如何使用Xamarin Android压缩图像但不调整大小？

Question

I am getting a image from a server which is pretty large around 15MB but i want to keep the Aspect Ratio but compress the size of the file because I am loading multiple files that around the same size? these Image are downloaded as BitMaps and Used SetImageBitmap to display the image

Answer 1

You can do this by converting the image to a jpeg or png. This is quick and dirty implementation of a Bitmap to PNG conversion routine:

public string ResizeImage(string sourceFilePath)
{
    Android.Graphics.Bitmap bmp = Android.Graphics.BitmapFactory.DecodeFile (sourceFilePath);

    string newPath = sourceFilePath.Replace(".bmp", ".png");
    using (var fs = new FileStream (newPath, FileMode.OpenOrCreate)) {
        bmp.Compress (Android.Graphics.Bitmap.CompressFormat.Png, 100, fs);
    }

    return newPath;
}

It makes assumptions on the file extension but that can modified fairly easily.

Here is the full sample I used to verify the compression:

public class MainActivity : Activity
{
    public const string BITMAP_URL = @"http://www.openjpeg.org/samples/Bretagne2.bmp";


    public string ResizeImage(string sourceFilePath)
    {
        Android.Graphics.Bitmap bmp = Android.Graphics.BitmapFactory.DecodeFile (sourceFilePath);

        string newPath = sourceFilePath.Replace(".bmp", ".png");
        using (var fs = new FileStream (newPath, FileMode.OpenOrCreate)) {
            bmp.Compress (Android.Graphics.Bitmap.CompressFormat.Png, 100, fs);
        }

        return newPath;
    }

    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);

        SetContentView (Resource.Layout.Main);

        Button button = FindViewById<Button> (Resource.Id.myButton);

        button.Click += delegate {
            System.Threading.Tasks.Task.Run( () => {
                RunOnUiThread( () => Toast.MakeText(this, "Downloading file", ToastLength.Long).Show());

                string downloadFile = DownloadSourceImage(BITMAP_URL);

                RunOnUiThread( () => Toast.MakeText(this, "Rescaling image: " + downloadFile, ToastLength.Long).Show());

                string convertedFile = ResizeImage(downloadFile);

                var bmpFileSize = (new FileInfo(downloadFile)).Length;
                var pngFileSize = (new FileInfo(convertedFile)).Length;

                RunOnUiThread( () => Toast.MakeText(this, "BMP is " + bmpFileSize + "B. PNG is " + pngFileSize + "B.", ToastLength.Long).Show());
            });
        };
    }

    public string DownloadSourceImage(string url)
    {
        System.Net.WebClient client = new System.Net.WebClient ();

        string fileName = url.Split ('/').LastOrDefault ();
        string downloadedFilePath = Path.Combine(Android.OS.Environment.ExternalStorageDirectory.AbsolutePath, fileName);

        if (File.Exists (downloadedFilePath) == false) {
            client.DownloadFile (url, downloadedFilePath);
        }

        return downloadedFilePath;
    }
}

Answer 2

You can compress the image using other file formats (eg. jpeg). Or you can resize the image while maintaining the aspect ratio.