C# 圖片處理三大實戰技巧：高清縮略圖、智能裁剪、文字水印

編輯：關於C語言

純 .NET Framework 實現，無第三方依賴。三大高頻圖片處理場景，代碼可直接落地項目。

1. 高清縮略圖：比原圖更銳利的秘密

常規縮放會模糊，關鍵在於 InterpolationMode.HighQualityBicubic + CompositingQuality.HighQuality + SmoothingMode.HighQuality 三重保障：

public static Image GetHighQualityThumbnail(Image source, int maxWidth, int maxHeight) {
    var destRect = new Rectangle(0, 0, maxWidth, maxHeight);
    var destImage = new Bitmap(maxWidth, maxHeight);

    destImage.SetResolution(source.HorizontalResolution, source.VerticalResolution);

    using (var graphics = Graphics.FromImage(destImage)) {
        graphics.CompositingMode = CompositingMode.SourceCopy;
        graphics.CompositingQuality = CompositingQuality.HighQuality;
        graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
        graphics.SmoothingMode = SmoothingMode.HighQuality;
        graphics.PixelOffsetMode = PixelOffsetMode.HighQuality;

        using (var wrapMode = new ImageAttributes()) {
            wrapMode.SetWrapMode(WrapMode.TileFlipXY);
            graphics.DrawImage(source, destRect, 0, 0, source.Width, source.Height, GraphicsUnit.Pixel, wrapMode);
        }
    }
    return destImage;
}

權衡：輸出文件可能比原圖大（因無壓縮），建議縮略圖 ≤ 200px 使用；大圖需配合 Encoder 參數壓縮。

2. 智能裁剪：以中心為基準，最大面積保留 + 等比縮放

算法：原圖中心為裁剪中心 → 取最大可能裁剪區域 (目標長寬比) → 等比縮放至目標尺寸。

public static Image SmartCrop(Image source, int targetWidth, int targetHeight) {
    double targetRatio = (double)targetWidth / targetHeight;
    double sourceRatio = (double)source.Width / source.Height;

    int cropWidth, cropHeight, cropX, cropY;

    if (sourceRatio > targetRatio) { // 原圖更寬 → 裁剪左右
        cropHeight = source.Height;
        cropWidth = (int)(cropHeight * targetRatio);
        cropX = (source.Width - cropWidth) / 2;
        cropY = 0;
    } else { // 原圖更高 → 裁剪上下
        cropWidth = source.Width;
        cropHeight = (int)(cropWidth / targetRatio);
        cropX = 0;
        cropY = (source.Height - cropHeight) / 2;
    }

    var cropRect = new Rectangle(cropX, cropY, cropWidth, cropHeight);
    var result = new Bitmap(targetWidth, targetHeight);

    using (var g = Graphics.FromImage(result)) {
        g.CompositingQuality = CompositingQuality.HighQuality;
        g.InterpolationMode = InterpolationMode.HighQualityBicubic;
        g.DrawImage(source, new Rectangle(0, 0, targetWidth, targetHeight), cropRect, GraphicsUnit.Pixel);
    }
    return result;
}

效果對比（原圖 256×192 → 目標 100×100/90×120/120×90）：始終保留主體核心區域，無拉伸變形。

3. 文字水印：靈活定位、透明度、字體全可控

public static Image AddTextWatermark(Image source, string text, Font font, Color color, 
                                     float opacity = 0.5f, ContentAlignment alignment = ContentAlignment.BottomRight,
                                     int margin = 10) {
    var result = new Bitmap(source.Width, source.Height);
    using (var g = Graphics.FromImage(result)) {
        g.DrawImage(source, 0, 0);
        g.TextRenderingHint = TextRenderingHint.AntiAliasGridFit;

        var brush = new SolidBrush(Color.FromArgb((int)(opacity * 255), color));
        var textSize = g.MeasureString(text, font);

        float x = alignment switch {
            ContentAlignment.TopLeft => margin,
            ContentAlignment.TopCenter => (source.Width - textSize.Width) / 2,
            ContentAlignment.TopRight => source.Width - textSize.Width - margin,
            ContentAlignment.MiddleLeft => margin,
            ContentAlignment.MiddleCenter => (source.Width - textSize.Width) / 2,
            ContentAlignment.MiddleRight => source.Width - textSize.Width - margin,
            ContentAlignment.BottomLeft => margin,
            ContentAlignment.BottomCenter => (source.Width - textSize.Width) / 2,
            _ => source.Width - textSize.Width - margin // BottomRight
        };

        float y = alignment switch {
            ContentAlignment.TopLeft or ContentAlignment.TopCenter or ContentAlignment.TopRight => margin,
            ContentAlignment.MiddleLeft or ContentAlignment.MiddleCenter or ContentAlignment.MiddleRight => (source.Height - textSize.Height) / 2,
            _ => source.Height - textSize.Height - margin
        };

        g.DrawString(text, font, brush, x, y);
    }
    return result;
}

進階擴展：支援圖片水印、對角線平鋪、旋轉文字、陰影效果。

完整調用示例

using (var original = Image.FromFile("photo.jpg")) {
    // 1. 高清縮略圖
    var thumb = GetHighQualityThumbnail(original, 200, 200);
    thumb.Save("thumb.jpg", ImageFormat.Jpeg);

    // 2. 智能裁剪為正方形頭像
    var avatar = SmartCrop(original, 100, 100);
    avatar.Save("avatar.jpg", ImageFormat.Jpeg);

    // 3. 加水印後保存
    var watermarked = AddTextWatermark(original, "© 2024 MyApp", 
        new Font("Microsoft YaHei", 24), Color.White, 0.4f);
    watermarked.Save("watermarked.jpg", ImageFormat.Jpeg);
}