Stencil Rendering

Here’s a little demo to show off a technique that Farbs posted about earlier this week.

yay yay2


Binaries : (3.6 Mb – Binaries)
Code : (1.8 Mb – Source Only, get DLLs for TV3D and SlimDX from the Binaries)


Every frame, a random color from the target image is sampled. This color will be used as a stencil, such that every pixel whole target color is close enough to that stencil’s color will be painted. It’s a constructive painting process; every frame paints a single color, but if you wait long enough in a single spot you’ll end up with the target image.

Actually, the post processing draw code is so concise that I can post it here :

public override void PostDraw()
    targetBuffer.SetSystemMemCopy(true, true); // Lolworkaround

    // 1) Pick a colour from target image
    var pickedColor = Globals.DecodeRGBA(TextureFactory.GetPixel(targetBuffer.GetTexture(), RandomHelper.Random.Next(0, targetBuffer.GetWidth()), RandomHelper.Random.Next(0, targetBuffer.GetHeight())));
    stencilShader.SetEffectParamVector3("stencilColor", new TV_3DVECTOR(pickedColor.r, pickedColor.g, pickedColor.b));
    Screen2DImmediate.Draw_FullscreenQuadWithShader(stencilShader, 0, 0, 1, 1, mainBuffer.GetTexture(), targetBuffer.GetTexture());

“targetBuffer” and “mainBuffer” are just two TVRenderSurfaces as big as the viewport. Since I sample from targetBuffer, it needs to be flagged with “system memory copy”. I thought this would slow things down, but it runs at very interactive framerates (60 and more).

And uh… The “lolworkaround” is a bug in the current build of TV3D. Usually you only need to set this once at initialization time, but a BltFromMainBuffer does not flag the surface as dirty and it prevents updates to the pixels that I sample. Resetting the memory copy mode makes the changes effective. Sylvain tells me it’s fixed in the current development build. :)

A pixel shader does the rest :

	float3 bufferSample = tex2D(MainBufferSampler, IN.TexCoord).rgb;
	float3 targetSample = tex2D(TargetSampler, IN.TexCoord).rgb;

	// 2) Calc difference between current screen and target (per channel subtraction, abs, then accumulate all three into one)
	float sampleDiff = distance(bufferSample, targetSample);

	// 3) As above, but between colour picked in step 1 and target image
	float stencilDiff = distance(stencilColor, targetSample);

	// 4) Where result of step 2 > result of step 3, draw colour picked in step 1
	float4 color;
	if (stencilDiff < sampleDiff)
		color = float4(stencilColor, 1);
		color = float4(bufferSample, 1);

	return color;

I think it’s a lovely effect. It’s very dependent on how colorful and contrasted the scene is, and it works differently for sharply-defined shapes or gradients… And of course camera movement is a big factor. In the video it gets confusing, the effect is more “painterly” if you just rotate the camera in small circles and wait for the effect to accumulate.


Stimergy is a game that I have made with Heather Kelley of Kokoromi for the Bivouac Urbain gamejam/competition last weekend in Québec city. Our team name was EMERGENCY HAMMER… don’t ask?

The point of the jam was to make a game in 3 days – 36 hours. And so we did.

splash stimergy final

Download (2.1 Mb – Binaries) (471 Kb – Source Only, get DLLs for TV3D, SlimDX and IrrKlang from the Binaries)

I thought it was so cool that Petri used Chronolapse to film the making of his game Post I.T. Shooter, so I used the same thing for Stimergy :


Heather did most of the game design, defined the graphics style and did the sound effects. I did the programming and some game design.

The game was made from scratch in C# 3.0 using the Truevision3D engine with no prior design, graphics or sound work. All the graphics in the game are procedural, and the gameplay itself is based on AI rules, basically a cellular automaton plus the notion of “stigmergy” from the insect world. (We dropped the G in the game title.)

I used a more recent version of my components system, which now has a sound interface via IrrKlang. It’s the same system I used for Super HYPERCUBE and Trouble in Euclidea, growing quite fond of it.


  • Guide the ants using suggestive pheromone trails towards or away from the picnic blanket, or killer antlions.

There are 4 levels. Each level has different goals, which are explained when it begins. Some have time limits. If you fail the objective, the level will restart until you get it.


Mouse Left Button : Attraction Pheromone
Mouse Right Button : Repulsion Pheromone

Escape key to restart the game from Level 1.


This game uses the .NET Framework 3.5, installing it is mandatory. I suggest you install the SP1 version just in case.
You will also need a bunch of DirectX DLLs that are provided by the DirectX Web Setup.

Because of the fancy blur effect, the game requires a Shader Model 2.0 compatible graphics card. This type of card is fairly commonplace now.

Post Mortem

This was my first real rapid game prototyping experience. The shortest project span I had seen for a game before was about 15 days… so this was something else. Here’s a couple of random thoughts about that :

  • I need a proper system for timers and interpolators, similar to what Nick Gravelyn posted, or maybe just steal that. It’s really annoying to have a dozen TimeSpan variables to keep track of what changes over time and how long the transition lasts.
  • I need to learn other languages than C#, and other frameworks than TV3D/XNA. I’m making big efforts in my “engines” to cut down the redundant code, but even then I feel like I’m programming much more than is needed to describe the game mechanics.
  • A big part of what makes a game actually fun, is how much direct feedback you get from interacting in the gameworld, and how much you feel like you can control these interactions. Stimergy is a pretty slow game, almost an RTS (funny, because I hate this genre), and I’ve seen other competitors value the responsiveness of player input more than the complexity of game systems, and it ended up being a lot more fun. Maybe it just fit the “jam” context better, too.
  • I need a system for game screens. Something that puts game components in a context, that gives them lifetime. I tend to make all the major components global and automatically-loaded, which is the easiest way, but it makes game state management pretty hard. And it’s dangerous for memory usage. There’s actually a good XNA example that I just need to look at in detail.
  • If I’m going to do more 2D games in TV3D, I need to build something that will load GIF animations and non-power-of-two textures. Material management for stuff that changes color or opacity was kind of a mess too. I may need to wrap it in something more concise. Switching engine would probably be the more logical choice.
  • I spent silly amounts of time on tweaking graphical things that I ended up not using. This time would have been better spent balancing the difficulty level or adding more gameplay elements. I’m used to making demos look pretty,… I have to remind myself that I’m making a game here.

I may sound like I’m complaining about everything, but I’m actually really happy about how the game turned out. It’s fairly fun/challenging and it looks pretty good. I’m still wondering whether I’ll release the code because it’s kind of a mess. I suppose I will if I get a request. The source is available up in the Download section!