使用System.Drawing2D LinearGradientBrush填充椭圆出现异常结果

你的核心问题出在椭圆边缘点的计算逻辑，以及对LinearGradientBrush渐变范围的匹配上，以下是具体问题和修复步骤：

1. 椭圆边缘点计算错误 #

你当前用单位向量u直接乘以ovalRect.Width/2来计算边缘偏移量，这只适用于正圆（宽高相等的情况）。如果是椭圆，X轴和Y轴的半径分别是ovalRect.Width/2和ovalRect.Height/2，需要分别对单位向量的X、Y分量进行缩放，才能得到真正的椭圆边缘点：

修正后的边缘点计算代码：

Vector3 u = FBody.Unit(ovalCenter);
// 分别按椭圆X/Y轴半径计算边缘偏移
Vector3 fromBodyCenterToEdge = new Vector3(
    u.X * ovalRect.Width / 2.0f,
    u.Y * ovalRect.Height / 2.0f,
    u.Z // 2D渲染可忽略Z分量
);
Vector3 darkSidePoint = ovalCenter + fromBodyCenterToEdge;
Vector3 sunSidePoint = ovalCenter - fromBodyCenterToEdge;

2. 确保渐变与椭圆边界匹配 #

LinearGradientBrush的渐变范围是从darkSide到sunSide的直线，但填充椭圆时，渐变会覆盖整个ovalRect区域。可以添加一行代码禁用渐变平铺，避免边缘出现重复渐变的异常效果：

bl.WrapMode = WrapMode.Clamp;

3. 验证坐标投影正确性 #

确认ovalCenter的3D向量转2D平面时，Z轴的投影逻辑正确（比如是否用了透视投影或正交投影），如果Z分量处理不当，会导致darkSidePoint和sunSidePoint的2D坐标偏移，进而影响渐变方向。

修复后的完整代码片段 #

Vector3 u = FBody.Unit(ovalCenter);
Vector3 fromBodyCenterToEdge = new Vector3(
    u.X * ovalRect.Width / 2.0f,
    u.Y * ovalRect.Height / 2.0f,
    u.Z
);
Vector3 darkSidePoint = ovalCenter + fromBodyCenterToEdge;
Vector3 sunSidePoint = ovalCenter - fromBodyCenterToEdge;

PointF darkSide = new PointF(darkSidePoint.X, darkSidePoint.Y);
PointF sunSide = new PointF(sunSidePoint.X, sunSidePoint.Y); 
Color darkSideColor = Color.Black;
Color sunSideColor = penBody.Color;

LinearGradientBrush bl = new LinearGradientBrush(darkSide, sunSide, darkSideColor, sunSideColor);
bl.WrapMode = WrapMode.Clamp; // 禁用平铺，避免多余渐变

g.FillEllipse(bl, ovalRect);

内容的提问来源于stack exchange，提问作者DMP