最近被这个问题搞的我很无语，不多说，直接贴代码了：
测试函数：
void test_gdiplus_SetBlendBellShape(HDC hdc, RECT* pRect, REAL angle = 0.0f)
{
Graphics myGraphics(hdc);
Rect rt(pRect->left, pRect->top, pRect->right-pRect->left, pRect->bottom-pRect->top);
LinearGradientBrush linGrBrush(
rt,
Color(255, 255, 0, 0), // red
Color(255, 0, 0, 255), // blue
angle);
linGrBrush.SetBlendBellShape(1.0f); // this line will make one extra "border" appear
myGraphics.FillRectangle(&linGrBrush, rt);
}
调用：
#define MWIDTH 102
#define MHEIGHT 100
void CXXXDlg::OnPaint()
{
CPaintDC dc(this);
CRect rt;
rt.left = 50;
rt.top = 50;
rt.right = rt.left + MWIDTH;
rt.bottom = rt.top + MHEIGHT;
test_gdiplus_SetBlendBellShape(dc.GetSafeHdc(), rt);
}
出来的效果如下（运行环境 VC6 + XP SP3）：
放大的效果：
可以看到有一条多余的蓝边出现在填充矩形的左侧，如果把 SetBlendBellShape 的调用注释掉则不会出现，按道理1.0就应该是等价于默认的设置，所以怀疑这是GDI+的bug
。
后来在测试过程中发现即使不调用 SetBlendBellShape，填充矩形在某些 size 下也会出现这条额外的边，比如在上述code中的size 102x100，简单的说这个问题还跟填充区域的大小甚至位置有关。
就这个问题Google了一下，找到一些references：
1) http://blog.csdn.net/mubingyun/archive/2008/12/09/3484260.aspx
2) http://www.codeproject.com/KB/GDI-plus/TurnThePage.aspx
弄了很久后发现要解决这个问题，貌似有几个解决方法：
1、调用 Graphics::SetPixelOffsetMode( PixelOffsetModeHalf )，好像在某些case下还是不work。
2、使 brush 的矩形比填充的矩形大一个像素单位。
3、调用 LinearGradientBrush::SetWrapMode( WrapModeTileFlipXY )，实际就是重新拿第一个颜色去覆盖那条多余的蓝边。
看来GDI+作为Bleeding Edge的技术就是会让使用者承担风险啊。
1/18/2010（在发布此随笔之前），在搜索了一天后找到一些资料:
1) http://stackoverflow.com/questions/110081/lineargradientbrush-artifact-workaround
2) http://www.experts-exchange.com/Microsoft/Development/.NET/Visual_CSharp/Q_23329115.html
3) http://www.tech-archive.net/Archive/DotNet/microsoft.public.dotnet.languages.csharp/2007-01/msg01592.html
你会看到有人（包括一个MS MVP）说这是一个已知的bug，并且建议说把brush弄得比要填充的矩形大一点（一个像素即可）可以解决问题。
但这样是不是真的就是最佳workaround了呢？看这个发帖提问的人的回复：
I had tried inflating the brush, and it seemed to eliminate the random
line. But it introduced its own problems:
Some of the shaded areas get as narrow as 3 pixels (perspective-shrink
during animation). That means that a 1-pixel miss results in a very
visible 33% change in the outer color. This is not as ugly as the
black lines, but still a noticeable glitch.
And with the inflated brush, there's now no way to get a pure white or
pure black row of pixels on the outer edge of a 3 pixel wide object,
as the crossfade is already at 20% (1 pixel out of 5) when the brush
hits the object edge. But then the bug will pop in and randomly draw a
pure white or black edge.
主要意思是说这个方法引入了其他问题，比如对比增大brush的矩形之前，终点的颜色不太一样了，更重要的是在矩形比较小的时候这个问题还是会出现：
上面是在 位置249,249 大小为7x100的时候的截图
就我看来，LinearGradientBrush::SetWrapMode( WrapModeTileFlipXY ) 虽然有点tricky，但至少目前都还没发现问题重现，看来这暂时是个最终解决方法了。
顺便贴下某人的complaints（来源: http://weblogs.mozillazine.org/tor/archives/2005/06/）：
I'm considering switching the renderer that Mozilla SVG uses on win32 to cairo from GDI+. The reasons for this:
- Less problems for the user, as they don't have to deal with downloading GDI+ if they're on an older system (pre-XP).
- Some bugs in GDI+ are unfixable (or at least, would take a lot of effort). For example the spread method for radial gradients, bug 296411.
- Consistent behavior between platforms, up to the quality of the respective cairo backends.
- Saving effort adding features for GDI+, which likely won't be used past 1.8.x. The patch for <svg:textPath>, bug 282579, is an example of this.
注意第二点，"Some bugs in GDI+ are unfixable"，that is so microsoft, heh?
