
        CX As Long
        CY As Long
End Type

Private Type rect
        Left As Long
        Top As Long
        Right As Long
        Bottom As Long
End Type

Private Declare Function GetStockObject Lib "gdi32" _
  (ByVal nIndex As Long) As Long
Private Declare Function SelectObject Lib "gdi32" _
  (ByVal hdc As Long, ByVal hObject As Long) As Long
Private Declare Function SetViewportExtEx Lib "gdi32" _
  (ByVal hdc As Long, ByVal nX As Long, ByVal nY As Long, _
  lpSize As size) As Long
Private Declare Function SetViewportOrgEx Lib "gdi32" _
  (ByVal hdc As Long, ByVal nX As Long, ByVal nY As Long, _
  lpPoint As POINTAPI) As Long
Private Declare Function SetMapMode Lib "gdi32" _
  (ByVal hdc As Long, ByVal nMapMode As Long) As Long
Private Declare Function SetWindowExtEx Lib "gdi32" _
  (ByVal hdc As Long, ByVal nX As Long, ByVal nY As Long, _
  lpSize As size) As Long
Private Declare Function SetWindowOrgEx Lib "gdi32" _
  (ByVal hdc As Long, ByVal nX As Long, ByVal nY As Long, _
  lpPoint As POINTAPI) As Long
Private Declare Function GetClientRect Lib "user32" _
  (ByVal hwnd As Long, lpRect As rect) As Long
Private Declare Function MyArc Lib "MyGDI" _
  (ByVal hdc As Long, ByVal CX As Long, _
  ByVal CY As Long, ByVal Radius As Long, _
  ByVal sDegree As Single, _
  ByVal SweepDegree As Single) As Long
     
Private Const LOGPIXELSX = 88        '  Logical pixels/inch in X
Private Const LOGPIXELSY = 90        '  Logical pixels/inch in Y
Private Const WHITE_PEN = 6
Private Const BLACK_PEN = 7
Private Const MM_ANISOTROPIC = 8

Private Const CirConst As Double = 3.14159265358979
Private Const PI As Double = CirConst / 180#

Private Sub Form_Load()
   Me.WindowState = 2
End Sub

Private Sub Form_Paint()
draw Picture1, False
End Sub

Private Sub draw(picTest As Object, bPrint As Boolean)
   Dim retval As Long
   Dim x As Integer, y As Integer
   Dim R As Long
   Dim a1 As Double, a2 As Double
   Dim usepen&, oldpen&
   Dim size As size
   Dim point As POINTAPI
   Dim rect As rect
        
   picTest.Move 0, 0
   picTest.Height = Me.ScaleHeight * 0.98
   picTest.Width = picTest.Height
   
   picTest.Cls
    
   usepen = GetStockObject(BLACK_PEN)
   oldpen = SelectObject(picTest.hdc, usepen)
        
   SetMapMode picTest.hdc, MM_ANISOTROPIC
        
   GetClientRect picTest.hwnd, rect
   SetViewportExtEx picTest.hdc, _
       rect.Right - rect.Left, _
       rect.Bottom - rect.Top, size
   SetWindowExtEx picTest.hdc, 30, -30, size
   SetWindowOrgEx picTest.hdc, -15, 30, point
    
   MyArc picTest.hdc, -1000, 0, 1000, 0, 1
End Sub

Run the application and you can see the correct Arc.

Additional query words:


Keywords          : kbAPI kbGDI kbGrpUser kbVBp kbVBp500 kbVBp600 kbVC500 kbVC600 kbGrpVB 
Version           : WINDOWS:5.0,6.0
Platform          : WINDOWS 
Issue type        : kbprb

Last Reviewed: March 13, 1999