使用AlphaBlend繪製稍微透明的矩形失敗

Question

我試圖在本地Win32 C++中繪製一個稍微透明的藍色矩形。我正在使用函數AlphaBlend（），但它沒有在窗口上繪製任何東西，沒有任何反應。

我的問題：當我運行我的函數繪製一個稍微透明的矩形時，它不會顯示在我的窗口上。我有一種感覺我做錯了，也許我應該使用HBITMAP？

你能告訴我需要做些什麼來讓我的函數在窗口上繪製一個稍微透明的矩形嗎？

另外我知道GDI +，但我現在要避免它，因爲當我使用該庫時，我收到了很多編譯/包含錯誤，並且我想盡可能低/本地，而沒有幫助圖書館，爲我做的一切。

bool paintRect(HDC hdc, RECT dim, COLORREF penCol, COLORREF brushCol, unsigned int opacity) 
{ 
    HDC tempHdc   = CreateCompatibleDC(hdc); 
    BLENDFUNCTION blend = {AC_SRC_OVER, 0, 127, AC_SRC_ALPHA}; 

    SetDCPenColor(tempHdc, RGB(255,255,0)); 
    SetDCBrushColor(tempHdc, RGB(255,255,0)); 
    Rectangle(tempHdc, dim.left, dim.top, dim.right, dim.bottom); 

    return bool(AlphaBlend(hdc, dim.left, dim.top, dim.right, dim.bottom, tempHdc, dim.left, dim.top, dim.right, dim.bottom, blend)); 
} 
// Usage 
case WM_PAINT: 
{ 
    HDC hdc; 
    PAINTSTRUCT ps; 
    hdc = BeginPaint(hwnd, &ps); 

    RECT a = {0,0,100,100}; 
    paintRect(hdc, a, RGB(255,255,0), RGB(255,255,0), 127); // 127 is 50% transparency right? 

    EndPaint(hwnd, &ps); 
} 
break; 

Answer 1

這將工作：

bool paintRect(HDC hdc, RECT dim, COLORREF penCol, COLORREF brushCol, unsigned int opacity) 
{ 
     HDC tempHdc   = CreateCompatibleDC(hdc); 
     BLENDFUNCTION blend = {AC_SRC_OVER, 0, 127, 0}; 

     HBITMAP hbitmap;  // bitmap handle 
     BITMAPINFO bmi;  // bitmap header 
     // zero the memory for the bitmap info 
     ZeroMemory(&bmi, sizeof(BITMAPINFO)); 

     // setup bitmap info 
     // set the bitmap width and height to 60% of the width and height of each of the three horizontal areas. Later on, the blending will occur in the center of each of the three areas. 
     bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER); 
     bmi.bmiHeader.biWidth = dim.right-dim.left; 
     bmi.bmiHeader.biHeight = dim.bottom-dim.top; 
     bmi.bmiHeader.biPlanes = 1; 
     bmi.bmiHeader.biBitCount = 32;   // four 8-bit components 
     bmi.bmiHeader.biCompression = BI_RGB; 
     bmi.bmiHeader.biSizeImage = (dim.right-dim.left) * (dim.bottom-dim.top) * 4; 

     // create our DIB section and select the bitmap into the dc 
     hbitmap = CreateDIBSection(tempHdc, &bmi, DIB_RGB_COLORS, NULL, NULL, 0x0); 
     SelectObject(tempHdc, hbitmap); 

     SetDCPenColor(tempHdc, RGB(0,0,255)); 
     SetDCBrushColor(tempHdc, RGB(0,0,255)); 
     FillRect(tempHdc, &dim, CreateSolidBrush(RGB(0,0,255))); 

     return bool(AlphaBlend(hdc, dim.left, dim.top, dim.right, dim.bottom, tempHdc, dim.left, dim.top, dim.right, dim.bottom, blend)); 
} 

Answer 2

CreateCompatibleDC函數返回一個尺寸爲1x1的defualt位圖兼容直流。爲了繪製一些有用的東西，首先你應該選擇一個合適的大小合適的位圖。您可以使用CreateCompatibleBitmap函數。

基本上你應該做這樣的事情：

HDC tempHdc   = CreateCompatibleDC(hdc); 
// create a bitmap of a size you need, let's say it 100x100 
int width = 100; 
int height = 100; 
HBITMAP canvas = CreateCompatibleBitmap(hdc, width, height); 
// select new bitmap into context, don't forget to save old bitmap handle 
HBITMAP oldBmp = SelectObject(tepmHdc, canvas); 

BLENDFUNCTION blend = {AC_SRC_OVER, 0, 127, AC_SRC_ALPHA}; 

SetDCPenColor(tempHdc, RGB(255,255,0)); 
SetDCBrushColor(tempHdc, RGB(255,255,0)); 
Rectangle(tempHdc, dim.left, dim.top, dim.right, dim.bottom); 

bool res = AlphaBlend(hdc, dim.left, dim.top, dim.right, dim.bottom, tempHdc, dim.left, dim.top, dim.right, dim.bottom, blend); 
// reset the old bitmap 
SelectObect(tempHdc, oldBmp); 
// canvas is no longer needed and should be deleted to avoid GDI leaks 
DeleteObject(canvas); 

return res; 

但嚴重的是，你應該重新考慮GDI +，揣摩爲什麼你所有的編譯器錯誤和解決這些問題。 GDI很老，不太友善，手動資源管理，使用起來非常痛苦。

祝你好運。