Transparency with JPEGs done right

A few months ago, Square published a great article on Transparency with JPEGs on Android. It's definitely worth reading! Just don't use the provided code yet :) .

Romain Guy suggested in the comments that you can do this in a much more efficient and simpler way, either by using a BitmapShader or by playing with Porter-Duff blending modes.

Using a bitmap shader is great for dynamic masks. To apply a static mask to a bitmap loaded from a JPEG, Porter-Duff is the way to go, as we will see in this article.

Principle

We just need to load the bitmap, and then draw the mask on top of it with the right Porter-Duff mode.

Here is what the Android documentation has to say about the different modes:

  • ADD Saturate(S + D)
  • CLEAR [0, 0]
  • DARKEN [Sa + Da - SaDa, Sc(1 - Da) + Dc*(1 - Sa) + min(Sc, Dc)]
  • DST [Da, Dc]
  • DST_ATOP [Sa, Sa * Dc + Sc * (1 - Da)]
  • DST_IN [Sa * Da, Sa * Dc]
  • DST_OUT [Da * (1 - Sa), Dc * (1 - Sa)]
  • DST_OVER [Sa + (1 - Sa)Da, Rc = Dc + (1 - Da)Sc]
  • LIGHTEN [Sa + Da - SaDa, Sc(1 - Da) + Dc*(1 - Sa) + max(Sc, Dc)]
  • MULTIPLY [Sa * Da, Sc * Dc]
  • OVERLAY
  • SCREEN [Sa + Da - Sa * Da, Sc + Dc - Sc * Dc]
  • SRC [Sa, Sc]
  • SRC_ATOP [Da, Sc * Da + (1 - Sa) * Dc]
  • SRC_IN [Sa * Da, Sc * Da]
  • SRC_OUT [Sa * (1 - Da), Sc * (1 - Da)]
  • SRC_OVER [Sa + (1 - Sa)Da, Rc = Sc + (1 - Sa)Dc]
  • XOR [Sa + Da - 2 * Sa * Da, Sc * (1 - Da) + (1 - Sa) * Dc]

Crystal clear! Hopefully the Xfermodes example in the API Demos demonstrates what the different modes do:

Porter-Duff

The yellow circle is the destination bitmap and the blue rectangle is the source bitmap. The destination is drawn normally, then the source is drawn on top of that using the given Porter-Duff mode.

To apply the alpha mask, we will therefore use DST_IN.

Applying the mask

Let's say we have a nice JPEG:

Golden Gate

And a PNG that we want to use as a mask:

Troll Face

First, we load the bitmap:

BitmapFactory.Options options = new BitmapFactory.Options();
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
  // Starting with Honeycomb, we can load the bitmap as mutable.
  options.inMutable = true;
}
// We could also use ARGB_4444, but not RGB_565 (we need an alpha layer).
options.inPreferredConfig = Bitmap.Config.ARGB_8888;
Resources res = getResources();
Bitmap source = BitmapFactory.decodeResource(res, R.drawable.golden_gate, options);
Bitmap bitmap;
if (source.isMutable()) {
  bitmap = source;
} else {
  bitmap = source.copy(Bitmap.Config.ARGB_8888, true);
  source.recycle();
}
// The bitmap is opaque, we need to enable alpha compositing.
bitmap.setHasAlpha(true);

Next we draw the mask on the bitmap with a canvas, using the DST_IN Porter-Duff mode:

Canvas canvas = new Canvas(bitmap);
Bitmap mask = BitmapFactory.decodeResource(res, R.drawable.troll_face);
Paint paint = new Paint();
paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
canvas.drawBitmap(mask, 0, 0, paint);
// We do not need the mask bitmap anymore.
mask.recycle();

Finally we just use the bitmap:

ImageView trollFace = (ImageView) findViewById(R.id.troll_face);
trollFace.setImageBitmap(bitmap);

Result

There you go, Troll Face Golden Gate! Troll Face

Here is a helper method to do this all at once:

public static Bitmap getMaskedBitmap(Resources res, int sourceResId, int maskResId) {
  BitmapFactory.Options options = new BitmapFactory.Options();
  if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
    options.inMutable = true;
  }
  options.inPreferredConfig = Bitmap.Config.ARGB_8888;
  Bitmap source = BitmapFactory.decodeResource(res, sourceResId, options);
  Bitmap bitmap;
  if (source.isMutable()) {
    bitmap = source;
  } else {
    bitmap = source.copy(Bitmap.Config.ARGB_8888, true);
    source.recycle();
  }
  bitmap.setHasAlpha(true);
  Canvas canvas = new Canvas(bitmap);
  Bitmap mask = BitmapFactory.decodeResource(res, maskResId);
  Paint paint = new Paint();
  paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
  canvas.drawBitmap(mask, 0, 0, paint);
  mask.recycle();
  return bitmap;
}

Update

If you want to understand Porter-Duff blending modes in details, you should read this great article: Porter/Duff Compositing and Blend Modes (special thanks to Cyril Mottier for the link).

Comments

DreamingInDroids

Brilliant! Thanks for the pictures demonstrating the different Porter-Duff modes! I had been wondering about them for a long time!