XNA Alpha Blending With Primitives and Sprites

Question

I've been using XNA for awhile now and I really enjoy using its interface. Right now we're trying to make a "fog of war" system. We're making an RTS and we're basically trying to obscure the map. Units on the map have influence, and show what's happening. They reveal a specific area on the map. Previously we used a tile based interface but we weren't happy with the appearance. Instead, we decided to draw a black mask on the screen, then reveal a transparent circle with DrawUserPrimitives. Here's the problem: When we draw a black mask on the screen, followed by the transparent circle, the screen is entirely black. Reversing the order doesn't change anything either. We've also tried every SpriteBatch BlendState we can find.

OUR CURRENT METHOD: Right now, we're drawing a full black texture onto the screen, then drawing a transparent circle. The result is an entirely black screen.

OUR DESIRED OUTCOME: Drawing the circle will reveal a section of the screen, but keep the rest obscured. We'd be able to draw multiple circles, all with their own radius, that would could be near each other without causing problems.

Basically, we're trying to draw a black mask, but reveal portions of the screen. We can't just make a circle texture and make the edges transparent, because we're trying to make the center of the circle transparent. We can't make the center transparent, because then when two such textures were near each other, their borders would show up. How would I go about making the mask, then revealing sections of it?

Regards

Ares

Answer 1

You could render the graphics normally to one render target, then paint the visible area in white on a black background in another render target, then render the second into the first with the Min BlendFunction.

Alternately, using only one render target, draw the graphics normally, then clear the alpha channel (by painting a rectangle over the whole screen with ColorWriteChannels = Alpha and alpha blend factors both 0), then draw the visible area with ColorWriteChannels = Alpha only.