Hello, I hope this is the right category.

I wanted to implement something like a fog of war for my 2d shooter game. I am using Java 2D graphics.

Currently, what I do is go over each pixel of the screen and compute whether the distance from that pixel to the player is more than some number (200 for example). Then, I just do a & bit operation with a mask which has all of its even bits on. This makes the pixel look gray.

However, there is lag. This is mainly because I go over each pixel. Since there are 800x600 pixels, the computation is quite slow. What do you think? How can I implement something like "fog of war" or "line of sight"?

I was thinking of trying to improve the other computations in the game so that it is left more time for this part of the game. For example, currently I check all objects for a collision. I can reduce it by using a binary space tree.

What do you think?

P.S. I cannot switch to Opengl at this point.

Here is an image of the game screen: http://imgur.com/1rCq1Qv

Here is the code where I do the fog of war and check whether a players sees another player/bullet: https://github.com/martinradev/2dShooter/blob/master/VirtualCommando/src/main/java/me/martin/radev/game/virtualcommando/view/gui/screens/gamescreen/GameScreenMap.java

I will do that with not computing actually the square root of a number though this would not improve it a lot. I am not quite sure though how to do the other things you mentioned since I always produce on the image every frame and then I have to add the mask again.

Just a quick thought, but you could have a precomputed bitmap with alpha that is the fog with the player at the center of the bitmap, and then just center that bitmap around the player and blit that on top of the gameworld.

Another, separate but basic optimization is to just check the pixels that have changed. You already know where the player is, where the player was, and the radius of their fog clearing, so you could just check that space. You could just check the min-max of that and do a for loop around that rectangle. Or you could make the whole screen grey (blitting a big texture that is grey with your 'out in the fog' alpha, and then just check the area around where the player currently is.

Thank you. It worked flawlessly.