一般都用GDI实现:
void CXiangpijinView::OnMouseMove(UINT nFlags, CPoint point) { // TODO: Add your message handler code here and/or call default CView::OnMouseMove(nFlags, point); if (Lbrndown) { CClientDC dc(this); int oldmode=dc.SetROP2(R2_NOTXORPEN); CPen pen(PS_SOLID, 2.0, RGB(255,0,0)), *oldpen; oldpen = dc.SelectObject(&pen); dc.MoveTo(m_ptOrigin); dc.LineTo(m_ptEnd); m_ptEnd=point; dc.MoveTo(m_ptOrigin); dc.LineTo(m_ptEnd); dc.SelectObject(oldpen); dc.SetROP2(oldmode); ReleaseDC(&dc); } } void CXiangpijinView::OnLButtonDown(UINT nFlags, CPoint point) { // TODO: Add your message handler code here and/or call default CView::OnLButtonDown(nFlags, point); Lbrndown=true; m_ptOrigin = m_ptEnd = point; }
主要得益于GDI支持SetROP2,绘图模式。
nDrawMode Specifies the new drawing mode. It can be any of the following values:
R2_BLACK Pixel is always black.
R2_WHITE Pixel is always white.
R2_NOP Pixel remains unchanged.
R2_NOT Pixel is the inverse of the screen color.
R2_COPYPEN Pixel is the pen color.
R2_NOTCOPYPEN Pixel is the inverse of the pen color.
R2_MERGEPENNOT Pixel is a combination of the pen color and the inverse of the screen color (final pixel = (NOT screen pixel) OR pen).
R2_MASKPENNOT Pixel is a combination of the colors common to both the pen and the inverse of the screen (final pixel = (NOT screen pixel) AND pen).
R2_MERGENOTPEN Pixel is a combination of the screen color and the inverse of the pen color (final pixel = (NOT pen) OR screen pixel).
R2_MASKNOTPEN Pixel is a combination of the colors common to both the screen and the inverse of the pen (final pixel = (NOT pen) AND screen pixel).
R2_MERGEPEN Pixel is a combination of the pen color and the screen color (final pixel = pen OR screen pixel).
R2_NOTMERGEPEN Pixel is the inverse of the R2_MERGEPEN color (final pixel = NOT(pen OR screen pixel)).
R2_MASKPEN Pixel is a combination of the colors common to both the pen and the screen (final pixel = pen AND screen pixel).
R2_NOTMASKPEN Pixel is the inverse of the R2_MASKPEN color (final pixel = NOT(pen AND screen pixel)).
R2_XORPEN Pixel is a combination of the colors that are in the pen or in the screen, but not in both (final pixel = pen XOR screen pixel).
R2_NOTXORPEN Pixel is the inverse of the R2_XORPEN color (final pixel = NOT(pen XOR screen pixel)).
原理:R2_NOTXORPEN画出来的颜色与R2_XORPEN相反,R2_XORPEN是屏幕颜色和画笔颜色的异或。OnMouseMove第一次被调用时,还没画线,所以屏幕的颜色是白色的,R2_XORPEN是当前画笔的颜色取反,那么R2_NOTXORPEN就是当前画笔颜色了。就是说第一次画的线是画笔的颜色。
第二次调用OnMouseMove时,m_ptOrigin和m_ptEnd两个点还没变,所以可以用这两个点再画线,将第一次画的线覆盖掉,变成画布的颜色,然后在新的point点和m_ptOrigin之间重新画线,颜色为画笔颜色。在旧的直线上面画线,因为线本来有颜色,所以R2_XORPEN(屏幕的颜色==画笔颜色)就会变成黑色(1 xor 1=0,0 xor 0=0),取反,即R2_NOTXORPEN为白色,就是画布的颜色,看起来就像消失了一样,其实只不过是线变成白色了(如果画笔不是白色,比如使用你系统设置了护眼配色,客户区变成不伤眼的浅绿色,这样显示出来的颜色还是白色,而不是客户区的颜色)。旧的直线删除了,就可以在新的点point上再次画线了。
注意:如果背景色是白色,那么画出来的线就是画笔的颜色,如果不是白色,线的颜色为笔色与屏幕颜色异或再取反的颜色。
同理只需替换绘图函数就可可以实现矩形、椭圆的橡皮筋效果。
而GDI+不支持SetRop2,
好像说是因为SetRop2只针对光栅,而GDI+不只是用于光栅,要支持打印机等原因。不懂。
一般来说,新的库应该有方法来取代SetRopt2原来的功能。
这就使得画橡皮条非常困难,网上提供了一些方法以及问题:
1/ 使用DllImport,导入SetRop2 : 结合GDI绘图,这违背了GDI+删去SetRop2的本意。
2/ 更新从起始点到结束点的整个块状区域 : 我觉得为了一条线而刷新整个区域是很愚蠢的事情,虽然可以使用双缓冲等等技术
3/ 使用一个ErasePen和一个DrawPen,ErasePen和背景同色 : 这个方法很有问题,当和其他线条相交时会擦去原来的线条
不知道为什么要把SetRop2去掉。
我自己实现的使用双缓冲来实现橡皮条
void CXiangpijinView::OnDraw(CDC* pDC) { CXiangpijinDoc* pDoc = GetDocument(); ASSERT_VALID(pDoc); // TODO: add draw code for native data here GetClientRect(&rc); Graphics Gp(pDC->m_hDC); SolidBrush mybrush(Color( 135 ,135,135)); Gp.FillRectangle(&mybrush,0,0,rc.Width(),rc.Height()); Gp.ReleaseHDC(hMemDC); HBITMAP hBitmap,hOldBitmap; //建立一个屏幕设备环境句柄 hMemDC=CreateCompatibleDC(pDC->m_hDC); //建立一个与屏幕设备环境句柄兼容、与鼠标指针所在窗口的区域等大的位图 hBitmap=CreateCompatibleBitmap(pDC->m_hDC,rc.Width(),rc.Height()); //把新位图选到内存设备描述表中 hOldBitmap=(HBITMAP)SelectObject(hMemDC,hBitmap); //把屏幕设备描述表拷贝到内存设备描述表中 BitBlt(hMemDC,0,0,rc.Width(),rc.Height(),pDC->m_hDC,0,0,SRCCOPY); } void CXiangpijinView::OnMouseMove(UINT nFlags, CPoint point) { // TODO: Add your message handler code here and/or call default CView::OnMouseMove(nFlags, point); if (Lbrndown) { BitBlt(GetDC()->m_hDC,0,0,rc.Width(),rc.Height(),hMemDC,0,0,SRCCOPY);
Graphics Gp3(GetDC()->m_hDC);
Pen mypen(Color(0,255,0),2.0);
Gp3.DrawLine(&mypen,m_ptOrigin.x,m_ptOrigin.y,point.x,point.y);
Gp3.ReleaseHDC(GetDC()->m_hDC); } } void CXiangpijinView::OnLButtonDown(UINT nFlags, CPoint point) { // TODO: Add your message handler code here and/or call default CView::OnLButtonDown(nFlags, point); Lbrndown=true; m_ptOrigin = m_ptEnd = point; } void CXiangpijinView::OnLButtonUp(UINT nFlags, CPoint point) { // TODO: Add your message handler code here and/or call default CView::OnLButtonUp(nFlags, point); //把新的DC拷贝到hMemDC BitBlt(hMemDC,0,0,rc.Width(),rc.Height(),GetDC()->m_hDC,0,0,SRCCOPY); }
主要原理是绘图前把屏幕DC内容拷贝到兼容的内存位图中,绘图时把兼容dC 上的内容拷贝到屏幕,然后在在当前DC上绘图,一次绘制结束再把 DC上拷贝到兼容DC上。
常用绘图操作
m_map = new Bitmap(rc.Width(),rc.Height());
Pen mypen(Color(0,255,0),2.0);
m_Gpp = new Graphics(m_map);
m_Gpp->DrawEllipse(&mypen,po.X,po.Y,8.0,8.0);
Graphics Gp3(GetDC()->m_hDC);
Gp3.DrawImage(m_map,0,0);
Gp3.ReleaseHDC(GetDC()->m_hDC);