使用位图工具裁剪和调整图像大小

 public class BitmapTools
    {
        /// <summary>
        /// Gets the cropped bitmap asynchronously.
        /// </summary>
        /// <param name="originalImage">The original image.</param>
        /// <param name="startPoint">The start point.</param>
        /// <param name="cropSize">Size of the corp.</param>
        /// <param name="scale">The scale.</param>
        /// <returns>The cropped image.</returns>
        public static async Task<WriteableBitmap> GetCroppedBitmapAsync(IRandomAccessStream originalImage,
            Point startPoint, Size cropSize, double scale)
        {
            if (double.IsNaN(scale) || double.IsInfinity(scale))
            {
                scale = 1;
            }

            // Convert start point and size to integer.
            var startPointX = (uint)Math.Floor(startPoint.X * scale);
            var startPointY = (uint)Math.Floor(startPoint.Y * scale);
            var height = (uint)Math.Floor(cropSize.Height * scale);
            var width = (uint)Math.Floor(cropSize.Width * scale);

            // Create a decoder from the stream. With the decoder, we can get 
            // the properties of the image.
            var decoder = await BitmapDecoder.CreateAsync(originalImage);

            // The scaledSize of original image.
            var scaledWidth = (uint)Math.Floor(decoder.PixelWidth * scale);
            var scaledHeight = (uint)Math.Floor(decoder.PixelHeight * scale);

            // Refine the start point and the size. 
            if (startPointX + width > scaledWidth)
            {
                startPointX = scaledWidth - width;
            }

            if (startPointY + height > scaledHeight)
            {
                startPointY = scaledHeight - height;
            }

            // Get the cropped pixels.
            var pixels = await GetPixelData(decoder, startPointX, startPointY, width, height,
                scaledWidth, scaledHeight);

            // Stream the bytes into a WriteableBitmap
            var cropBmp = new WriteableBitmap((int)width, (int)height);
            var pixStream = cropBmp.PixelBuffer.AsStream();
            pixStream.Write(pixels, 0, (int)(width * height * 4));

            return cropBmp;
        }

        /// <summary>
        /// Gets the pixel data.
        /// </summary>
        /// <remarks>
        /// If you want to get the pixel data of a scaled image, set the scaledWidth and scaledHeight
        /// of the scaled image.
        /// </remarks>
        /// <param name="decoder">The bitmap decoder.</param>
        /// <param name="startPointX">The X coordinate of the start point.</param>
        /// <param name="startPointY">The Y coordinate of the start point.</param>
        /// <param name="width">The width of the source rect.</param>
        /// <param name="height">The height of the source rect.</param>
        /// <param name="scaledWidth">The desired width.</param>
        /// <param name="scaledHeight">The desired height.</param>
        /// <returns>The image data.</returns>
        private static async Task<byte[]> GetPixelData(BitmapDecoder decoder, uint startPointX, uint startPointY,
            uint width, uint height, uint scaledWidth, uint scaledHeight)
        {
            var transform = new BitmapTransform();
            var bounds = new BitmapBounds();
            bounds.X = startPointX;
            bounds.Y = startPointY;
            bounds.Height = height;
            bounds.Width = width;
            transform.Bounds = bounds;

            transform.ScaledWidth = scaledWidth;
            transform.ScaledHeight = scaledHeight;

            // Get the cropped pixels within the bounds of transform.
            var pix = await decoder.GetPixelDataAsync(
                BitmapPixelFormat.Bgra8,
                BitmapAlphaMode.Straight,
                transform,
                ExifOrientationMode.IgnoreExifOrientation,
                ColorManagementMode.ColorManageToSRgb);
            var pixels = pix.DetachPixelData();
            return pixels;
        }

        /// <summary>
        /// Resizes the specified stream.
        /// </summary>
        /// <param name="sourceStream">The source stream to resize.</param>
        /// <param name="newWidth">The width of the resized image.</param>
        /// <param name="newHeight">The height of the resized image.</param>
        /// <returns>The resized image stream.</returns>
        public static async Task<InMemoryRandomAccessStream> Resize(IRandomAccessStream sourceStream, uint requestedMinSide)
        {
            var decoder = await BitmapDecoder.CreateAsync(sourceStream);
            uint originalPixelWidth = decoder.OrientedPixelWidth;
            uint originalPixelHeight = decoder.OrientedPixelHeight;

            double widthRatio = (double)requestedMinSide / originalPixelWidth;
            double heightRatio = (double)requestedMinSide / originalPixelHeight;
            uint aspectHeight = (uint)requestedMinSide;
            uint aspectWidth = (uint)requestedMinSide;
            var scaledSize = (uint)requestedMinSide;
            if (originalPixelWidth < originalPixelHeight)
            {
                aspectWidth = (uint)(heightRatio * originalPixelWidth);
            }
            else
            {
                aspectHeight = (uint)(widthRatio * originalPixelHeight);
            }

            var destinationStream = new InMemoryRandomAccessStream();

     
            var transform = new BitmapTransform { ScaledWidth = aspectWidth, ScaledHeight = aspectHeight };

            var pixelData = await decoder.GetPixelDataAsync(
                BitmapPixelFormat.Bgra8,
                BitmapAlphaMode.Straight,
                transform,
                ExifOrientationMode.RespectExifOrientation,
                ColorManagementMode.DoNotColorManage);

            var encoder =
                await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, destinationStream);

            if(decoder.OrientedPixelHeight!=decoder.PixelHeight && decoder.OrientedPixelWidth!=decoder.PixelWidth)
            encoder.BitmapTransform.Rotation = BitmapRotation.Clockwise270Degrees;

            encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Premultiplied, aspectWidth, aspectHeight, 96, 96,
                pixelData.DetachPixelData());
            await encoder.FlushAsync();

            return destinationStream;
        }

        /// <summary>
        /// Rotates the given stream.
        /// </summary>
        /// <param name="randomAccessStream">The random access stream.</param>
        /// <param name="rotation">The rotation.</param>
        /// <returns>The stream.</returns>
        public static async Task<InMemoryRandomAccessStream> Rotate(IRandomAccessStream randomAccessStream,
            BitmapRotation rotation)
        {
            var decoder = await BitmapDecoder.CreateAsync(randomAccessStream);

            var rotatedStream = new InMemoryRandomAccessStream();

            var encoder = await BitmapEncoder.CreateForTranscodingAsync(rotatedStream, decoder);

            encoder.BitmapTransform.Rotation = rotation;
            encoder.BitmapTransform.InterpolationMode = BitmapInterpolationMode.Fant;

            await encoder.FlushAsync();

            return rotatedStream;
        }

        /// <summary>
        /// Resizes and crops source file image so that resized image width/height are not larger than <param name="requestedMinSide"></param>
        /// </summary>
        /// <param name="sourceFile">Source StorageFile</param>
        /// <param name="requestedMinSide">Width/Height of the output image</param>
        /// <param name="resizedImageFile">Target StorageFile</param>
        /// <returns></returns>
        public static async Task<IStorageFile> CreateThumbnaiImage(StorageFile sourceFile, int requestedMinSide, StorageFile resizedImageFile)
        {
            var imageStream = await sourceFile.OpenReadAsync();
            var decoder = await BitmapDecoder.CreateAsync(imageStream);
            var originalPixelWidth = decoder.PixelWidth;
            var originalPixelHeight = decoder.PixelHeight;
            using (imageStream)
            {
                //do resize only if needed
                if (originalPixelHeight > requestedMinSide && originalPixelWidth > requestedMinSide)
                {
                    using (var resizedStream = await resizedImageFile.OpenAsync(FileAccessMode.ReadWrite))
                    {
                        //create encoder based on decoder of the source file
                        var encoder = await BitmapEncoder.CreateForTranscodingAsync(resizedStream, decoder);
                        double widthRatio = (double)requestedMinSide / originalPixelWidth;
                        double heightRatio = (double)requestedMinSide / originalPixelHeight;
                        uint aspectHeight = (uint)requestedMinSide;
                        uint aspectWidth = (uint)requestedMinSide;
                        uint cropX = 0, cropY = 0;
                        var scaledSize = (uint)requestedMinSide;
                        if (originalPixelWidth > originalPixelHeight)
                        {
                            aspectWidth = (uint)(heightRatio * originalPixelWidth);
                            cropX = (aspectWidth - aspectHeight) / 2;
                        }
                        else
                        {
                            aspectHeight = (uint)(widthRatio * originalPixelHeight);
                            cropY = (aspectHeight - aspectWidth) / 2;
                        }
                        //you can adjust interpolation and other options here, so far linear is fine for thumbnails
                        encoder.BitmapTransform.InterpolationMode = BitmapInterpolationMode.Linear;
                        encoder.BitmapTransform.ScaledHeight = aspectHeight;
                        encoder.BitmapTransform.ScaledWidth = aspectWidth;
                        encoder.BitmapTransform.Bounds = new BitmapBounds()
                        {
                            Width = scaledSize,
                            Height = scaledSize,
                            X = cropX,
                            Y = cropY,
                        };
                        await encoder.FlushAsync();
                    }
                }
                else
                {
                    //otherwise just use source file as thumbnail
                    await sourceFile.CopyAndReplaceAsync(resizedImageFile);
                }
            }
            return resizedImageFile;
        }

    }