Question

I’m making a simple drawing app on Android. I’m using the FingerPaint.java provided with the ApiDemos sample as a base, adding some features and getting rid of others. It basically creates a canvas with a bitmap that you can draw on top.

I’ve added “open” and “save” features. The “save” feature stores the bitmap into a file using the getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES) function, and the “open” feature launches startActivityForResult(photoPickerIntent, 1) and loads the selected image.

The problem is that I can’t draw on a bitmap that I’ve opened manually. A trace is shown when drawing with the finger, but once the finger is lifted from the screen the trace disappears. If I save this bitmap, it will show up completely unmodified in the gallery. But when starting the app, I can draw just fine on the canvas.

I’m pretty sure this is a very simple problem due to my lack of understanding of how Canvas and Bitmap work. Anyway, here’s my code.

When creating my custom view:

    public MyView(Context c){

        super(c);

        mBitmap = Bitmap.createBitmap(480, 800, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);
        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);

    }

The OnDraw Override:

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(0xFFAAAAAA);
        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
        canvas.drawPath(mPath, mPaint);
    }

My “open” function:

    private void open(){
    //checkSD: 1 if read only, 2 if RW
    if(checkSD()==1 || checkSD()==2){
        Intent photoPickerIntent = new Intent(Intent.ACTION_PICK);
        photoPickerIntent.setType("image/*");
        startActivityForResult(photoPickerIntent, 1);
    }
}

And finally my onActivityResult:

    @Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    //RequestCode:1 image
    if (requestCode == 1) {
        if (resultCode == Activity.RESULT_OK) {
            Uri selectedImage = data.getData();
            try{
                InputStream fIn = getContentResolver().openInputStream(selectedImage);
                mBitmap = BitmapFactory.decodeStream(fIn);
                fIn.close();
            } catch (Exception e){
                e.printStackTrace();
            }

        }
    }
}

I think my error is in this code, but if anyone thinks it could be in the way I actually draw, I haven’t modified the touch_xxx and the onTouchEvent from the FingerPaint.java example, so you can take a look at it (or ask me to post it here if you can’t see it for some reason).

Thanks!

Answer 1

Ok, solved it!

I did 2 things. The first one was adding the following line right after “fIn.close();” inside onActivityResult:

mCanvas = new Canvas(mBitmap);

I’m still not quite sure why this is necessary but I know that it doesn’t work without this line.

The 2nd thing I did was copying the bitmap I was loading. Turns out that BitmapFactory.decodeStream() returns an inmutable bitmap and in order to create a new canvas a mutable bitmap is needed. I found a good explanation in this blog post:

http://sudarnimalan.blogspot.com.es/2011/09/android-convert-immutable-bitmap-into.html

I used the second method, which is copying the bitmap into a new one, like this:

mBitmap = mBitmap.copy(Config.ARGB_8888, true);

I know from the post that this is not a safe method, but for now it works.

If only I had used “inmutable bitmap” in my search I’d have come up with an answer by myself. Thank you anyways!