Floating-Point Formats Cheatsheet
Floating-point numbers (or floats in short) are not as simple as integer numbers. There is much to be understood when dealing with these numbers on low level - basic things like the sign + exponent + significand representation (and that exponent is biased, while significand has implicit leading 1), why you should never compare calculation results operator ==, that some fractions with finite decimal representation cannot be represented exactly in binary etc., as well as why there are two zeros -0 and +1, what are infinite, NaN (Not a Number) and denorm (denormal numbers) and how they behave. I won't describe it here. It's not an arcane knowledge - you can find many information about this on the Web, starting from Wikipedia article.
But after you understand these concepts, quantitative questions come to mind, like: how many significant decimal digits can we expect from precision of particular float representation (half, single, double)? What is the minimum non-zero value representable in that format? What range of integers can we represent exactly? What is the maximum value? And finally: if our game crashes with "Access violation, reading location 0x3f800000", what chances are that we mistaken pointer for a float number, as this is one of common values, meaning 1.0?
So to organize such knowledge, I created a "Floating-Point Formats" cheatsheet:
SBX - Scale-Bias Transform
A class of 2D or 3D point and vector transformations called affine transformations (that is - linear transformation plus translation) can be conveniently represented by matrix. But sometimes we don't need possibility of input x to influence output y or vice versa. For example, that's the case when we want to convert coordinates of mouse cursor from pixels, like we take it from WinAPI (where X goes right from 0 to e.g. 1280 and Y goes down from 0 to e.g. 720) to post-projective 3D space (where X goes right from -1.0 to +1.0 and Y goes up from -1.0 on the bottom to +1.0 on the top). Then every component can be transformed independently using linear transform, like this:
Parameters are: scale.xy, bias.xy.
Given input point i.xy, output point o.xy is:
o.x = i.x * scale.x + bias.x
o.y = i.y * scale.y + bias.y
This seems trivial, but what if we wanted to encapsulate such transformation in a new data structure and define operations on it, like we do on matrices? Let's call it SBX - Scale-Bias Transform. I propose following structure:
static const ScaleBiasXform IDENTITY;
And following functions:
void Construct(ScaleBiasXform& out,
const vec2& input1, const vec2& output1,
const vec2& input2, const vec2& output2);
void Scaling(ScaleBiasXform& out, float scale);
void Scaling(ScaleBiasXform& out, const vec2& scale);
void Translation(ScaleBiasXform& out, const vec2& vec);
void Inverse(ScaleBiasXform& out, const ScaleBiasXform& sbx);
void Combine(ScaleBiasXform& out, const ScaleBiasXform& sbx1, const ScaleBiasXform& sbx2);
void TransformPoint(vec2& out, const vec2& point, const ScaleBiasXform& sbx);
void UntransformPoint(vec2& out, const vec2& point, const ScaleBiasXform& sbx);
void TransformVector(vec2& out, const vec2& vec, const ScaleBiasXform& sbx);
void UntransformVector(vec2& out, const vec2& vec, const ScaleBiasXform& sbx);
Particle System - How to Store Particle Age?
Particle effects are nice because they are simple and look interesting. Besides, coding it is fun. So I code it again :) Particle systems can have state (when parameters of each particle are calculated based on previous values and time step) or stateless (when parameters are always recalculated from scratch using fixed function and current time). My current particle system has the state.
Today a question came to my mind about how to store age of a particle to delete it after some expiration time, determined by the emitter and also unique for each particle. First, let's think for a moment about the operations we need to perform on this data. We need to: 1. increment age by time step 2. check if particle expired and should be deleted.
If that was all, the solution would be simple. It would be enough to store just one number, let's call it TimeLeft. Assigned to the particle life duration at the beginning, it would be:
Step with dt: TimeLeft = TimeLeft - dt
Delete if: TimeLeft <= 0
But what if we additionally want to determine the progress of the particle lifetime, e.g. to interpolate its color of other parameters depending on it? The progress can be expressed in seconds (or whatever time unit we use) or in percent (0..1). My first idea was to simply store two numbers, expressed in seconds: Age and MaxAge. Age would be initialized to 0 and MaxAge to particle lifetime duration. Then:
Step with dt: Age = Age + dt
Delete if: Age > MaxAge
Percent progress: Age / MaxAge
Looks OK, but it involves costly division. So I came up with an idea of pre-dividing everything here by MaxAge, thus defining new parameters: AgeNorm = Age / MaxAge (which goes from 0 to 1 during particle lifetime) and AgeIncrement = 1 / MaxAge. Then it gives:
Step with dt: AgeNorm = AgeNorm + dt * AgeIncrement
Delete if: AgeNorm > 1
Progress: Age / AgeIncrement
Percent progress: AgeNorm
This needs additional multiplication during time step and division when we want to determine progress in seconds. But as I consider progress in percent more useful than the absolute progress value in seconds, that's my solution of choice for now.
DirectXMath - A First Look
Programmers using DirectX have access to the accompanying library full of routines that support various 3D math calculations. First it was D3DX - auxilliary library tightly coupled with DirectX itself. It's still there, but some time ago Microsoft released a new library called XNA Math. I've written about it here. This library also ships with latest versions of DirectX SDK (which was last updated in Jun 2010), but it is something completely new - a more independent, small, elegant and efficient library, portable between Windows (where it can use SSE2 or old FPU) and Xbox 360.
Now Microsoft comes up with another library called DirectXMath. See the official documentation and introductory blog post for details. As a successor of XNA Math, it looks very similar. Main differences are:
Sounds great? Well, I didn't tell yet how to get it. It's not shipped with DirectX SDK. You need the SDK for Windows 8. They say that:
The library is annotated using SAL2 (rather than the older VS-style or Windows-style annotations), so requires the standalone Windows SDK for Windows 8 Consumer Preview or some additional fixups to use with Visual C++ 2010.
So probably I'm not going to switch to this new library anytime soon :P
Checking collision between different kinds of 2D or 3D shapes is a subject I deal with for some time. It is useful in game development to determine if some object is visible or affected by light and this if it should be rendered. I have lots of function to check such collisions in the math module of CommonLib library.
But that is only the beginning if you want to make physics. Adding physical behavior to your game requires additional calculations to correct positions and apply forces to colliding bodies. I prepared a small code snippet that implements physical 2D collision between moving circle and another moving circle (calculated by CircleToCircleCollision function), static line (CircleToLineCollision function) and static axis-aligned rectangle (CircleToRectangleCollision function).
The code uses some good practices (like fixed time step) as well as bad practices (like Euler integration). It should be enough for a simple physics in game like a platformer.
Vector Register Size - Diagram
It may be hard to imagine and remember what is the exact number of bits, bytes, words or floats in some piece of data, like a SIMD register. So today I've made following diagram/cheatsheet:
Here you can find its "source" in OpenOffice Draw format: Vector_register_size.odg.
DevMeeting and Demoscene Night
Second event was Noc z Demoscen± - Demoscene Night. It took place in Fabryka Kot³ów club (former No Mercy). During that night we could see some olschool, as well as newschool demos presented on a a big screen and, what is most important, meet some nice people there. Thanks Voyager for organizing this event! It was great especially because the 3-day demoscene party RiverWash, which took place here in Warsaw in last 2 years, this year is in £ód¼ and I can't attend it. This night was very inspiring and now I feel like developing a technology to make a demo :)
Bit Tricks with Modulo
I like to emphasize that programming is not so similar to maths as some people say. We are used to thinking about numbers in decimal numeral system and the operations that seem most natural for us are arithmetic computations like additions, multiplication and division. On the other hand, computer works with numbers stored in binary system and bitwise operations are most efficient and natural for it. Another difference is modulo operation (the remainder after division) - a function not so frequently used in math classes, but very important in programming. That's what I'd like to talk about here.
Modulo in C++ can be done using % operator, but just as division, it is quite slow. So when doing a common operation of cycling between subsequent numbers from a limited range:
x2 = (x1 + 1) % n;
few alternatives have been developed for some special cases, using bitwise operations.
First, when the divisor is a power of 2, we can use bitwise and (operator &) to mask only least significant bits of the addition result. But this optimization can be done automatically by the compiler (at least my Visual C++ 2010).
// 7 = 0b111, so it masks 3 least significant bits.
// Equal to (x1 + 1) % 8
x2 = (x1 + 1) & 7;
When we only cycle between two values - 0 and 1 - we can use XOR (operator ^):
// Equal to (x1 + 1) % 2, for values 0, 1.
x2 = x1 ^ 1;
Finally, when we cycle between three values - 0, 1, 2 (like when indexing x, y, z components of a 3D vector), we can use the trick invented by Christer Ericson (at the end of the linked page). I came across it recently on Twitter and that's what inspired me to write this article.
// Equal to (x1 + 1) % 3, for values 0, 1, 2.
x2 = (1 << x1) & 3;