SelectObject(dc, hBMP) 的作用是什么？

发布于 2024-10-12 04:57:09 字数 782 浏览 6 评论 0原文

我想我确实明白如果我在“设备连接”中选择“一个对象”，并且该对象是画笔、字体、笔< /em>：这些对象的特征因此用于在设备上下文上进行绘图。

但是，我不明白在设备上下文中选择位图后会发生什么，更不用说为什么有必要这样做了。

我有一个程序，有些人做了

HDC dc = ::GetDC(hWnd);

ffBitmap   = ::CreateCompatibleBitmap(dc, windowWidth, windowHeight);
ffMemoryDc = ::CreateCompatibleDC(dc);

hOldBitmap          = (HBITMAP) ::SelectObject(ffMemoryDc, ffBitmap);

，然后绘制到 ffMemoryDc 然后 *BitBlt*s 将 ffMemoyDc 放入窗口的真实设备上下文。

BitBlt ( dc, 
         0, 0, windowWidth, windowHeight,
         ffMemoryDc,
         0, 0,
         SRCCOPY
       );

变量ffBitmap在整个程序中没有被其他地方引用，但是，如果我不选择SelectObject(ffBitmap)，则不会绘制任何内容，因此这是必要的。

如果有人能阐明这里发生的事情，我将不胜感激。

原文

I guess I do understand what happens if I select "an object" into a "device contect", and if this object is a brush, font, pen: The charactersicts of these objects are hence forth used for the drawing onto the device context.

However, I don't understand what happens after I select a bitmap into the device context, much less, why it is necessary to do so.

I have a program that somehwere does

HDC dc = ::GetDC(hWnd);

ffBitmap   = ::CreateCompatibleBitmap(dc, windowWidth, windowHeight);
ffMemoryDc = ::CreateCompatibleDC(dc);

hOldBitmap          = (HBITMAP) ::SelectObject(ffMemoryDc, ffBitmap);

and later draws unto the ffMemoryDc and then *BitBlt*s the ffMemoyDc into the
real device context of a window.

BitBlt ( dc, 
         0, 0, windowWidth, windowHeight,
         ffMemoryDc,
         0, 0,
         SRCCOPY
       );

The variable ffBitmap is referenced nowhere else in the entire program, yet, if I don't SelectObject(ffBitmap), nothing is drawn, so it is necessary.

I'd appreciate if someone could shed some light what happens here.

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