C# 圖片放大模糊問題解決：插值模式與像素偏移修正

問題現象

使用 Graphics.DrawImage 放大圖片時，預設插值模式為 InterpolationMode.Default (雙三次插值)，導致：

• 邊緣模糊、鋸齒明顯
• 像素風格圖片 (像素藝術、遊戲素材) 失去銳利邊緣
• 放大後圖片左上角缺少半個像素 (座標偏移問題)

解決方案：最近鄰居插值 + 半像素偏移

using System.Drawing;
using System.Drawing.Drawing2D;

public static Bitmap ScaleImageNearestNeighbor(Image source, int scaleFactor) {
    int newWidth = source.Width * scaleFactor;
    int newHeight = source.Height * scaleFactor;
    
    Bitmap result = new Bitmap(newWidth, newHeight);
    using (Graphics g = Graphics.FromImage(result)) {
        // 關鍵 1：最近鄰居插值 - 保留硬邊緣，適合像素圖
        g.InterpolationMode = InterpolationMode.NearestNeighbor;
        // 關鍵 2：半像素偏移 - 修正左上角缺失半像素問題
        g.PixelOffsetMode = PixelOffsetMode.Half;
        
        // 可選：關閉抗鋸齒以獲得純淨像素風格
        g.SmoothingMode = SmoothingMode.None;
        
        g.DrawImage(source, 0, 0, newWidth, newHeight);
    }
    return result;
}

插值模式比較

像素偏移模式說明

完整封裝工具類別

public static class ImageScaler {
    public enum ScaleMode {
        PixelArt,      // 像素風格：NearestNeighbor + Half
        Photo,         // 照片：HighQualityBicubic
        Fast,          // 快速：Bilinear
    }

    public static Bitmap Scale(Image source, int targetWidth, int targetHeight, ScaleMode mode = ScaleMode.Photo) {
        Bitmap result = new Bitmap(targetWidth, targetHeight);
        using (Graphics g = Graphics.FromImage(result)) {
            switch (mode) {
                case ScaleMode.PixelArt:
                    g.InterpolationMode = InterpolationMode.NearestNeighbor;
                    g.PixelOffsetMode = PixelOffsetMode.Half;
                    g.SmoothingMode = SmoothingMode.None;
                    break;
                case ScaleMode.Photo:
                    g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                    g.PixelOffsetMode = PixelOffsetMode.HighQuality;
                    g.SmoothingMode = SmoothingMode.HighQuality;
                    g.CompositingQuality = CompositingQuality.HighQuality;
                    break;
                case ScaleMode.Fast:
                    g.InterpolationMode = InterpolationMode.Bilinear;
                    break;
            }
            g.DrawImage(source, 0, 0, targetWidth, targetHeight);
        }
        return result;
    }

    // 等比例縮放
    public static Bitmap ScaleByFactor(Image source, int factor, ScaleMode mode = ScaleMode.Photo) {
        return Scale(source, source.Width * factor, source.Height * factor, mode);
    }
}

使用範例

// 像素圖放大 8 倍
using (Image pixelArt = Image.FromFile("sprite.png")) {
    Bitmap scaled = ImageScaler.ScaleByFactor(pixelArt, 8, ImageScaler.ScaleMode.PixelArt);
    scaled.Save("sprite_8x.png", ImageFormat.Png);
}

// 照片高品質放大 2 倍
using (Image photo = Image.FromFile("photo.jpg")) {
    Bitmap scaled = ImageScaler.ScaleByFactor(photo, 2, ImageScaler.ScaleMode.Photo);
    scaled.Save("photo_2x.jpg", ImageFormat.Jpeg);
}