4 minute read

Previously, I’ve described a basic architecture for accessing and manipulating pixel data from a Xamarin.Forms project. A Xamarin.Forms PCL project defines the IBitmap interface, which specifies the operations that must be implemented in each platform-specific project to access and manipulate pixel data. I then explained the operation of the Bitmap class in the Xamarin.Forms.iOS project, which implements the IBitmap interface.

In this blog post I’ll explain the operation of the Bitmap class in the Xamarin.Forms Android project, which implements the IBitmap interface. The DependencyService can be used to invoke a method from the Xamarin.Forms PCL project, which in turn invokes IBitmap operations.

Disclaimer: The code featured here is alpha code, so all the usual caveats apply.

Implementation

The following code shows the Bitmap class implemented on Android:

 1 public class Bitmap : IBitmap<Android.Graphics.Bitmap>
 2 {
 3     const byte bytesPerPixel = 4;
 4     readonly int height;
 5     readonly int width;
 6     byte[] pixelData;
 7     Android.Graphics.Bitmap bitmap;
 8     string photoFile;
 9 
10     public Bitmap (string photo)
11     {
12         photoFile = photo;
13         var options = new BitmapFactory.Options {
14             InJustDecodeBounds = true
15         };
16 
17         // Bitmap will be null because InJustDecodeBounds = true
18         bitmap = BitmapFactory.DecodeFile (photoFile, options);
19         width = options.OutWidth;
20         height = options.OutHeight;
21     }
22 
23     public void ToPixelArray ()
24     {
25         bitmap = BitmapFactory.DecodeFile (photoFile);
26 
27         int size = width * height * bytesPerPixel;
28         pixelData = new byte[size];
29         var byteBuffer = Java.Nio.ByteBuffer.AllocateDirect (size);
30         bitmap.CopyPixelsToBuffer (byteBuffer);
31         Marshal.Copy (byteBuffer.GetDirectBufferAddress (), pixelData, 0, size);
32         byteBuffer.Dispose ();
33     }
34 
35     public void TransformImage (Func<byte, byte, byte, double> pixelOperation)
36     {
37         byte r, g, b;
38 
39         try {
40             // Pixel data order is RGBA
41             for (int i = 0; i < pixelData.Length; i += bytesPerPixel) {
42                 r = pixelData [i];
43                 g = pixelData [i + 1];
44                 b = pixelData [i + 2];
45 
46                 // Leave alpha value intact
47                 pixelData [i] = pixelData [i + 1] = pixelData [i + 2] = (byte)pixelOperation (r, g, b);
48             }
49         } catch (Exception ex) {
50             Console.WriteLine (ex.Message);
51         }
52     }
53 
54     public Android.Graphics.Bitmap ToImage ()
55     {
56         var byteBuffer = Java.Nio.ByteBuffer.AllocateDirect (width * height * bytesPerPixel);
57         Marshal.Copy (pixelData, 0, byteBuffer.GetDirectBufferAddress (), width * height * bytesPerPixel);
58         bitmap.CopyPixelsFromBuffer (byteBuffer);
59         byteBuffer.Dispose ();
60         return bitmap;
61     }
62 
63     public void Dispose ()
64     {
65         if (bitmap != null) {
66             bitmap.Recycle ();
67             bitmap.Dispose ();
68             bitmap = null;
69         }
70         pixelData = null;
71     }
72 }

The constructor reads and decodes the photo. Setting the InJustDecodeBounds property to true while decoding avoids allocating memory for the bitmap and hence returns null for the bitmap object, but does set the OutWidth, and OutHeight properties. This technique allows you to read the dimensions and type of the image data prior to construction (and memory allocation) of the bitmap.

The ToPixelArray method is used to read the photo to a Android.Graphics.Bitmap and convert it to a byte array of raw pixel data. The image’s pixel data is placed into a continuous sequence of bytes. It’s formatted as 32 bits per pixel, with each pixel being composed of a Red, Green, Blue, and Alpha channel. It’s then copied to the pixelData array. Therefore, the pixelData array stores each pixel over four array elements (R, G, B, A). It’s likely that the pixelData array is not required, and that the pixel data can be accessed directly through the ByteBuffer instance. I’ll be exploring this performance optimization at a later date.

The TransformImage method is used to perform an imaging operation on the pixel data, such as convert to greyscale. The method specifies a Func parameter that’s used to perform a per-pixel operation. The Func is passed from the Xamarin.Forms PCL project into the method, and operates on each pixel component. At the moment the TransformImage method is hard-coded to place the result of the Func into each pixel component. This is purely for my own test purposes, for implementing a ConvertToGreyscale Func. It can easily be modified to not duplicate the result of the Func across all four colour components.

The ToImage method is then used to place the transformed pixel data back into the Android.Graphics.Bitmap. Note that the ToImage method creates a 32 bit image. An overload could be provided to create a 8 bit image.

A Dispose method is also provided to clean up the Android.Graphics.Bitmap instance and byte array of pixels.

The Bitmap class can then be invoked as follows:

 1 public Task<Stream> TransformPhotoAsync (Func<byte, byte, byte, double> pixelOperation)
 2 {
 3     return Task.Run (() => {
 4         var bitmap = new Bitmap (photoPath);
 5         bitmap.ToPixelArray ();
 6         bitmap.TransformImage (pixelOperation);
 7         var androidBitmap = bitmap.ToImage ();
 8 
 9         var memoryStream = new MemoryStream ();
10         androidBitmap.Compress (Android.Graphics.Bitmap.CompressFormat.Jpeg, 80, memoryStream);
11         memoryStream.Seek (0L, SeekOrigin.Begin);
12         bitmap.Dispose ();
13 
14         return (Stream)memoryStream;
15     });
16 }

A new Bitmap instance is created, with the path to the photo being passed into the constructor. The photo is read and converted to an array of pixels, transformed by the pixelOperation Func, and then converted into a new Android.Graphics.Bitmap before being returned for display by a Xamarin.Forms Image control. All of this work is wrapped in a Task.Run in order for it to be executed on a background thread.

Summary

This blog post has described how to access and manipulate pixel data in a Xamarin.Forms Android project, by implementing the Bitmap class. The Bitmap class implements the IBitmap interface that’s provided by the Xamarin.Forms PCL project. My next blog post will explain the implementation of the IBitmap interface in WinRT.

Updated: