从FillRectangle中略微不需要的透明度 [英] Slight undesired transparency from FillRectangle

问题描述

我有一个用 WS_EX_LAYERED 窗口样式创建的窗口。我目前使用GDI +绘制一个内存位图，并使用 UpdateLayeredWindow 更新分层窗口的图形内容。

I have a window created with the WS_EX_LAYERED window style. I am currently drawing onto a memory bitmap using GDI+, and using UpdateLayeredWindow to update the graphical content of my layered window.

我的代码片段：

void Redraw(HWND hWnd, int width, int height) {
    static bool floppy = true;

    floppy = !floppy;

    HDC hScreenDC = GetDC(HWND_DESKTOP);
    HDC hMemDC = CreateCompatibleDC(hScreenDC);
    HBITMAP hBmp = CreateCompatibleBitmap(hScreenDC, width, height);
    HGDIOBJ hObj = SelectObject(hMemDC, hBmp);

    Graphics gfx(hMemDC);

    SolidBrush b(Color(254, (floppy ? 255 : 0), (floppy ? 0 : 255), 0));
    gfx.FillRectangle(&b, Rect(0, 0, width, height));

    BLENDFUNCTION blend;
    blend.BlendOp = AC_SRC_OVER;
    blend.BlendFlags = 0;
    blend.SourceConstantAlpha = 255;
    blend.AlphaFormat = AC_SRC_ALPHA;

    POINT src = { 0, 0 };

    SIZE size;
    size.cx = width;
    size.cy = height;

    Assert(UpdateLayeredWindow(
        hWnd,
        hScreenDC,
        NULL,
        &size,
        hMemDC,
        &src,
        RGB(0, 0, 0),
        &blend,
        ULW_ALPHA
    ));

    SelectObject(hMemDC, hObj);
    DeleteObject(hBmp);
    DeleteDC(hMemDC);
    ReleaseDC(HWND_DESKTOP, hScreenDC);
}

当创建我的 SolidBrush ，我为alpha组件指定了254的值。这导致了99.6％的不透明填充，这不是我想要的。

When creating my SolidBrush, I specified the value of 254 for the alpha component. This results in a 99.6% opaque fill, which is not what I want.

当我指定255作为alpha组件，似乎没有填充;我的窗口变得完全透明。这是一个问题，因为我希望绘制100％不透明的形状，但我也希望绘制一些不是。

When I specify 255 as the alpha component, there appears to be no fill; my window becomes completely transparent. This is an issue because I wish to draw shapes that are 100% opaque, but I also wish to draw some that aren't.

推荐答案

似乎有一些qwerks与 FillRectangle 。当我们观察到使用 FillEllipse 与α分量为255的 SolidBrush 时，会导致形状被呈现完全（不透明）。

There seems to be some qwerks with FillRectangle. This becomes apparent when we observe that using FillEllipse with a SolidBrush whose alpha component is 255, results in the shape being rendered perfectly (opaque).

这里有两个解决方案，每个解决问题为我：

Here are two work-arounds that I came up with, which each solve the issue for me:

• 致电 FillRectangle 两次 b
  $ b

• Call FillRectangle twice

SolidBrush b(Color(254, 255, 0, 0));
gfx.FillRectangle(&b, Rect(0, 0, width, height));
gfx.FillRectangle(&b, Rect(0, 0, width, height));

由于相同区域被填充两次，它们将混合并创建RGB（255,0,0 ）无论窗口后面的内容（它现在是100％不透明）。

Since the same area is being filled twice, they will blend and create RGB(255, 0, 0) regardless of the content behind the window (it's now 100% opaque). I do not prefer this method, as it requires every rectangle to be drawn twice.

使用 FillPolygon 而非

与 FillEllipse ， FillPolygon 似乎没有颜色问题，除非您像这样调用

Just as with FillEllipse, FillPolygon doesn't seem to have the colour issue, unless you call it like so:

SolidBrush b(Color(255, 255, 0, 0));
Point points[4];
points[0] = Point(0, 0);
points[1] = Point(width, 0);
points[2] = Point(width, height);
points[4] = Point(0, height);
gfx.FillPolygon(&b, points, 4); //don't copy and paste - this won't work

上述代码将导致100％透明窗。我猜这是由于某种形式的优化，通过调用 FillRectangle 。或者 - 很可能 - FillPolygon 有一些问题，它由 FillRectangle 调用。但是，如果你向数组添加一个额外的 Point ，你可以绕过它：

The above code will result in a 100% transparent window. I am guessing that this is either due to some form of optimisation that passes the call to FillRectangle instead. Or - most likely - there is some problem with FillPolygon, which is called by FillRectangle. However, if you add an extra Point to the array, you can get around it:

SolidBrush b(Color(255, 255, 0, 0));
Point points[5];
points[0] = Point(0, 0);
points[1] = Point(0, 0); //<-
points[2] = Point(width, 0);
points[3] = Point(width, height);
points[4] = Point(0, height);
gfx.FillPolygon(&b, points, 5);

上面的代码确实会绘制一个100％不透明的形状，这解决了我的问题。

The above code will indeed draw a 100% opaque shape, which fixes my problem.

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