[伊蒙 dev c++] Graph 繪製圓弧線－面向陽光，陰影就會在身後

還記得在 Graph 繪製圖形這一篇裡，為了畫一個圓而第一次接觸到了畫圓弧的函式，
Arc(HDC hdc, int nLeftRect, int nTopRect, int RightRect, int nBottomRect,
int nXStartArc, int nYStartArc, int nXEndArc, nYEndArc)，
雖然只要把nXEndArc﹦nXStartArc，nYEndArc﹦nYStartArc就可以得到我想要的圓，但要怎樣使用nXEndArc、nXStartArc、nYEndArc、﹦nYStartArc來控制圓弧的位置，卻一直摸索不出來。

而又在繪製圖形中的實線、虛線與線寬這一篇裡再次地畫了一個圓，這次刻意的將nXEndArc、nXStartArc、nYEndArc、nYStartArc通通改為0來測試一下，
Arc(hdc,230,180,250,200,0,0,0,0)，畫圓沒問題，但還是沒能搞懂nXEndArc、nXStartArc、nYEndArc、nYStartArc，我一直認為圓弧也是二元二次方程式，起點與終點的X、Y座標的變化該如何精準的填上呢？

因此，假設整個圓弧是繞著圓心在走的，所以何不加一條輔助線，從圓心拉到起點和終點來看看會發生甚麼事情，就定義兩組座標取任意值

int nXStartArc = 150;
int nYStartArc = 380;

int nXEndArc = 120;
int nYEndArc = 260;

這兩組座標每次執行完後都再更改過數值，再編譯再執行，看看他的變化

定義好兩種顏色的畫筆，比較好區分起點與終點
hpen = CreatePen(PS_SOLID,1,RGB(0,0,0));
hpen1 = CreatePen(PS_SOLID,1,RGB(0,100,255));

畫上輔助線
SelectObject(hdc,hpen);
MoveToEx(hdc,280,280,NULL);
LineTo(hdc,nXStartArc,nYStartArc);

SelectObject(hdc,hpen1);
MoveToEx(hdc,280,280,NULL);
LineTo(hdc,nXEndArc,nYEndArc);

看看設計這個畫圓弧的函式的人當初是怎麼想的，幾趟的重複編譯執行下來總該有個譜吧！

。，、＇﹕︰﹔﹖﹑•！？！﹪%*﹡﹢ 我是可愛的分隔線〈〉『』〖〗［］《》〔〕「」【】﹁﹃︻﹂﹄︼

main.cpp

#define WIN32_LEAN_AND_MEAN
#include <windows.h>

#define IDM_Demo1 1001
#define IDM_Bottom 1002 //

LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR szCmdLine, int iCmdShow)
{
char szAppName[] = "HelloWin";
HWND hwnd ;
MSG msg ;
WNDCLASSEX wndclass ;
wndclass.cbSize = sizeof(WNDCLASSEX) ;
wndclass.style = CS_HREDRAW | CS_VREDRAW ;
wndclass.lpfnWndProc = WndProc ;
wndclass.cbClsExtra = 0 ;
wndclass.cbWndExtra = 0 ;
wndclass.hInstance = hInstance ;
wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
wndclass.hbrBackground = GetSysColorBrush(COLOR_3DFACE); //3DFACE);
wndclass.lpszMenuName = "DODO";
wndclass.lpszClassName = szAppName ;
wndclass.hIconSm = 0 ;

RegisterClassEx (&wndclass);

hwnd = CreateWindowEx (
0,
szAppName,
"Graph 繪製圖形",
WS_OVERLAPPEDWINDOW,

0,0,800,500,
NULL,
NULL,
hInstance,
NULL) ;

ShowWindow (hwnd, iCmdShow) ;
UpdateWindow (hwnd) ;

while (GetMessage (&msg, NULL, 0, 0))
{
TranslateMessage (&msg) ;
DispatchMessage (&msg) ;
}

return msg.wParam ;
}

LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam,
LPARAM lParam)
{
PAINTSTRUCT ps;
HPEN hpen, hpen1;
int nXStartArc = 150;
  int nYStartArc = 380;

  int nXEndArc = 120;
  int nYEndArc = 260;

switch (message)
{
HDC hdc;

case WM_PAINT:
hpen = CreatePen(PS_SOLID,1,RGB(0,0,0));
hpen1 = CreatePen(PS_SOLID,1,RGB(0,100,255));




//   hdc = GetDC(hwnd);
   hdc = BeginPaint(hwnd,&ps); // 若改用這樣則不會閃爍 //*

  SelectObject(hdc,hpen1);
   MoveToEx(hdc,270,280,NULL); //畫圓心
   LineTo(hdc,290,280);   //畫圓心
  MoveToEx(hdc,280,270,NULL); //畫圓心
   LineTo(hdc,280,290);   //畫圓心
   SelectObject(hdc,hpen);
   Arc(hdc,220,220,340,340,0,0,0,0); // 這是畫一個圓

   SelectObject(hdc,hpen1);
  Arc(hdc,210,210,350,350,nXStartArc,nYStartArc,nXEndArc,nYEndArc); // 這是畫圓弧
  SelectObject(hdc,hpen);
   MoveToEx(hdc,280,280,NULL);   //畫輔助線
   LineTo(hdc,nXStartArc,nYStartArc);
   SelectObject(hdc,hpen1);
  MoveToEx(hdc,280,280,NULL);
   LineTo(hdc,nXEndArc,nYEndArc);

// ReleaseDC(hwnd,hdc);

EndPaint(hwnd,&ps); // 若改用這樣則不會閃爍 //*
return 0;

case WM_CREATE:
return 0 ;

case WM_COMMAND:
       switch(LOWORD(wParam))
   {
           case IDM_Demo1:

       break;

       case IDM_Bottom: //
           {

               InvalidateRect(hwnd, NULL, TRUE);

               }
            break;
}

return 0 ;

case WM_DESTROY:

DeleteObject(hpen); //不用畫筆時記得要刪除
DeleteObject(hpen1); //不用畫筆時記得要刪除
PostQuitMessage (0) ;
return 0 ;
}

return DefWindowProc (hwnd, message, wParam, lParam) ;
}