Guardian Menus

Spruced up the menus quite a bit with some floaty asteroids instead of just the raw text. Looks much nicer I think. The todo list is pretty much empty now. Mostly some final balancing and play testing left to go, and some art work for the various icons and other app store images. If all goes well over the next couple of days it’ll be complete and ready to test when I hopefully get my hardware on Monday.

Spruced up the menus quite a bit with some floaty asteroids instead of just the raw text. Looks much nicer I think. The todo list is pretty much empty now. Mostly some final balancing and play testing left to go, and some art work for the various icons and other app store images. If all goes well over the next couple of days it’ll be complete and ready to test when I hopefully get my hardware on Monday.

User Interface for Guardian High Scores and Challenges

Here is my attempt at using XNA to duplicate some of the UI functionality on WP7. Well, not really duplicate so much as “create a passing resemblance to”.

The menu is pretty boring. I’ve been trying to come up with a good idea to make it interesting, and think I finally have something that will be worth the effort. That’ll probably be the next movie.

The first WP7 phones are supposed to be released in the U.S. on November 8th – just one week away. Here’s hoping Guardian works well on the hardware without needing too many optimizations.

My name is Crappy Coding Guy, and I use Texture2D.GetData

In a previous post about texture modification, I mentioned the evils of transferring data from the GPU to the CPU, and then presented an example showing one way to avoid doing it. The post wasn’t really about deformable 2D terrain or collision detection, but was intended to help newer game programmers open up a new way of thinking when it comes to using the GPU to accomplish tasks.

Since that post, and the one showing a video of my WP7 game, I’ve received a couple of questions about how I do the collision detection in Guardian, which would seem to require the use of Texture2D.GetData.

In a previous post about texture modification, I mentioned the evils of transferring data from the GPU to the CPU, and then presented an example showing one way to avoid doing it.  The post wasn’t really about deformable 2D terrain or collision detection, but was intended to help newer game programmers open up a new way of thinking when it comes to using the GPU to accomplish tasks.

Since that post, and the one showing a video of my WP7 game, I’ve received a couple of questions about how I do the collision detection in Guardian, which would seem to require the use of Texture2D.GetData.

As it turns out, I am evil, and I do use GetData.  But, my evilness is optimized based on information from here, here, and here.

  • Crater drawing is batched, meaning that rather than draw each one as it’s created, I add them to a list and draw all of them every few frames. This reduces the number of GetData calls – one per batch of craters rather than one per crater.
  • After drawing craters to the render target, I wait a few frames before calling GetData to make sure the GPU has processed all of the drawing commands. This minimizes pipeline stalls.
  • If I have a pending GetData call to make and more craters come in, the craters will stay batched until the GetData call is complete.  In other words, the drawing and getting are synchronized so that a GetData call always happens several frames after drawing a batch of craters, and any new crater draw requests wait until after a pending GetData.

If there are a lot of craters being created the built-in delays can cause some slightly inaccurate collision detection since we may be looking at collision data that’s outdated by several frames. At least in this particular game there are never huge numbers of crater adds going on so this isn’t a problem. If there are more than several crater adds they tend to be bunched close together, so the explosion animation hides any visual oddities.

There is one other optimization that I have available but haven’t needed to use.  The collision data doesn’t need to be at the same resolution as the drawing data.  Basically have two sets of render targets – one for the visual texture, and a lower resolution set for the collision data.  Do the GetData on the collision texture and scale everything appropriately when doing the collision check. You have to draw twice – once for the visual data and once for the collision data – but you’re pulling much less data from the GPU which would possibly offset the extra drawing time (this isn’t something I’ve tested yet). You won’t be pixel perfect, but for this type of game that isn’t necessary. As I write this it seems using multiple render targets would eliminate the “draw twice” issue here, but I’ve never done that so some research would be required.

So there you have it. Is this the best or most efficient way?  I don’t know – I’m far from an expert on any of this. To be honest, I never actually tested doing this just on the CPU, so it’s entirely possible that that approach is better if there are collision detection requirements. There are also other considerations, such as whether your game is CPU or GPU bound, which would go into determining which method is better suited to your needs. Ultimately, whatever works in your situation is the right method.

Guardian WP7 Bosses

Added some tough “boss” asteroids. This one just has a single kill point so it’s relatively simple. Later waves will require destroying multiple targets for the kill. Still need to add a destruction sequence when it’s destroyed, rather than just fading away like it currently does.

Added some tough “boss” asteroids. This one just has a single kill point so it’s relatively simple. Later waves will require destroying multiple targets for the kill. Still need to add a destruction sequence when it’s destroyed, rather than just fading away like it currently does.

WP7 Planet Defender Progress Video

I’ve been working on a Windows Phone 7 port of Guardian and finally have something to show for it. All of the systems are in place now, just need to do a lot of tuning and game play tweaking. Of course, at this point I have no idea how it’s going to perform on an actual phone, but it should do fairly well. Hopefully there won’t be too much optimization required after I get my hands on some hardware.

I recorded the video using Fraps while running the version compiled for Windows. Keeping a version working in Windows has made testing and debugging work much more smoothly than having to deploy to the simulator each time I make a change. It’s also a fairly simple matter to simulate the touch functionality using the mouse.

Texture Modification in XNA 3.1

I’ve had a couple of questions about what changes are needed to get the texture modification tutorial to work in XNA 3.1.

So, here’s a 3.1 version of the project, and a quick overview of the major things that need to change.

  • You need to create the depth/stencil buffer yourself, set it on the GraphicsDevice when setting the render target, and restore the previous buffer when you’re done.
  • RenderTarget2D can’t be used directly as a texture, you must call RenderTarget2D.GetTexture instead and use that when drawing.
  • Render states are all set in GraphicsDevice.RenderState instead of the various classes used in 4.0.
  • Various minor syntax changes.

Texture Modification using Render Targets, with some Stencil Buffer Action

Sometimes you need to modify a texture while your game is running, and there are a number of ways to do this. One of the first things newer game programmers often try to do is use Texture2D.GetData to copy the texture data from the GPU to an array on the CPU, modify the bytes, and then send it back to the GPU with Texture2D.SetData.

This is a bad idea on many, levels. Beyond issues with pipeline stalls, GetData and SetData can be slow, especially when working with a large texture. Any time you’re tempted grab data from the GPU for use on the CPU you should very carefully consider all of your options. There are often other solutions that let you keep the data entirely on the GPU and accomplish the same thing.

This tutorial will use an example that could be solved with GetData and SetData, and show you another alternative using render targets and the stencil buffer that will let you perform the same function entirely on the GPU.

Sometimes you need to modify a texture while your game is running, and there are a number of ways to do this. One of the first things newer game programmers often try to do is use Texture2D.GetData to copy the texture data from the GPU to an array on the CPU, modify the bytes, and then send it back to the GPU with Texture2D.SetData.

This is a bad idea on many levels. Beyond issues with pipeline stalls, GetData and SetData can be slow, especially when working with a large texture. Any time you’re tempted grab data from the GPU for use on the CPU you should very carefully consider all of your options. There are often other solutions that let you keep the data entirely on the GPU and accomplish the same thing.

This tutorial will use an example that could be solved with GetData and SetData, and show you another alternative using render targets and the stencil buffer that will let you perform the same function entirely on the GPU.

CPU Craters

Let’s pretend you want to draw 2D planet, and periodically add a crater to it. You want a hole to appear somewhere on the planet, so it looks like part of it was removed.

You could do this using the GetData/SetData method by getting the data from a texture into an array, setting the color to the background (or alpha to 0) in the shape of the crater, then writing the data back to the texture. Or you could be a little cleverer and eliminate GetData by always keeping the data in the array, but you still have to do the SetData to get it into the texture on the GPU each time it’s changed.

GPU Craters

The method we’ll use to do this entirely on the GPU involves several steps. First, we need a couple of resources. We’ll use a simple textured circle for a planet, and a crater shaped texture for the crater.

It’s important to note that the black areas on these have an alpha value of 0, meaning completely transparent. For the planet this just lets us draw the round shape over the background without looking like a square image. But for the crater image the alpha value is very important since it will control what part of the crater image is removed from the planet.

Next, we need to set up two render targets (these will be referred to later as Render Target A, and Render Target B). When we need to add a crater, one of these will be used as a target for drawing to, while the other used as a texture. The next time we add a crater they will swap roles – the texture will become the target, and the target will become the texture. This is called “ping-ponging” and will be discussed more fully later.

Once we have these resources ready to go, the method for adding a crater goes like this:

  1. Activate Render Target A using GraphicsDevice.SetRenderTarget.
  2. Clear the graphics device, setting the color to solid black, and the stencil buffer to 0.
  3. Set up the stencil buffer state so whatever we draw writes a value of 1 to the stencil buffer.
  4. Set up the alpha test state so we only draw where the alpha value is zero.
  5. Draw the crater texture. Because of the way we’ve set up the graphics device, only the parts of the crater texture that have alpha = 0 will be drawn, and those parts will write a 1 to the stencil buffer. So what we have at this point is a “mask” in the stencil buffer that we can use in the next step. The white area in the following image represents the stencil mask we’ve set up – the stencil buffer contains “1” in the white area, and “0” everywhere else.
  6. Set up the stencil buffer so when we draw, anything that has a value of 1 in the stencil buffer will be masked out – meaning it won’t draw.
  7. Draw the “planet texture”. Because of the way we’ve set up the graphics device, anything with a 1 in the stencil buffer won’t be drawn – since these 1’s are in the shape of a crater, that shape will be masked out of the planet texture, leaving holes that look like craters.
  8. Set the render target to the backbuffer. We can now access Render Target A as a texture, and that texture contains the planet texture with a crater-shaped hole in it.
Step 5
Step 7

From now on, until we need to add another crater, we can treat Render Target A as a texture and draw it using SpriteBatch, and we’ll have a nice crater. Now, what if we need to add another crater? This is where the ping-ponging comes in. Since Render Target A is now the “planet texture”, we need to be able to draw somewhere else when we’re filling in the stencil buffer with our crater shape. It just so happens that we set up another place to draw to, Render Target B.

So now, in Step 1, instead of activating Render Target A we need to activate Render Target B and draw the crater shapes into that. But what happens when we get to Step 7? Well, the “planet texture” is now in Render Target A, so we draw that. And in Step 8, Render Target B now contains our new planet texture with two craters.

And if we add a third crater then we’re back to where we started – drawing to Render Target A, and using Render Target B as the source texture. In other words, we “ping-pong” between the two render targets – each time we need to modify the texture, one is used for a texture, and one is used for drawing to, and then those roles are swapped.

You may have noticed that there’s one issue here. The first time through, Render Target B has nothing in it, so we can’t use it as the planet texture. This can be handled by using the actual planet texture the first time, and the render target thereafter.

The Code

Now let’s walk through the code involved, using XNA 4.0. You can do this in 3.1, but you’ll have to make significant changes when creating the render targets and setting the render states.

The complete code is in the downloadable project linked at the end of the tutorial. We’ll just go through the highlights here, referring to the steps mentioned above as we go.

The XNA 4.0 API has been changed substantially where render states are concerned, and for the better. Render states have been grouped by functionality into several classes. You create instances of these classes to represent the state you want, then set them on the graphics device, or pass them to SpriteBatch. So first we need to create these render state objects.

Set Up Render State Objects

For Step 3, we need to use the DepthStencilState class to set up the device to always set the stencil buffer to 1. We enable the stencil buffer, set the stencil function to Always, the pass operation to Replace, and ReferenceStencil to 1. This means that as we’re drawing, each pixel will Always pass, and the value in the stencil buffer will be Replaced with 1.

stencilAlways = new DepthStencilState();
stencilAlways.StencilEnable = true;
stencilAlways.StencilFunction = CompareFunction.Always;
stencilAlways.StencilPass = StencilOperation.Replace;
stencilAlways.ReferenceStencil = 1;
stencilAlways.DepthBufferEnable = false;

And for Step 4 we need to use the standard AlphaTestEffect so we can draw the asteroid texture only where the alpha value is 0.

Matrix projection = Matrix.CreateOrthographicOffCenter(0, PlanetDataSize, PlanetDataSize, 0, 0, 1);
Matrix halfPixelOffset = Matrix.CreateTranslation(-0.5f, -0.5f, 0);
alphaTestEffect = new AlphaTestEffect(GraphicsDevice);
alphaTestEffect.VertexColorEnabled = true;
alphaTestEffect.DiffuseColor = Color.White.ToVector3();
alphaTestEffect.AlphaFunction = CompareFunction.Equal;
alphaTestEffect.ReferenceAlpha = 0;
lphaTestEffect.World = Matrix.Identity;
alphaTestEffect.View = Matrix.Identity;
alphaTestEffect.Projection = halfPixelOffset * projection;

We first set up an orthographic projection matrix that matches SpriteBatch. We set AlphaFunction to Equal, and ReferenceAlpha to 0. This means the alpha test will pass whenever the alpha value we’re drawing is equal to 0. In our crater texture, the crater area has an alpha value of 0, while the surrounding area has 1, so only the crater area will be drawn.

For Step 6 we need a stencil buffer state that allows drawing only where the stencil buffer contains a 0. We enable the stencil buffer, set the stencil function to Equal, the pass operation to Keep, and the reference stencil to 0. This means that when we’re drawing, each pixel will pass if the value in the stencil buffer is Equal to 0.

stencilKeepIfZero = new DepthStencilState();
stencilKeepIfZero.StencilEnable = true;
stencilKeepIfZero.StencilFunction = CompareFunction.Equal;
stencilKeepIfZero.StencilPass = StencilOperation.Keep;
stencilKeepIfZero.ReferenceStencil = 0;
stencilKeepIfZero.DepthBufferEnable = false;

Create Render Targets

Now that we have the render state objects created, it’s time to create the render targets. Both are the same, so just one is shown here. This creates a render target with a Color format, and a depth format that includes a stencil buffer.

renderTargetA = new RenderTarget2D(GraphicsDevice, PlanetDataSize, 
  PlanetDataSize, false, SurfaceFormat.Color, 
  DepthFormat.Depth24Stencil8, 0, 
  RenderTargetUsage.DiscardContents);

Draw the Crater Mask

Next up is drawing the crater masks (Steps 2-5). First we activate the render target, clear it to solid black, and clear the stencil buffer to 0.

GraphicsDevice.SetRenderTarget(activeRenderTarget);
GraphicsDevice.Clear(ClearOptions.Target | ClearOptions.Stencil,
                     new Color(0, 0, 0, 1), 0, 0);

Next we begin a SpriteBatch, passing in the stencilAlways and alphaTestEffect objects that we created earlier. Calculate some random rotation, size the crater texture using a Rectangle, and call SpriteBatch.Draw to draw the crater.

spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Opaque,
                  null, stencilAlways, null, alphaTestEffect);
Vector2 origin = new Vector2(craterTexture.Width * 0.5f,
                             craterTexture.Height * 0.5f);
float rotation = (float)random.NextDouble() * MathHelper.TwoPi;
Rectangle r = new Rectangle((int)position.X, (int)position.Y, 50, 50);

spriteBatch.Draw(craterTexture, r, null, Color.White, rotation,
                 origin, SpriteEffects.None, 0);

spriteBatch.End();

Draw the Planet Texture

Now we need to draw the latest planet texture, using the stencil buffer to mask out the craters (Steps 6-7). We begin a SpriteBatch, passing in the stencilKeepIfZero object we created earlier. Note that the first time we draw the actual planet texture, but subsequently we draw using the texture from the previous iteration.

spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Opaque,
                  null, stencilKeepIfZero, null, null);

if (firstTime)
{
  spriteBatch.Draw(planetTexture, Vector2.Zero, Color.White);
  firstTime = false;
}
else
  spriteBatch.Draw(textureRenderTarget, Vector2.Zero, Color.White);

spriteBatch.End();

Swap Render Targets

Finally we activate the backbuffer render target.

GraphicsDevice.SetRenderTarget(null);

And then swap the render targets as discussed previously.

RenderTarget2D t = activeRenderTarget;
activeRenderTarget = textureRenderTarget;
textureRenderTarget = t;

In the main Draw function, you draw the latest cratered planet using the textureRenderTarget. Of course, you need to deal with using the planet texture the first time through though. The downloadable code shows one simple way to do that.

GraphicsDevice.Clear(Color.CornflowerBlue);
spriteBatch.Begin();
spriteBatch.Draw(textureRenderTarget, planetPosition, Color.White);
spriteBatch.End();

Conclusion

And there you have it, a powerful technique for altering textures during your game. Doing this entirely on the GPU is quite a bit more complex than GetData/SetData, but is well worth the extra trouble.

There are some things you can do to improve this technique. If you need to add a lot of craters, rather than adding them one at a time you can batch them up for a while, then in Step 5 draw all of them at once.

I hope you found this tutorial informative. Learning about render targets and stencil buffers opens up a whole new world of possibilities beyond just making craters. What other uses can you think of?

Download the sample XNA 4.0 project

Download the sample XNA 3.1 project