Triangle/AABB Intersection Test in C#

TriangleBoxIntersect.cs

public static bool IntersectsBox( Vector3 a, Vector3 b, Vector3 c, Vector3 boxCenter, Vector3 boxExtents )
{
	// From the book "Real-Time Collision Detection" by Christer Ericson, page 169
	// See also the published Errata at http://realtimecollisiondetection.net/books/rtcd/errata/
	
	// Translate triangle as conceptually moving AABB to origin
	var v0 = ( a - boxCenter );
	var v1 = ( b - boxCenter );
	var v2 = ( c - boxCenter );

	// Compute edge vectors for triangle
	var f0 = ( v1 - v0 );
	var	f1 = ( v2 - v1 );
	var	f2 = ( v0 - v2 );

	#region Test axes a00..a22 (category 3)

	// Test axis a00
	var a00 = new Vector3( 0, -f0.z, f0.y );
	var p0 = Vector3.Dot( v0, a00 );
	var p1 = Vector3.Dot( v1, a00 );
	var p2 = Vector3.Dot( v2, a00 );
	var r = boxExtents.y * Math.Abs( f0.z ) + boxExtents.z * Math.Abs( f0.y );
	if( Math.Max( -fmax( p0, p1, p2 ), fmin( p0, p1, p2 ) ) > r )
	{
		return false;
	}

	// Test axis a01
	var a01 = new Vector3( 0, -f1.z, f1.y );
	p0 = Vector3.Dot( v0, a01 );
	p1 = Vector3.Dot( v1, a01 );
	p2 = Vector3.Dot( v2, a01 );
	r = boxExtents.y * Math.Abs( f1.z ) + boxExtents.z * Math.Abs( f1.y );
	if( Math.Max( -fmax( p0, p1, p2 ), fmin( p0, p1, p2 ) ) > r )
	{
		return false;
	}

	// Test axis a02
	var a02 = new Vector3( 0, -f2.z, f2.y );
	p0 = Vector3.Dot( v0, a02 );
	p1 = Vector3.Dot( v1, a02 );
	p2 = Vector3.Dot( v2, a02 );
	r = boxExtents.y * Math.Abs( f2.z ) + boxExtents.z * Math.Abs( f2.y );
	if( Math.Max( -fmax( p0, p1, p2 ), fmin( p0, p1, p2 ) ) > r )
	{
		return false;
	}

	// Test axis a10
	var a10 = new Vector3( f0.z, 0, -f0.x );
	p0 = Vector3.Dot( v0, a10 );
	p1 = Vector3.Dot( v1, a10 );
	p2 = Vector3.Dot( v2, a10 );
	r = boxExtents.x * Math.Abs( f0.z ) + boxExtents.z * Math.Abs( f0.x );
	if( Math.Max( -fmax( p0, p1, p2 ), fmin( p0, p1, p2 ) ) > r )
	{
		return false;
	}

	// Test axis a11
	var a11 = new Vector3( f1.z, 0, -f1.x );
	p0 = Vector3.Dot( v0, a11 );
	p1 = Vector3.Dot( v1, a11 );
	p2 = Vector3.Dot( v2, a11 );
	r = boxExtents.x * Math.Abs( f1.z ) + boxExtents.z * Math.Abs( f1.x );
	if( Math.Max( -fmax( p0, p1, p2 ), fmin( p0, p1, p2 ) ) > r )
	{
		return false;
	}

	// Test axis a12
	var a12 = new Vector3( f2.z, 0, -f2.x );
	p0 = Vector3.Dot( v0, a12 );
	p1 = Vector3.Dot( v1, a12 );
	p2 = Vector3.Dot( v2, a12 );
	r = boxExtents.x * Math.Abs( f2.z ) + boxExtents.z * Math.Abs( f2.x );
	if( Math.Max( -fmax( p0, p1, p2 ), fmin( p0, p1, p2 ) ) > r )
	{
		return false;
	}

	// Test axis a20
	var a20 = new Vector3( -f0.y, f0.x, 0 );
	p0 = Vector3.Dot( v0, a20 );
	p1 = Vector3.Dot( v1, a20 );
	p2 = Vector3.Dot( v2, a20 );
	r = boxExtents.x * Math.Abs( f0.y ) + boxExtents.y * Math.Abs( f0.x );
	if( Math.Max( -fmax( p0, p1, p2 ), fmin( p0, p1, p2 ) ) > r )
	{
		return false;
	}

	// Test axis a21
	var a21 = new Vector3( -f1.y, f1.x, 0 );
	p0 = Vector3.Dot( v0, a21 );
	p1 = Vector3.Dot( v1, a21 );
	p2 = Vector3.Dot( v2, a21 );
	r = boxExtents.x * Math.Abs( f1.y ) + boxExtents.y * Math.Abs( f1.x );
	if( Math.Max( -fmax( p0, p1, p2 ), fmin( p0, p1, p2 ) ) > r )
	{
		return false;
	}

	// Test axis a22
	var a22 = new Vector3( -f2.y, f2.x, 0 );
	p0 = Vector3.Dot( v0, a22 );
	p1 = Vector3.Dot( v1, a22 );
	p2 = Vector3.Dot( v2, a22 );
	r = boxExtents.x * Math.Abs( f2.y ) + boxExtents.y * Math.Abs( f2.x );
	if( Math.Max( -fmax( p0, p1, p2 ), fmin( p0, p1, p2 ) ) > r )
	{
		return false;
	}

	#endregion 

	#region Test the three axes corresponding to the face normals of AABB b (category 1)

	// Exit if...
	// ... [-extents.x, extents.x] and [min(v0.x,v1.x,v2.x), max(v0.x,v1.x,v2.x)] do not overlap
	if( fmax( v0.x, v1.x, v2.x ) < -boxExtents.x || fmin( v0.x, v1.x, v2.x ) > boxExtents.x )
	{
		return false;
	}

	// ... [-extents.y, extents.y] and [min(v0.y,v1.y,v2.y), max(v0.y,v1.y,v2.y)] do not overlap
	if( fmax( v0.y, v1.y, v2.y ) < -boxExtents.y || fmin( v0.y, v1.y, v2.y ) > boxExtents.y )
	{
		return false;
	}
			
	// ... [-extents.z, extents.z] and [min(v0.z,v1.z,v2.z), max(v0.z,v1.z,v2.z)] do not overlap
	if( fmax( v0.z, v1.z, v2.z ) < -boxExtents.z || fmin( v0.z, v1.z, v2.z ) > boxExtents.z )
	{
		return false;
	}

	#endregion 

	#region Test separating axis corresponding to triangle face normal (category 2)

	var planeNormal = Vector3.Cross( f0, f1 );
	var planeDistance = Vector3.Dot( planeNormal, v0 );

	// Compute the projection interval radius of b onto L(t) = b.c + t * p.n
	r = boxExtents.x * Math.Abs( planeNormal.x )
		+ boxExtents.y * Math.Abs( planeNormal.y )
		+ boxExtents.z * Math.Abs( planeNormal.z );

	// Intersection occurs when plane distance falls within [-r,+r] interval
	if( planeDistance > r )
	{
		return false;
	}

	#endregion

	return true;
}

private static float fmin( float a, float b, float c )
{
	return Math.Min( a, Math.Min( b, c ) );
}

private static float fmax( float a, float b, float c )
{
	return Math.Max( a, Math.Max( b, c ) ); 
}
		

Yup, straight out of his book Real-Time Collision Detection. I highly recommend that book, it's a fantastic resource. The code above is just my attempt to port it to C# for my own use. I've not made any claim to ownership or invention, nor any other representation at all. I'm pretty sure he didn't invent it either, for whatever that's worth.

I'll review and test your suggested changes if I ever need to use code like this again. It's doubtful that I will, so the code will likely remain unchanged here, and people will have to investigate any potential issues themselves.

Here's the notes published on the Errata page for the book: