Question

I need to “compute” the resulting bitmap obtained from overlapping two different bitmaps that have an alpha value somewhere between 0 and 255. I need to do this in java code, not xml, because bitmaps are being loaded dinamically and not from resources.

Here is my first try (which always yields a black bitmap…):

private Drawable composeBitmaps(int alpha1, BitmapDrawable bm1, 
                                int alpha2, BitmapDrawable bm2)
{
  Canvas c = new Canvas();
  Bitmap b1 = bm1.getBitmap();
  BitmapDrawable draw1 = new BitmapDrawable(b1.copy(b1.getConfig(), true));
  draw1.setAlpha(alpha1);
  c.setBitmap(draw1.getBitmap());

  Paint p = new Paint();
  p.setAlpha(alpha2);
  c.drawBitmap(bm2.getBitmap(), 0, 0, p);

  return draw1;
}

...

  View v = // whatever
  v.setBackgroundDrawable(composeBitmaps(100, bdrawable1, 150, bdrawable2));

And the view goes black background. What am I doing wrong?

Answer 1

My code above is absolutely correct. The bug that made the bitmap go black was elsewhere. Found it, fixed it, now it works.

The only thing to note is that my code is slow as hell and it cannot be used to crossfade two fullscreen bitmaps at a reasonable fps rate.