windows api（GDI）实现图片旋转

GDI实现图片旋转，博主在网上找了好多资料，都不太如意。

并且在尝试中发现，如果先用SetViewportOrgEx将HDC上的坐标原点移动到图片中心；再在HDC上的获取每个点，用三角函数进行变换，算得新坐标进行点绘制。理论可行，但是因为double与int转换的关系，会丢失精度，旋转后图像会很模糊。

附：三角函数公式

逆时针：
x1=xcos(β)-ysin(β);
y1=ycos(β)+xsin(β);

顺时针：
x1=xcos(β)+ysin(β);
y1=ycos(β)-xsin(β);

笔者兜兜转转找了很多资料，发现有大神利用函数PlgBlt实现图片的旋转，但是用的是VB。根据理解，本人进行了修改，并译成了C++。代码如下，重点在于Rotate函数，并附上使用方法：

#include <cmath>
using namespace std;

extern "C"
{
    #include <windows.h>
    #include <tchar.h>
    #include "resource.h"
}

#define ID_TIMER 1

/*
逆时针
x1=xcos(β)-ysin(β);
y1=ycos(β)+xsin(β);

顺时针
x1=xcos(β)+ysin(β);
y1=ycos(β)-xsin(β);
*/

LRESULT CALLBACK WndProc(HWND hwnd,UINT message,WPARAM wParam,LPARAM lParam);
void Rotate(HDC hdcDest,int xPos,int yPos,int angle,HDC hdcSrc,int xSrc,int ySrc,int srcWidth,int srcHeight);

HWND hwnd;
HINSTANCE hInst;
int WINAPI WinMain(HINSTANCE hInstance,HINSTANCE hPrevInstance,LPSTR lpCmdLine,int nShowCmd)
{
    TCHAR szAppName[]=TEXT("first win");

    MSG msg;
    WNDCLASS wndclass;
    hInst=hInstance;

    wndclass.style=CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc=WndProc;
    wndclass.cbClsExtra=;
    wndclass.cbWndExtra=;
    wndclass.hInstance=hInstance;
    wndclass.hIcon=LoadIcon(NULL,IDI_APPLICATION);
    wndclass.hCursor=LoadCursor(NULL,IDC_ARROW);
    wndclass.hbrBackground=(HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName=NULL;
    wndclass.lpszClassName=szAppName;

    if(!RegisterClass(&wndclass))
    {
        MessageBox(NULL,TEXT("error"),szAppName,MB_ICONERROR);
        return ;
    }

    hwnd=CreateWindow(szAppName,
                      TEXT("hello"),
                      WS_OVERLAPPEDWINDOW^WS_MAXIMIZEBOX^WS_THICKFRAME,
                      ,    //CW_USEDEFAULT
                      ,
                      ,    //CW_USEDEFAULT,
                      ,    //CW_USEDEFAULT,
                      NULL,
                      NULL,
                      hInstance,
                      NULL);

    ShowWindow(hwnd,nShowCmd);
    UpdateWindow(hwnd);

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

    return msg.wParam;
}

HDC hdc;
PAINTSTRUCT ps;

HDC dcWin,dcBmp1,dcBmp2;
HBITMAP hBmp1,hBmp2;

POINT pt;
RECT rect;

int angle=;

void Rotate(HDC hdcDest,int xPos,int yPos,int angle,HDC hdcSrc,int xSrc,int ySrc,int srcWidth,int srcHeight,COLORREF col)
{
    POINT pt[];
    POINT defPt[];
    double notPI=3.14/;

    double thetS,thetC;
    int ret;

    pt[].x=-srcWidth * 0.5;
    pt[].y=-srcHeight * 0.5;

    pt[].x = pt[].x + srcWidth;
    pt[].y = pt[].y;

    pt[].x = pt[].x;
    pt[].y = pt[].y + srcHeight;

    thetS = sin(angle * notPI);
    thetC = cos(angle * notPI);
    defPt[].x = (pt[].x * thetC - pt[].y * thetS) + xPos;
    defPt[].y = (pt[].x * thetS + pt[].y * thetC) + yPos;

    defPt[].x = (pt[].x * thetC - pt[].y * thetS) + xPos;
    defPt[].y = (pt[].x * thetS + pt[].y * thetC) + yPos;

    defPt[].x = (pt[].x * thetC - pt[].y * thetS) + xPos;
    defPt[].y = (pt[].x * thetS + pt[].y * thetC) + yPos;

    HBRUSH hBrush=CreateSolidBrush(col);
    RECT rect;
    rect.left=rect.top=;
    rect.right=rect.left+srcWidth;
    rect.bottom=rect.top+srcHeight;
    FillRect(hdcDest,&rect,hBrush);
    DeleteObject(hBrush);

    PlgBlt(hdcDest, &defPt[], hdcSrc, xSrc, ySrc, srcWidth, srcHeight, , , );
}

LRESULT CALLBACK WndProc(HWND hwnd,UINT message,WPARAM wParam,LPARAM lParam)
{
    switch(message)
    {
        case WM_CREATE:
            dcWin=GetDC(hwnd);
            dcBmp1=CreateCompatibleDC(dcWin);
            dcBmp2=CreateCompatibleDC(dcWin);

            hBmp1=LoadBitmap(hInst,MAKEINTRESOURCE(IDB_BITMAP1));
            hBmp2=CreateCompatibleBitmap(dcWin,,);

            SelectObject(dcBmp1,hBmp1);
            SelectObject(dcBmp2,hBmp2);

            //将dcBmp1以(49,49)为坐标中心旋转angle角度后后画到dcBmp2中
            Rotate(dcBmp2,,,angle,dcBmp1,,,,,0xffffff);

            SetTimer(hwnd,ID_TIMER,,NULL);

            break;
        case WM_PAINT:
            hdc=BeginPaint(hwnd,&ps);
            SetBkMode(hdc,TRANSPARENT);

            BitBlt(hdc,,,,,dcBmp1,,,SRCCOPY);
            BitBlt(hdc,,,,,dcBmp2,,,SRCCOPY);

            EndPaint(hwnd,&ps);
            break;
        case WM_TIMER:
            if(wParam==ID_TIMER)
            {
                angle=(angle+)%;
                Rotate(dcBmp2,,,angle,dcBmp1,,,,,0xffffff);
                InvalidateRect(hwnd,NULL,true);
                UpdateWindow(hwnd);
            }
            break;
        case WM_DESTROY:
            ReleaseDC(hwnd,dcWin);

            DeleteDC(dcBmp1);
            DeleteDC(dcBmp2);

            DeleteObject(hBmp1);
            DeleteObject(hBmp2);

            KillTimer(hwnd,ID_TIMER);
            PostQuitMessage();
            return ;

    }

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