为什么在Paint事件中绘制的是小正方形而非圆形？

1. 缩放因子计算方向完全颠倒 #

你当前的缩放因子用图像尺寸 / 控件尺寸计算，这会导致椭圆尺寸被错误缩小（当图像比控件大时），甚至因整数截断变成极小的正方形。正确逻辑应该是将图像坐标转换为控件显示坐标，缩放因子应为控件尺寸 / 图像尺寸：

// 修正后的缩放因子计算
double xFactor = (double)pictureBoxPoints.Width / bitmap.Width;
double yFactor = (double)pictureBoxPoints.Height / bitmap.Height;

2. 未适配PictureBox的SizeMode #

如果SizeMode设为Zoom或CenterImage，图像会在控件内居中，存在空白偏移；若设为StretchImage，PictureBox会自动对Paint事件的Graphics应用拉伸变换，手动二次缩放会导致圆形被拉伸变形。

• 针对Zoom模式，需先计算图像显示偏移量，再调整椭圆坐标：

  int offsetX = (pictureBoxPoints.Width - (int)(bitmap.Width * scaleFactor)) / 2;
  int offsetY = (pictureBoxPoints.Height - (int)(bitmap.Height * scaleFactor)) / 2;
  int ellipseX = offsetX + (int)(point.X * scaleFactor) - ellipseSize / 2;
  int ellipseY = offsetY + (int)(point.Y * scaleFactor) - ellipseSize / 2;
  

  此时scaleFactor应取Math.Min(xFactor, yFactor)（Zoom模式按最小比例保持图像比例）。

3. 整数截断导致尺寸失真 #

直接用(int)强制转换会截断小数部分，导致椭圆宽高虽代码中一致，但实际显示因误差变成近似正方形。建议用Math.Round取整后再转换：

int ellipseSize = (int)Math.Round(5 * scaleFactor);
int ellipseX = (int)Math.Round(point.X * scaleFactor) - ellipseSize / 2;
int ellipseY = (int)Math.Round(point.Y * scaleFactor) - ellipseSize / 2;

4. 可选优化：开启图形平滑模式 #

虽不影响形状，但开启后椭圆边缘更圆润，避免锯齿：

e.Graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;

private void PictureBoxPoints_Paint(object sender, PaintEventArgs e)
{
    if (pictureBoxPoints.Image != null)
    {
        Bitmap bitmap = pictureBoxPoints.Image as Bitmap;
        double xFactor = (double)pictureBoxPoints.Width / bitmap.Width;
        double yFactor = (double)pictureBoxPoints.Height / bitmap.Height;
        double scaleFactor = 1;
        int offsetX = 0;
        int offsetY = 0;

        // 根据SizeMode调整参数
        if (pictureBoxPoints.SizeMode == PictureBoxSizeMode.Zoom)
        {
            scaleFactor = Math.Min(xFactor, yFactor);
            offsetX = (pictureBoxPoints.Width - (int)(bitmap.Width * scaleFactor)) / 2;
            offsetY = (pictureBoxPoints.Height - (int)(bitmap.Height * scaleFactor)) / 2;
        }
        else if (pictureBoxPoints.SizeMode == PictureBoxSizeMode.StretchImage)
        {
            scaleFactor = 1;
            xFactor = yFactor = 1;
        }

        e.Graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;

        foreach (System.Drawing.Point point in _dotPoints)
        {
            int ellipseSize = (int)Math.Round(5 * scaleFactor);
            int ellipseX = offsetX + (int)Math.Round(point.X * xFactor) - ellipseSize / 2;
            int ellipseY = offsetY + (int)Math.Round(point.Y * yFactor) - ellipseSize / 2;

            e.Graphics.FillEllipse(Brushes.Red, new Rectangle(ellipseX, ellipseY, ellipseSize, ellipseSize));
        }
    }
}

内容的提问来源于stack exchange，提问作者Daniel Lip