问题描述

我正在创建一个非常简单的 Win32 C++ 应用程序，其唯一目的是仅显示半透明的 PNG.窗口不应有任何镀铬，并且所有不透明度都应在 PNG 本身中控制.

I'm creating what should be a very simple Win32 C++ app whose sole purpose it to ONLY display a semi-transparent PNG. The window shouldn't have any chrome, and all the opacity should be controlled in the PNG itself.

我的问题是当窗口下的内容发生变化时窗口不会重新绘制，因此 PNG 的透明区域卡住"了应用程序最初启动时窗口下的内容.

My problem is that the window doesn't repaint when the content under the window changes, so the transparent areas of the PNG are "stuck" with what was under the window when the application was initially started.

这是我设置新窗口的行:

Here's the line where I setup the new window:

hWnd = CreateWindowEx(WS_EX_TOPMOST, szWindowClass, szTitle, WS_POPUP, 0, height/2 - 20, 40, 102, NULL, NULL, hInstance, 0);

对于 RegisterClassEx 的调用，我为后台设置了这个:

For the call to RegisterClassEx, I have this set for the background:

wcex.hbrBackground = (HBRUSH)0;

这是我的 WM_PAINT 消息处理程序:

Here is my handler for WM_PAINT message:

 case WM_PAINT:
 {
   hdc = BeginPaint(hWnd, &ps);
   Gdiplus::Graphics graphics(hdc);
   graphics.DrawImage(*m_pBitmap, 0, 0);
   EndPaint(hWnd, &ps);
   break;
 }

需要注意的一点是，应用程序始终停靠在屏幕左侧并且不会移动.但是，应用程序下方的内容可能会随着用户打开、关闭或移动其下方的窗口而发生变化.

One thing to note is that the application is always docked to the left of the screen and doesn't move. But, what's underneath the application may change as the user opens, closes or moves windows under it.

当应用程序第一次启动时，它看起来很完美.PNG 的透明(和半透明)部分完美地显示出来.但是，当应用程序下面的背景发生变化时，背景不会更新，它只是与应用程序刚启动时保持不变.实际上，WM_PAINT(或 WM_ERASEBKGND 在背景变化时不会被调用).

When the application first starts, it looks perfect. The transparent (and simi-transparent) parts of the PNG show through perfectly. BUT, when the background underneath the application changes, the background DOESN'T update, it just stays the same from when the application first started. In fact, WM_PAINT (or WM_ERASEBKGND does not get called when the background changes).

我已经玩了很长一段时间，并且已经接近 100% 正确，但还没有完全正确.例如，我尝试将背景设置为 (HBRUSH) NULL_BRUSH，并且尝试处理 WM_ERASEBKGND.

I've been playing with this for quite a while and have gotten close to getting 100% right, but not quite there. For instance, I've tried setting the background to (HBRUSH) NULL_BRUSH and I've tried handling WM_ERASEBKGND.

当窗口下的内容发生变化时，如何让窗口重新绘制?

What can be done to get the window to repaint when the contents under it changes?

推荐答案

通过使用本系列第 1 部分和第 2 部分中的代码，我能够完全按照自己的意愿行事:

I was able to do exactly what I wanted by using the code from Part 1 and Part 2 of this series:

使用 C++ 显示启动画面

• 第 1 部分: 创建 HBITMAP ^存档
• 第 2 部分: 显示窗口 ^存档

那些博客文章谈论的是在 Win32 C++ 中显示启动画面，但这与我需要做的几乎相同.我相信我缺少的部分是，我需要使用 UpdateLayeredWindow 函数和正确的 BLENDFUNCTION 参数.我将粘贴下面的 SetSplashImage 方法，该方法可以在上面链接的第 2 部分中找到:

Those blog posts are talking about displaying a splash screen in Win32 C++, but it was almost identical to what I needed to do. I believe the part that I was missing was that instead of just painting the PNG to the window using GDI+, I needed to use the UpdateLayeredWindow function with the proper BLENDFUNCTION parameter. I'll paste the SetSplashImage method below, which can be found in Part 2 in the link above:

void SetSplashImage(HWND hwndSplash, HBITMAP hbmpSplash)
{
  // get the size of the bitmap
  BITMAP bm;
  GetObject(hbmpSplash, sizeof(bm), &bm);
  SIZE sizeSplash = { bm.bmWidth, bm.bmHeight };

  // get the primary monitor's info
  POINT ptZero = { 0 };
  HMONITOR hmonPrimary = MonitorFromPoint(ptZero, MONITOR_DEFAULTTOPRIMARY);
  MONITORINFO monitorinfo = { 0 };
  monitorinfo.cbSize = sizeof(monitorinfo);
  GetMonitorInfo(hmonPrimary, &monitorinfo);

  // center the splash screen in the middle of the primary work area
  const RECT & rcWork = monitorinfo.rcWork;
  POINT ptOrigin;
  ptOrigin.x = 0;
  ptOrigin.y = rcWork.top + (rcWork.bottom - rcWork.top - sizeSplash.cy) / 2;

  // create a memory DC holding the splash bitmap
  HDC hdcScreen = GetDC(NULL);
  HDC hdcMem = CreateCompatibleDC(hdcScreen);
  HBITMAP hbmpOld = (HBITMAP) SelectObject(hdcMem, hbmpSplash);

  // use the source image's alpha channel for blending
  BLENDFUNCTION blend = { 0 };
  blend.BlendOp = AC_SRC_OVER;
  blend.SourceConstantAlpha = 255;
  blend.AlphaFormat = AC_SRC_ALPHA;

  // paint the window (in the right location) with the alpha-blended bitmap
  UpdateLayeredWindow(hwndSplash, hdcScreen, &ptOrigin, &sizeSplash,
      hdcMem, &ptZero, RGB(0, 0, 0), &blend, ULW_ALPHA);

  // delete temporary objects
  SelectObject(hdcMem, hbmpOld);
  DeleteDC(hdcMem);
  ReleaseDC(NULL, hdcScreen);
}

这篇关于在 C++ Win32 中创建透明窗口的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！