Xamarin Android 无法从旋转图像中读取 EXIF 数据以检查图像是否旋转

Question

Description

To check whether the image is rotated or not by reading the EXIF meta data based on the orientation rotate then create the new bitmap and set to ImageView control.

Actual Image EXIF data:

[C#]

    protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            Xamarin.Essentials.Platform.Init(this, savedInstanceState);
            ImageView image = new ImageView(this);
            var bitmap = BitmapFactory.DecodeResource(Resources, Resource.Drawable.test);
            ExifInterface originalMetadata;
            using (var stream = new MemoryStream())
            {
                bitmap.Compress(Bitmap.CompressFormat.Jpeg, 0, stream);
                originalMetadata = new ExifInterface(stream);
            }

            int orientation = GetRotation(originalMetadata);

            Matrix matrix = new Matrix();
            if (orientation != 0)
            {
                matrix.PreRotate(orientation);
                bitmap = Bitmap.CreateBitmap(bitmap, 0, 0, bitmap.Width, bitmap.Height, matrix, true);
            }
            image.SetImageBitmap(bitmap);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.activity_main);
        }

Expected Behavior

To read the rotated image ExifData to get the orientation. Based on the orientation rotate then create the new bitmap and set to ImageView control.

Actual Behavior

Getting the ExifData orientation value as 0.

Basic Information

Tested device are Huawei PRA, Infinix Pro

Environment

Visual Studio 2019

Screenshots

Sample Link

ImageDemo.zip

Is there any solution to resolve this problem?

Answer 1

var bitmap = BitmapFactory.DecodeResource(Resources, Resource.Drawable.test);

Now you have a bitmap.

But bitmaps do not contain an exif header or orientation information.

You compress that bitmap to a stream and then let an Exifinterface read from that stream.

bitmap.Compress(Bitmap.CompressFormat.Jpeg, 0, stream);

originalMetadata = new ExifInterface(stream);

But as the bitmap did not contain exif info or orientation info the stream does not have them too.

So no valid exif info and no orientation information. Nothing.

Instead you should read exif info from the original resource file.

Answer 2

First,as blackapps said above, after you decode the picture for Bitmap,the Exif message will be lost.

Second, Resources/ in android usually refers to resources. ExifInterface only loads files off of the filesystem, not resources. After all, it would be completely pointless for a developer to want to use ExifInterface on a resource. Resources are read-only, so you cannot use ExifInterface to modify EXIF headers in a resource. And rather than wasting users' time, CPU, and battery reading in a read-only image file to extract metadata, developers would do that ahead of time, putting that data somewhere else.

So you could move the test.jpg into Assets folder,and then use your method to get its EXIF data:

var stream = Assets.Open("test.jpg");
originalMetadata = new ExifInterface(stream);
int orientation = GetRotation(originalMetadata);