March 2012

Warning! Some information on this page is older than 5 years now. I keep it for reference, but it probably doesn't reflect my current knowledge and beliefs.

# IGK-9'2012 Starts Tomorrow

Mar 2012

9th Polish Game Engineering Conference IGK-9'2012 starts tomorrow in Siedlce, Poland. Just like in previous years, we expect interesting lectures on Friday evening and whole Saturday. I especially look forward to hear about NoSQL databases and sphere tracing, as these are subjects that I always wanted to learn about but I didn't find enought time to do it before. On Sunday there will be traditional competition where teams of up to 4 people have to develop a game from scratch during 8 hours. But first of all this is a great opportunity for passionate game developers to meet and socialize. So see you in Siedlce!

Comments | #warsztat #events #igk Share

# Ball Physics

Mar 2012

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).

Download: ball_physics.cpp

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.

Comments | #math #physics Share

# Iterating Backward using Unsigned Numbers

Mar 2012

I've had a discussion recently with my colleagues about what is the best way of writing a loop in C++ that would iterate backward: Count-1, Count-2, ..., 0, using unsigned integer numbers. I'd like to share all possible solutions I managed to gather.

Iterating forward is simple:

unsigned count = 10;
// Forward: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
for(unsigned i = 0; i < count; ++i)
    printf("%d ", i);

Iterating backward is equally simple and looks similar, if you use signed integers:

// Backward: 9, 8, 7, 6, 5, 4, 3, 2, 1, 0
// Using signed integer
for(int i = count - 1; i >= 0; --i)
    printf("%d ", i);

Same code cannot be used with unsigned integers, because it creates infinite loop, as unsigned number is always >= 0 and just wraps around to maximum value 0xFFFFFFFF:

// WRONG: Inifinte loop, unsigned int wraps around and is always >= 0.
for(unsigned i = count - 1; i >= 0; --i)
    printf("%d ", i);

Changing condition to i > 0 is also invalid, because then loop is not executed for i == 0:

// WRONG: Not executed for i == 0.
for(unsigned i = count - 1; i > 0; --i)
    printf("%d ", i);

Here comes a bunch of possible valid solutions. My favourite is following for loop:

for(unsigned i = count; i--; )
    printf("%d ", i);

Following while loop does same thing. Some people say that it looks more clear. I don't think so, while I believe the for loop is better because it creates local scope for variable i, while the following code does not:

unsigned i = count;
    printf("%d ", i);

Another way of implementing exactly same logic is to write the condition like this. It may look like C++ had the "goes to zero" arrow operator -->, but it is actually just (i-- > 0) :)

unsigned i = count;
while(i --> 0)
    printf("%d ", i);

Another possible solution of loop condition (a one that does not use the postdecrementation operator) is to compare i with maximum integer value. One disadvantage of it is that you have to carefully match the value of the max constant with type of your iterator - whether it is unsigned int, unsigned short etc.

for(unsigned i = count - 1; i != UINT_MAX; --i)
    printf("%d ", i);

Another way of doing this, without using maximum value, is:

for(unsigned i = count - 1; i < count; --i)
    printf("%d ", i);

Finally there are solutions possible that introduce second iterator:

// Option 1 using second iterator:
// i goes 0...9, j is inverted i.
for(unsigned i = 0; i < count; ++i)
    unsigned j = count - i - 1;
    printf("%d ", j);

// Option 2 using second iterator:
// i goes 10...1, j is decramented i.
for(unsigned i = count; i > 0; --i)
    unsigned j = i - 1;
    printf("%d ", j);

Which is your favorite loop? Please leave a comment :)

Comments | #algorithms #c++ Share

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