android：如何旋转位图前景而不旋转背景

Question

I have a project that needs to rotate a bitmap, the bitmap is in GRAYSCALE/Black and White mode. When I rotate that bitmap, it works but the background (the white part of the bitmap) is rotated too. Is there anyway to rotate just the foreground (the non white part of the bitmap) only? or if I must create a new function to do that, can you explain the algorithm?

Thanks for your help..

I edit for the relevant code :

int i;
        InputStream inStream = context.getResources().openRawResource(R.raw.register2);
        BufferedInputStream bis = new BufferedInputStream(inStream, 8000);

        DataInputStream dis=new DataInputStream(bis);
        byte[] music=null;
        try {
            music = new byte[inStream.available()];
            i=0;
            while(dis.available()>0)
            {
                music[i]=dis.readByte();
                i++;
            }

            dis.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        Bitmap bm = Bitmap.createBitmap(260, 300, Bitmap.Config.ARGB_8888);
        ByteBuffer byteBuff=ByteBuffer.allocate(260*300);
        bm.setHasAlpha(false);
        if(false)
            byteBuff.put(music);
         int[] intbuffer = new int[260*300];
         for (int x=0; x<intbuffer.length; ++x)
            intbuffer[x] = (int) music[x];
        bm.setPixels(intbuffer, 0, 260, 0, 0, 260, 300);
        //img1.setImageBitmap(bm);
        //Canvas canvas=new Canvas(bm);
        Matrix matrix=new Matrix();
        matrix.postRotate(-90);
        Bitmap rotated=Bitmap.createBitmap(bm, 0, 0, 260, 300, matrix, true);
        //matrix.setRotate(4,bm.getWidth()/2,bm.getHeight()/2);
        //canvas.drawBitmap(bm, matrix, new Paint());
        img1.setImageBitmap(rotated);

Answer 1

Given that your foreground and background pixels are diff in color- You need to extract/separate out your foreground pixels and background pixels in two different bitmaps. The two new bitmaps that you create should only have the pixels extracted from your original bitmap rest everything should be transparent.

Once you have your foreground and background bitmap separated , just rotate your foreground and then merge back.