July 2009 – The Instruction Limit

Fast Uniform Poisson-Disk Sampling in C#

Updated July 26th : Added a circle sampling method.

This appears to be tool-class weekend! Here’s another that I just finished up.

I’ve been wanting uniform poisson-disk distribution generation code for ages, ever since I started working with shadow map filtering in October 2006. I had difficulty decrypting the whitepapers that popped in the first results of Google at the time, so I just gave up for a little bit until… well… someone did a simple implementation for me to grab.

Thankfully, it happened!

The fine gentlemen at Luma labs (Herman Tulleken is credited for the class I converted) released a Java, Python and Ruby implementation of uniform and non-uniform poisson-disk sampling. It’s based on an oddly straightforward whitepaper by Robert Bridson at the UBC (in Canada!).

It took me under an hour to make it work in C#. This will prove very useful in my PCF filtering code, and I will be using it immediately in a Depth-of-Field shader I’m working on.

Download

UniformPoissonDiskSampler.cs (4 kb – C# class)

This one uses Vector2’s from the SlimDX namespace, but it would be easy to just replace them with XNA Vector2’s or TV3D TV_2DVECTOR’s.

The original Java code used a non-static class, I decided to make it static and use structures for settings and state. This will reduce garbage if it’s used repeatedly, and since I send the structures as references, it’s just as fast as instance variables. I feel it’s a bit cleaner too.

The pointsPerIteration parameter defaulted to 30 in the original code, I did this too and it works well. You can reduce it to have potentially less dense distributions but much faster generation, or make it higher to make sure the whole space is filled.

Again, I won’t post a sample because the class usage is as simple as can be : it returns a List<Vector2> in the specified domain.

Enjoy!

Flash-style Tween/Easing Functions in C#

Easing functions make any movement look pretty. So you’re going to need them at some point.

And implementations are so widely available that you don’t have an excuse not to use them. I found this ActionScript reference to be very helpful in creating my own C# easing functions library, which I’d like to share… so here it is!

First line is EaseIn, second is EaseOut, third is EaseInOut

Download

Easing.cs (4 kb – C# class)

It’s a static class and it’s framework-agnostic, completely standalone. The screenshot you see above is my TV3D test class. I won’t post the whole sample, but here’s the code if you want to see how I used it. In order to run, one would need a version of my components framework that I haven’t released yet.

I also didn’t implement each and every function that Robert Penner presents, just the ones I figured I’d use. Bounce and Elastic sound like physics to me, I don’t think I’d use easing functions to achieve that.

Quick Tip : Stencil Buffer Comparisons

I just realized something today about the stencil buffer, which I love and use extensively in Fez to do tricky screen-space effects and keep everything single-pass.
Even though the way it works is really standard, documented in the APIs and even has the exact same behavior in both DirectX and OpenGL… it kinda goes against what you’d expect.

Let’s say you want to only draw pixels where the current stencil buffer value is greater than 4. In your mind, it probably goes like this (in a fictitious “foreach pixel in screen” loop) :

if (stencilBuffer > 4) draw();

…and if so, I’m totally with you. You already wrote to the stencil buffer, and now you want to compare against it. Intuitively, the reference value would be at the right-hand-side of the comparison operator.

But nope, wrong way around! This is how the GPU expects you to think :

if (4 > stencilBuffer) draw();

This OpenGL page on the stencil buffer states it clearly :

GL_GREATER : [stencil test] passes if reference value is greater than stencil buffer

This applies for Less, LessEqual, Greater and GreaterEqual operators. When you test a reference value against the stencil buffer, you test it in that precise order : the reference value goes at the left-hand-side of the comparison operator.

Hope I saved you some confusion! I sure wasted an hour on this one…