How To Crop Bitmap For UWP

裁剪图片主要是借助于 BitmapDecoder.GetPixelDataAsync() 以及 BitmapTransform对象来实现。

实现的代码如下：

 using System;
 using System.Threading.Tasks;
 using Windows.Storage.Streams;
 using Windows.Foundation;
 using Windows.Graphics.Imaging;

 namespace ScanQRCode
 {
     public static class ImageHelper
     {
         /// <summary>
         /// Asynchronously get the cropped stream.
         /// </summary>
         /// <param name="inputStream">The input stream</param>
         /// <param name="rect"></param>
         /// <returns></returns>
         public static async Task<IRandomAccessStream> GetCroppedStreamAsync(IRandomAccessStream inputStream, Rect rect)
         {
             if (inputStream == null)
                 return null;

             var startPointX = (uint)Math.Floor(rect.X);
             var startPointY = (uint)Math.Floor(rect.Y);
             var width = (uint)Math.Floor(rect.Width);
             var height = (uint)Math.Floor(rect.Height);

             return await GetCroppedStreamAsync(inputStream, startPointX, startPointY, width, height);
         }

         /// <summary>
         /// Asynchronously get the cropped stream.
         /// </summary>
         /// <param name="inputStream">The input stream</param>
         /// <param name="x"></param>
         /// <param name="y"></param>
         /// <param name="width"></param>
         /// <param name="height"></param>
         /// <returns></returns>
         public static async Task<IRandomAccessStream> GetCroppedStreamAsync(IRandomAccessStream inputStream, uint x, uint y, uint width, uint height)
         {
             if (inputStream == null)
                 return null;

             var pixelData = await GetCroppedPixelDataAsync(inputStream, x, y, width, height);
             if (pixelData == null)
                 return null;

             var outputStream = new InMemoryRandomAccessStream();
             var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, outputStream);
             encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore, width, height, , , pixelData);
             await encoder.FlushAsync();

             return outputStream;
         }

         /// <summary>
         /// Asynchronously get the cropped pixel data.
         /// </summary>
         /// <param name="inputStream">The input stream</param>
         /// <param name="rect"></param>
         /// <returns></returns>
         public static async Task<byte[]> GetCroppedPixelDataAsync(IRandomAccessStream inputStream, Rect rect)
         {
             if (inputStream == null)
                 return null;

             var startPointX = (uint)Math.Floor(rect.X);
             var startPointY = (uint)Math.Floor(rect.Y);
             var width = (uint)Math.Floor(rect.Width);
             var height = (uint)Math.Floor(rect.Height);

             return await GetCroppedPixelDataAsync(inputStream, startPointX, startPointY, width, height);
         }

         /// <summary>
         /// Asynchronously get the cropped pixel data.
         /// </summary>
         /// <param name="inputStream">The input stream</param>
         /// <param name="x"></param>
         /// <param name="y"></param>
         /// <param name="width"></param>
         /// <param name="height"></param>
         /// <returns></returns>
         public static async Task<byte[]> GetCroppedPixelDataAsync(IRandomAccessStream inputStream, uint x, uint y, uint width, uint height)
         {
             if (inputStream == null)
                 return null;

             var decoder = await BitmapDecoder.CreateAsync(inputStream);

             // Refine the start point
             if (x + width > decoder.PixelWidth)
                 x = decoder.PixelWidth - width;
             if (y + height > decoder.PixelHeight)
                 y = decoder.PixelHeight - height;

             var transform = new BitmapTransform()
             {
                 Bounds = new BitmapBounds
                 {
                     X = x,
                     Y = y,
                     Width = width,
                     Height = height
                 },
                 InterpolationMode = BitmapInterpolationMode.Fant,
                 ScaledWidth = decoder.PixelWidth,
                 ScaledHeight = decoder.PixelHeight
             };

             var pixelProvider = await decoder.GetPixelDataAsync(decoder.BitmapPixelFormat, decoder.BitmapAlphaMode, transform,
                                                      ExifOrientationMode.RespectExifOrientation, ColorManagementMode.ColorManageToSRgb);

             return pixelProvider.DetachPixelData();
         }
     }
 }