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Yet another way to create a mutable bitmap (Xamarin Android)


Current API provided by Android Bitmap doesn’t provide a constructor to create a mutable bitmap object, however you can make it mutable by copying the existing one with a mutable flag set.

Bitmap bitmap = new Bitmap(100, 100, Bitmap.Config.Argb8888);
Bitmap mutableBitmap = bm.Copy(Bitmap.Config.Argb8888, true);

Another way is to use a BitmapFactory and create bitmap object from stream or byte array with a mutable flag set.So for byte array it goes like this:

BitmapFactory.Options options = new BitmapFactory.Options();
options.InMutable = true;
options.InPreferredConfig = Bitmap.Config.Argb8888;
Bitmap result = BitmapFactory.DecodeByteArray(imageData, 0, imageData.Length, options);

But imageData is required to contain an encoded image not the raw pixel data. One of the solutions here that doesn’t require bitmap copying or reading from disk is to provide BitmapFactory with some data it’s able to recognize as an image and therefore decode.

The code

/// <summary>
/// A bitmap helper class by Apitron.
/// </summary>
static class BitmapHelper
    /// <summary>
    /// Creates mutable bitmap with ARGB32 pixel format.
    /// </summary>
    /// <param name="width">The width of the bitmap.</param>
    /// <param name="height">The height of the bitmap.</param>   
    /// <returns>Created bitmap.</returns>
    public static Bitmap CreateMutableBitmapARGB32(int width, int height)
        // pixel data
        uint pixelDataSize = (uint) (width*height*4);

        byte[] imageData = new byte[pixelDataSize+54];

        // "BM" +2
        imageData[0] = 0x42;
        imageData[1] = 0x4D;

        // bitmapsize +4; pixeldata size + bitmap file header(14 bytes) + bitmapinfo header(40 bytes)
        Array.Copy(BitConverter.GetBytes(pixelDataSize+54), 0, imageData, 2, 4);

        // reserved +4
        imageData[6] = 0;
        imageData[7] = 0;
        imageData[8] = 0;
        imageData[9] = 0;

        // pixel offset +4
        imageData[10] = 54;
        imageData[11] = 0;
        imageData[12] = 0;
        imageData[13] = 0;

        // header size offset +4
        imageData[14] = 40;
        imageData[15] = 0;
        imageData[16] = 0;
        imageData[17] = 0;

        // bitmapWidth +4
        Array.Copy(BitConverter.GetBytes(width), 0, imageData, 18, 4);

        // bitmapHeight +4
        Array.Copy(BitConverter.GetBytes(height), 0, imageData, 22, 4);

        // planes +2
        imageData[26] = 1;
        imageData[27] = 0;

        // bitcount +2
        imageData[28] = 32;
        imageData[29] = 0;

        // compression +4, BI_RGB
        imageData[30] = 0;
        imageData[31] = 0;
        imageData[32] = 0;
        imageData[33] = 0;

        // pixel data size +4
        Array.Copy(BitConverter.GetBytes(pixelDataSize), 0, imageData, 34, 4);

        // dpiX  +4
        Array.Copy(BitConverter.GetBytes(2835), 0, imageData, 38, 4);

        // dpiY +4
        Array.Copy(BitConverter.GetBytes(2835), 0, imageData, 42, 4);

        // clrUsed +4
        imageData[46] = 0;
        imageData[47] = 0;
        imageData[48] = 0;
        imageData[49] = 0;

        // clr important +4
        imageData[50] = 0;
        imageData[51] = 0;
        imageData[52] = 0;
        imageData[53] = 0;

        BitmapFactory.Options options = new BitmapFactory.Options();
        options.InMutable = true;
        options.InPreferredConfig = Bitmap.Config.Argb8888;
        Bitmap result = BitmapFactory.DecodeByteArray(imageData, 0, imageData.Length, options);
        result.HasAlpha = true;

        return result;

So it just creates a byte array sized to fit the requested bitmap data and adds BITMAPFILEHEADER and BITMAPINFOHEADER to make it look like an image in BMP format for BitmapFactory. Hope it will help someone looking for a mutable bitmap.

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