C++ MFC How to Draw Alpha transparent Rectangle

in a C++ MFC application. using the dc of ( CPaintDC dc(this); )

How do i draw a rectangle ( LPRECT ) with an alpha transparency that i can adjust.?

Following is an example c# code which i need to convert into C++

private void pictureBox1_Paint(object sender, PaintEventArgs e)  
{
    Graphics g = e.Graphics;
    Color color = Color.开发者_StackOverflow社区FromArgb(75,Color.Red); //sets color Red with 75% alpha transparency

    Rectangle rectangle = new Rectangle(100,100,400,400);
    g.FillRectangle(new SolidBrush(color), rectangle); //draws the rectangle with the color set.
} 

You need to look into GDI+. Its a bit of a faff but you can create a "Graphics" object as follows:

Gdiplus::Graphics g( dc.GetSafeHdc() );
Gdiplus::Color color( 192, 255, 0, 0 );

Gdiplus::Rect rectangle( 100, 100, 400, 400 );
Gdiplus::SolidBrush solidBrush( color );
g.FillRectangle( &solidBrush, rectangle );

Don't forget to do

#include <gdiplus.h>

and to call

 GdiplusStartup(...);

somewhere :)

You'll notice it's pretty damned similar to your C# code ;)

Its worth noting that the 75 you put in your FromArgb code doesn't set 75% alpha it actually sets 75/255 alpha or ~29% alpha.

GDI (and thus MFC) has no decent support for drawing with an alpha. But GDI+ is available in C++ code as well. Use #include <gdiplus.h> and initialize it with GdiplusStartup(). You can use the Graphics class, create one with its Graphics(HDC) constructor from your CPaintDC. And use its FillRectangle() method. The SDK docs are here.

int StartHoriz,StartVert,BarWidth,BarHeight; // rect start, width and height
StartHoriz=0;
StartVert=100;
width = 100;
height=120;
CDC* pCDC = GetDC();      // Get CDC pointer
CRect Rect(StartHoriz,StartVert,BarWidth,BarHeight);  //create rectangle dimensions
pCDC->Rectangle(Rect);   //draw rectangle