windows C++/Win32：如何从 HBITMAP 获取 alpha 通道？
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C++/Win32: How to get the alpha channel from an HBITMAP?
提问by mackenir
I have an HBITMAPcontaining alpha channel data. I can successfully render this using the ::AlphaBlendGDI function.
我有一个HBITMAP包含 alpha 通道数据。我可以使用::AlphaBlendGDI 函数成功渲染它。
However, when I call the ::GetPixelGDI function, I never get back values with an alpha component. The documentation does say that it returns the RGB value of the pixel.
但是，当我调用::GetPixelGDI 函数时，我永远不会返回带有 alpha 组件的值。文档确实说它返回像素的 RGB 值。
Is there a way to retrieve the alpha channel values for pixels in an HBITMAP?
有没有办法检索 中像素的 alpha 通道值HBITMAP？
I want to be able to detect when to use ::AlphaBlend, and when to use an old-school method for treating a particular colour in the source HBITMAP as transparent.
我希望能够检测何时使用 ::AlphaBlend，以及何时使用老式方法将源 HBITMAP 中的特定颜色视为透明。
HDC sourceHdc = ::CreateCompatibleDC(hdcDraw);
::SelectObject(sourceHdc, m_hbmp);
// This pixel has partial transparency, but ::GetPixel returns just RGB.
COLORREF c = ::GetPixel(sourceHdc, 20, 20);
// Draw the bitmap to hdcDraw
BLENDFUNCTION bf1;
bf1.BlendOp = AC_SRC_OVER;
bf1.BlendFlags = 0;
bf1.SourceConstantAlpha = 0xff;
bf1.AlphaFormat = AC_SRC_ALPHA;
::AlphaBlend(di.hdcDraw, x, 10, 64, 64, sourceHdc, 0, 0, 64, 64, bf1);
::DeleteDC(sourceHdc);
Answer
回答
Use GetDIBits to retrieve the first (or more) scan line(s) of the image:
使用 GetDIBits 检索图像的第一个（或多个）扫描线：
byte* bits[1000];// = new byte[w * 4];
BITMAPINFO bmi;
memset(&bmi, 0, sizeof(BITMAPINFO));
bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bmi.bmiHeader.biWidth = w;
bmi.bmiHeader.biHeight = -h;
bmi.bmiHeader.biBitCount = 32;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biCompression = BI_RGB;
bmi.bmiHeader.biSizeImage = 0;
bmi.bmiHeader.biXPelsPerMeter = 0;
bmi.bmiHeader.biYPelsPerMeter = 0;
bmi.bmiHeader.biClrUsed = 0;
bmi.bmiHeader.biClrImportant = 0;
int rv = ::GetDIBits(sourceHdc1, m_hbmp, 0, 1, (void**)&bits, &bmi, DIB_RGB_COLORS);
//bits[3] == alpha of topleft pixel;
//delete[] bits;
采纳答案by Roel
Use GetDIBits. That way you get an array of RGBQUAD's which have as you can probably guess an alpha channel next to the R, G and B components.
使用 GetDIBits。这样你就得到了一个 RGBQUAD 数组，你可以猜到 R、G 和 B 分量旁边的一个 alpha 通道。
