android Paint.setShadowLayer() vs Canvas.drawbitmap() problems

Question

I use the following code to draw a bitmap during onDraw of a custom View

Paint p = new Paint(Paint.ANTI_ALIAS_FLAG|Paint.FILTER_BITMAP_FLAG);
p.setShadowLayer(3,3,3,0xff000000);
Bitmap bmp = BitmapFactory.decodeResource(some drawable jpg);
canvas.drawBitmap(bmp,null,new Rect(blah blah),p);

and the shadow looks funny. What do I mean funny? I mean that I get a different shadow then if I was calling canvas.drawRect(). It looks sorta like the android shadow code is treating the bitmap as an alpha channel or something.

have tried adding p.setColor(0xff000000); and p.setStyle(Paint.Style.FILL); but not difference.

I guess I could drawRect with shadow on, then drawBitmap with shadow off, but that seems silly as it would be rendering pixels twice.

Answer 1

Basically the shadow layer doesn't work for anything except text. It's real dumb. Check the Android hardware acceleration supported operations chart.

This question was asked back in 2010 when hardware accelerated view trees didn't exist.. based on things I've read, even then the shadow layer only worked for simple shapes (if at all), and others got weird results using it on anything but text. You might be out of luck.

Finally, there are many ways you can fake a shadow layer. You can wrap a view in another view and draw underneath it. You can write a view that draws the shadow yourself as a radial gradient, etc. But you probably just want to make a 9patch that looks like a shadow and use that.

2018 Update

It looks like most of the operations are supported as of api 28! I haven't had a chance to play around with shadow layers recently, but things are looking much more optimistic now.