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Member Since 05 Nov 2008
Offline Last Active Oct 12 2014 07:52 AM

Posts I've Made

In Topic: Culling points from a polygon.

28 August 2014 - 11:46 PM

I have solved problems very similar to this by using the "Signed triangle area" formula.




All you need to do is loop over the points in the shape, and test the triangle at each vertex. i.e. for vertex v, you form the triangle with vertices {v-1, v, v+1}. You know the shape has vertices in a straight line if the triangle area is very close to zero. Unfortunately you can't rely on using an exact comparison against zero when dealing with floating point numbers, so you compare against epsilon (+-0.000001 or whatever suits).

In Topic: how to delete two objects that are circularly linked

08 January 2014 - 03:29 AM

Are the references actually circular or are you just talking about a node which may belong to multiple entities? To handle the latter you can use a std::shared_ptr wrapper for your node objects. It will automatically count the number of references to the node and call the delete operator when the object containing the last reference is deleted.


Dealing with circular references is much harder. Some approaches are to use weak pointers (std::weak_ptr) to allow an object to know about another without having ownership (i.e. holding the weak reference will not prevent the object from being deleted). If you can't design the solution in this way, there are more elaborate methods where you scan your objects, detect the circular reference and delete objects which are unreachable in the graph. But it's normally much simpler to just not use (strong) circular references at all.

In Topic: make an object that is pointing in a direction rotate to point in another dir...

24 February 2013 - 01:15 AM

You have your old and new direction vectors, so the problem is simply a matter of building the rotation matrix to rotate the first vector onto the second. You use the cross and dot product to get the rotation axis and angle.


Assuming that oldV and newV are normalised:

axisOfRotation = crossProduct(oldV, newV).normalise(); // Depending on whether your library requires normalised rotation axes

angleOfRotation = arccos(dotProduct(oldV, newV));


Whatever library you're using should have a function that builds a rotation matrix from an axis and angle of rotation, so there's no need to manually code a whole bunch of math.

In Topic: Calculating angles of rotation

01 November 2012 - 07:32 AM

I'd do it like this:

1) Project the destination vector onto the XZ axis. Do this simply by setting the y component to zero. The projected vector will no longer be normalised.
2) Get the angle between the object's vector and the vector resulting from 1), and rotate around the y axis by this amount.
3) Now you can rotate your object onto the destination vector using the method outlined earlier in this thread (cross product vectors to get rotation axis, dot product to find angle). This will cause the object to point at the destination vector without any undesired rotation along the object's local x axis.

In Topic: Bit Flags vs. Boolean

13 October 2012 - 09:26 PM

One thing to keep in mind is that accessing bit fields will require the compiler to produce additional bit masking instructions, which will *increase* the memory footprint of your instruction code. If you try to use bit fields as a general rule instead of a situational optimisation, you'll end up with slower code.

Also, the alignment issues are subtle. Most types want to align on 4 byte boundaries, so AFAIK if you have a structure containing, say, an integer, and any less than 5 flags, then converting these flags to single bits won't save any space at all.