How to create a 'fog of war' bitmap overlay in Android

I have a simple 2D game with a handful of characters which move around the screen. I'd like to create a fog of war, whereby only the background around each character is visible.

Hopefully that's clear (not too foggy?!?!). My thoughts were to create a black bitmap, then draw a 'transparent' disk centered on each of the characters. Then I can draw the background, draw the characters and finally draw this overlay bitmap over the top.

Just not su开发者_如何学Pythonre how to draw the 'transparent' disks on to the black bitmap.

Any ideas, or alternative designs?

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Use Canvas.drawCircle() with a Paint object whose alpha is set to 0. Alternatively you could create a transparent circle with a Bitmap and draw it just like another bitmap. The Bitmap based circle would be more advantageous if you wanted strong control with a fade to black.

1. Create a bitmap to hold the black overlay
2. Fill bitmap with black
3. Paint a transparent circle over it of where you whant to punch holes
4. Paint the overlay over your game canvas
5. Repeat.

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Actually, since the fog-of-war needs to be persistent, I've had to go for an opaque tiled solution, rather than a straight bitmap, as the bitmap was too large and took too long to load/save between activations.