使用 C# System.Drawing 生成缩略图时居中裁剪图像

Posted 2023-03-29

【中文标题】使用 C# System.Drawing 生成缩略图时居中裁剪图像

【英文标题】：Center crop image while using C# System.Drawing to generate thumbnail

【发布时间】：2018-11-16 00:09:23

【问题描述】：

我有一个场景，我使用 System.Drawing 通过将原始图像的大小调整为提供的大小（正在使用的大小）来生成缩略图。如果源图像是矩形，则生成的缩略图需要是方形的，而不会破坏图像（拉伸它）。到目前为止，我有以下代码：

//Obtain original image from input stream
using (var sourceImage = new Bitmap(Image.FromStream(inStream)))

    //Setting thumbnail aspect ratio based on source image
    int destWidth, destHeight;

    if (sourceImage.Width > sourceImage.Height)
    
        destWidth = providedSize;
        destHeight = Convert.ToInt32(sourceImage.Height * providedSize/ (double)sourceImage.Width);
    
    else
    
        destWidth = Convert.ToInt32(sourceImage.Width * providedSize/ (double)sourceImage.Height);
        destHeight = providedSize;
    

    //Initialize thumbnail bitmap
    var thumbnail = new Bitmap(destWidth, destHeight);

    //Create thumbnail
    using (var graphics = Graphics.FromImage(thumbnail))
    
        graphics.CompositingQuality = CompositingQuality.HighSpeed;
        graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
        graphics.CompositingMode = CompositingMode.SourceCopy;
        graphics.DrawImage(sourceImage, 0, 0, destWidth, destHeight);

        using (MemoryStream stream = new MemoryStream())
        
            var encoderParameters = new EncoderParameters(1);
            encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, Convert.ToInt64(Environment.GetEnvironmentVariable("ThumbnailQuality")));
            var codecInfo = ImageCodecInfo.GetImageDecoders().FirstOrDefault(c => c.FormatID == ImageFormat.Jpeg.Guid);
            thumbnail.Save(stream, codecInfo, encoderParameters);
            stream.Position = 0;

            //Upload thumbnail
        
    

【参考方案1】：

我已经找到了解决方案，需要将 Graphics.DrawImage 用作可以与目标矩形一起提供计算偏移量的点，以创建完美的中心裁剪方形结果。以下是改进后的代码：

//Obtain original image from input stream
using (var sourceImage = new Bitmap(Image.FromStream(inStream)))

    //Obtain source dimensions and initialize scaled dimensions and crop offsets
    int sourceWidth = sourceImage.Width;
    int sourceHeight = sourceImage.Height;
    int scaledSourceWidth = 0;
    int scaledSourceHeight = 0;
    int offsetWidth = 0;
    int offsetHeight = 0;

    //Calculate cropping offset
    if (sourceWidth > sourceHeight)
    
        offsetWidth = (sourceWidth - sourceHeight) / 2;
        scaledSourceWidth = sourceWidth / (sourceHeight / thumbSize);
        scaledSourceHeight = thumbSize;
    
    else if (sourceHeight > sourceWidth)
    
        offsetHeight = (sourceHeight - sourceWidth) / 2;
        scaledSourceHeight = sourceHeight / (sourceWidth / thumbSize);
        scaledSourceWidth = thumbSize;
    

    //Create new thumbnail image of height and width defined in thumbSize
    Bitmap thumbnail = new Bitmap(thumbSize, thumbSize, PixelFormat.Format24bppRgb);
    thumbnail.SetResolution(sourceImage.HorizontalResolution, sourceImage.VerticalResolution);

    using (var graphics = Graphics.FromImage(thumbnail))
    
        //Draw source image scaled down with aspect ratio maintained onto the thumbnail with the offset
        graphics.CompositingQuality = CompositingQuality.HighSpeed;
        graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
        graphics.CompositingMode = CompositingMode.SourceCopy;
        graphics.DrawImage(sourceImage, new Rectangle(0, 0, scaledSourceWidth, scaledSourceHeight), offsetWidth, offsetHeight, sourceWidth, sourceHeight, GraphicsUnit.Pixel);

        //Push thumbnail onto stream for upload
        using (MemoryStream stream = new MemoryStream())
        
            var encoderParameters = new EncoderParameters(1);
            encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, Convert.ToInt64(Environment.GetEnvironmentVariable("ThumbnailQuality")));
            var codecInfo = ImageCodecInfo.GetImageDecoders().FirstOrDefault(c => c.FormatID == ImageFormat.Jpeg.Guid);
            thumbnail.Save(stream, codecInfo, encoderParameters);
            stream.Position = 0;

            //Upload thumbnail
        
    

欢迎任何改进或优化。