c++ - 如何使用 GDI 绘制透明文本？

Question

我的目标是动态地将一些任意文本放入 HICON 图像(在运行时)。我正在使用以下代码:

//Error checks are omitted for brevity

//First create font
LOGFONT lf = {0};
lf.lfHeight = -58;
lf.lfWeight = FW_NORMAL;
lf.lfOutPrecision = OUT_TT_PRECIS;  //Use TrueType fonts for anti-alliasing
lf.lfQuality = CLEARTYPE_QUALITY;
lstrcpy(lf.lfFaceName, L"Segoe UI");

HFONT hFont = ::CreateFontIndirect(&lf);


//HICON hIcon = original icon to use as a source
//I'm using a large 256x256 pixel icon
hIcon = (HICON)::LoadImage(theApp.m_hInstance, MAKEINTRESOURCE(IDI_ICON_GREEN_DIAMOND), IMAGE_ICON, 256, 256, LR_DEFAULTCOLOR);

ICONINFO ii = {0};
::GetIconInfo(hIcon, &ii);

BITMAP bm = {0};
::GetObject(ii.hbmColor, sizeof(bm), &bm);
SIZE szBmp = {bm.bmWidth, bm.bmHeight};

HDC hDc = ::GetDC(hWnd);
HDC hMemDC = ::CreateCompatibleDC(hDc);

HGDIOBJ hOldBmp = ::SelectObject(hMemDC, ii.hbmColor);
HGDIOBJ hOldFont = ::SelectObject(hMemDC, hFont);

::SetBkMode(hMemDC, TRANSPARENT);
::SetTextColor(hMemDC, RGB(255, 0, 0));     //Red text

//Draw text
//NOTE that DrawText API behaves in a similar way
::TextOut(hMemDC, 0, 0, L"Hello", 5);

::SelectObject(hMemDC, hOldFont);
::SelectObject(hMemDC, hOldBmp);


//We need a simple mask bitmap for the icon
HBITMAP hBmpMsk = ::CreateBitmap(szBmp.cx, szBmp.cy, 1, 1, NULL);

ICONINFO ii2 = {0};
ii2.fIcon = TRUE;
ii2.hbmColor = ii.hbmColor;
ii2.hbmMask = hBmpMsk;

//Create updated icon
HICON hIcon2 = ::CreateIconIndirect(&ii2);


//Cleanup
::DeleteObject(hBmpMsk);
::DeleteDC(hMemDC);
::ReleaseDC(hWnd, hDc);
::DeleteObject(ii.hbmColor);
::DeleteObject(ii.hbmMask);

::DeleteObject(hFont);

然后我可以通过 OnPaint() 处理程序在我的窗口中显示图标(以便我可以看到结果如何)，如下所示:

::DrawIconEx(dc.GetSafeHdc(), 0, 0,
    hIcon2, 
    256, 256, NULL, 
    ::GetSysColorBrush(COLOR_BTNFACE),
    DI_NORMAL);

这就是我得到的:

为了查看我的 hIcon2 中像素方面发生了什么，我调用了 GetDIBits在上面代码的 ii.hbmColor 上。应该显示我的单词“Hello”的结果像素阵列如下所示:

像素在该内存转储中被编码为 BGRA，因此每个 DWORD 中的第 4 个字节代表透明度:0=透明，FF=不透明。但在这种情况下，TextOut 不会填充透明度，或者将其保留为 0，这被解释为“完全透明”。相反，它似乎将其预乘为 RGB 颜色本身。

请注意，如果我继续向下查看同一个位图，即绿色菱形开始的位置，图像像素似乎已正确设置透明度字节:

知道如何绘制文本以便 API 可以设置那些透明字节吗？

编辑:按照下面的建议，我尝试了以下 GDI+ 方法:

HGDIOBJ hOldBmp = ::SelectObject(hMemDC, ii.hbmColor);

Graphics grpx(hMemDC);

RectF rcfTxt(0.0f, 0.0f, (REAL)szBmp.cx, (REAL)szBmp.cy);
Font gdiFont(L"Segoe UI", 58.0f, FontStyleRegular, UnitPixel);

SolidBrush gdiBrush(Color(255, 0, 0));

StringFormat gdiSF;
gdiSF.SetAlignment(StringAlignmentNear);
gdiSF.SetFormatFlags(StringFormatFlagsNoWrap);
gdiSF.SetHotkeyPrefix(HotkeyPrefixNone);

//The reason I was using GDI was because I was setting
//spacing between letters using SetTextCharacterExtra()
//Unfortunately with GDI+ this does not work!
HDC hTmpDC = grpx.GetHDC();
::SetTextCharacterExtra(hTmpDC, -4);  //This doesn't do anything!
grpx.ReleaseHDC(hTmpDC);

grpx.DrawString(L"Hello", 5, &gdiFont, rcfTxt, &gdiSF, &gdiBrush);

::SelectObject(hMemDC, hOldBmp);

除了无法设置字符间距(我可以使用 GDI 使用 SetTextCharacterExtra)之外，这是我得到的(为了可见性而略微放大):

显然仍然是透明度问题。

Answer 1

摘自 Microsoft MVP Mike D Sutton 的旧帖子 here .

When you create a DC it initially has default 'stock' objects selected into it, including the stock 1*1*1 Bitmap. Since there is a Bitmap already selected into the DC when you call DrawText() it will still try and render to it even though pretty much everything (apart from one pixel) will be clipped.

What you need to do is to create a Bitmap, either DDB or DIBSection, and select that into your DC before drawing to it.

First though you need to find the size of your Bitmap since you want it large enough to display your text in, so for that you use the DrawText() call again on the initial DC but include the DT_CALCRECT flag. What this does is rather than drawing anything it simply measures how large the text is and dumps that into the RECT you pass the call. From here you can go ahead and create your DIBSection using those dimensions and select it into your DC. Finally perform your existing DrawText ()call (you may also want to use SetBkMode/Color()) which will render the text to the DIBSection from which you can get at the data.

This seems to work pretty well here:

HBITMAP CreateAlphaTextBitmap(LPCSTR inText, HFONT inFont, COLORREF inColour) {
    int TextLength = (int)strlen(inText);
    if (TextLength <= 0) return NULL;

    // Create DC and select font into it
    HDC hTextDC = CreateCompatibleDC(NULL);
    HFONT hOldFont = (HFONT)SelectObject(hTextDC, inFont);
    HBITMAP hMyDIB = NULL;

    // Get text area
    RECT TextArea = {0, 0, 0, 0};
    DrawText(hTextDC, inText, TextLength, &TextArea, DT_CALCRECT);

    if ((TextArea.right > TextArea.left) && (TextArea.bottom > TextArea.top)) {
        BITMAPINFOHEADER BMIH;
        memset(&BMIH, 0x0, sizeof(BITMAPINFOHEADER));

        void *pvBits = NULL;

        // Specify DIB setup
        BMIH.biSize = sizeof(BMIH);
        BMIH.biWidth = TextArea.right - TextArea.left;
        BMIH.biHeight = TextArea.bottom - TextArea.top;
        BMIH.biPlanes = 1;
        BMIH.biBitCount = 32;
        BMIH.biCompression = BI_RGB;

        // Create and select DIB into DC
        hMyDIB = CreateDIBSection(hTextDC, (LPBITMAPINFO)&BMIH, 0, (LPVOID*)&pvBits, NULL, 0);
        HBITMAP hOldBMP = (HBITMAP)SelectObject(hTextDC, hMyDIB);

        if (hOldBMP != NULL) {
            // Set up DC properties
            SetTextColor(hTextDC, 0x00FFFFFF);
            SetBkColor(hTextDC, 0x00000000);
            SetBkMode(hTextDC, OPAQUE);

            // Draw text to buffer
            DrawText(hTextDC, inText, TextLength, &TextArea, DT_NOCLIP);

            BYTE* DataPtr = (BYTE*)pvBits;
            BYTE FillR = GetRValue(inColour);
            BYTE FillG = GetGValue(inColour);
            BYTE FillB = GetBValue(inColour);
            BYTE ThisA;

            for (int LoopY = 0; LoopY < BMIH.biHeight; LoopY++) {
                for (int LoopX = 0; LoopX < BMIH.biWidth; LoopX++) {
                    ThisA = *DataPtr; // Move alpha and pre-multiply with RGB
                    *DataPtr++ = (FillB * ThisA) >> 8;
                    *DataPtr++ = (FillG * ThisA) >> 8;
                    *DataPtr++ = (FillR * ThisA) >> 8;
                    *DataPtr++ = ThisA; // Set Alpha
                }
            }

            // De-select bitmap
            SelectObject(hTextDC, hOldBMP);
        }
    }

    // De-select font and destroy temp DC
    SelectObject(hTextDC, hOldFont);
    DeleteDC(hTextDC);

    // Return DIBSection
    return hMyDIB;
}

If you need an example of how to call it then try something like this (inDC is the DC to render to):

void TestAlphaText(HDC inDC, int inX, int inY) {
    const char *DemoText = "Hello World!\0";

    RECT TextArea = {0, 0, 0, 0};
    HFONT TempFont = CreateFont(50, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, "Arial\0");
    HBITMAP MyBMP = CreateAlphaTextBitmap(DemoText, TempFont, 0xFF);
    DeleteObject(TempFont);

    if (MyBMP) { // Create temporary DC and select new Bitmap into it
        HDC hTempDC = CreateCompatibleDC(inDC);
        HBITMAP hOldBMP = (HBITMAP)SelectObject(hTempDC, MyBMP);

        if (hOldBMP) {
            BITMAP BMInf; // Get Bitmap image size
            GetObject(MyBMP, sizeof(BITMAP), &BMInf);

            // Fill blend function and blend new text to window
            BLENDFUNCTION bf;
            bf.BlendOp = AC_SRC_OVER;
            bf.BlendFlags = 0;
            bf.SourceConstantAlpha = 0x80;
            bf.AlphaFormat = AC_SRC_ALPHA;
            AlphaBlend(inDC, inX, inY, BMInf.bmWidth, BMInf.bmHeight,
                hTempDC, 0, 0, BMInf.bmWidth, BMInf.bmHeight, bf);

            // Clean up
            SelectObject(hTempDC, hOldBMP);
            DeleteObject(MyBMP);
            DeleteDC(hTempDC);
        }
    }
}

答案和代码的所有功劳都归功于该论坛上的原始发帖者，我只是重新发布了它，以便在链接失效时该答案仍然有效。