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Moe

C++ macro return? (converting C++ macro to C# code)

4 posts in this topic

First of all, please forgive me, as my C++ is really quite rusty. It's been... seven years or so since I've really had to use it.
 

I"m in the process of converting some C++ code to C# (in particular some code for checking an axis-aligned bounding box against a triangle, in 3D).  The original source in question can be found here.

 

The particular portion that I'm having a bit of trouble is this:

#define AXISTEST_X01(a, b, fa, fb)			   \

	p0 = a*v0[Y] - b*v0[Z];			       	   \

	p2 = a*v2[Y] - b*v2[Z];			       	   \

        if(p0<p2) {min=p0; max=p2;} else {min=p2; max=p0;} \

	rad = fa * boxhalfsize[Y] + fb * boxhalfsize[Z];   \

	if(min>rad || max<-rad) return 0;

Now from what I remember, I thought C++ macros were basically pre-processor bits that get substituted in at compile time.  The thing I'm perplexed at is the return statement in the macro.  In this case, what happens if min < rad?  Does the statement still return any sort of value, or is there some sort of default return value on a macro that I'm not aware of?

Here's what the rough C# equivalent is, as far as I have been able to figure:

 

	//Assume variables are declared/initialized properly.  Unlike what I've done here...
        float p0;
        float p2;
        Vector3 v0;
        Vector3 v1;
        Vector3 v2;
        Vector3 boxHalfSize;
        float min;
        float max;

        private int AXISTEST_X01(float a, float b, float fa, float fb)
        {
            p0 = a * v0.Y - b * v0.Z;
            p2 = a * v2.Y - b * v2.Z;

            if (p0 > p2)
            {
                min = p0;
                max = p2;
            }
            else
            {
		min = p2;
		max = p2;
            }

	    float rad = fa * boxHalfSize.Y + fb * boxHalfSize.Z;

	    if (min > rad || max < -rad)
	    {
		return 0;
	    }
	
            //Otherwise... return something else?
           return 0;
    }

Any thoughts/ideas on how it should actually look?  I can't say I've ever dealt much with macros, and this one has me a bit stumped.

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Awesome guys.  Many thanks on the uber-quick reply.  Also, frob, good catch on my mistake there.  Thinking about it, it now makes sense that the macro isn't returning anything, but is calling return in the code where it is being used.  I also agree that this is a pretty not-nice use of a macro.  

I haven't seen too many other samples out there that use the separating axis theorem for AABB-triangle detection.  The only other sample I've seen had issues as well.  

Again, thanks a bunch you guys.  

(I feel like it's been quite a while since I've been around here, and I'm glad to see the community is still alive and kicking, like back in the good ol' days.)

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