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Calculating Mesh Normals

Started by sonka May 1, 2007 at 11:54 AM 8 replies 4.6k views

Original Post

May 1, 2007 at 11:54 AM

Hi! After calculating normals of a terrain, I want to do the same with my .3ds model. I've found a topic in the forum and in it somebody wrote this algorithm:


for each vertex in the mesh
  find all the triangles that has the vertex
    calculate its surface normal
    add it to a variable
  end
  avarage the variable, and normalize it
end

Based on the algorithm above, I wrote this function:


void Mesh3ds::CreateNormals( void ) {
	if (!VertexCount || !TriangleCount) return;
	crVec3 normal, v1, v2;

	Normals = new crVec3[VertexCount];

	normal = crVec3( 0, 0, 0 );

	int k=0;
	for(unsigned long i=0; i<VertexCount; i++) {
		for(unsigned long f=0; f<TriangleCount; f++) {
			if (Triangles[f].a == i ) {
					v1 = Vertices[Triangles.b] - Vertices[Triangles.a];
					v2 = Vertices[Triangles.c] - Vertices[Triangles.a];
			}

			if (Triangles[f].b == i ) {
					v1 = Vertices[Triangles.a] - Vertices[Triangles.b];
					v2 = Vertices[Triangles.c] - Vertices[Triangles.b];
			}

			if (Triangles[f].c == i ) {
					v1 = Vertices[Triangles.a] - Vertices[Triangles.c];
					v2 = Vertices[Triangles.b] - Vertices[Triangles.c];
			}

				normal += normalize(cross( v1, v2 ));
				k++;
		}
		Normals = normalize (normal/float(k));
		k=0;
	}

}

I tried not to divide it with k, and when adding to 'normal', not to normalize it. ( normal += cross(v1, v2); then Normals = normalize(normal); ) But it does not work. I don't have any idea about this, please help!

haemonculus

May 1, 2007 at 1:19 PM

you have to set v1 and v2 in clock or counter clock order as are your triangles.
you have to change the part:

if (Triangles[f].b == i ) {					v1 = Vertices[Triangles.a] - Vertices[Triangles.b];					v2 = Vertices[Triangles.c] - Vertices[Triangles.b];			}

if (Triangles[f].b == i ) {					v1 = Vertices[Triangles.c] - Vertices[Triangles.b];					v2 = Vertices[Triangles.a] - Vertices[Triangles.b];			}

that's the part that goes against the clock order.
the cross product changes the direction if the 2 vectors are in another order, so the sum of the normals is wrong.

if the clock order is another, you get normals, that are facing inside the mesh but thats not as big a problem.
best you let your program show the normals ;)

Palidine

May 1, 2007 at 1:24 PM

for each vertex in the mesh  find all the triangles that has the vertex    calculate its surface normal    add it to a variable  end  avarage the variable, and normalize itend

You don't need the average the variable step. just add all the normal vectors of the adjacent triangles together and normalize that.

I also found that the best way to do this for an old project the first time I did it was to draw the normals after the finding the normals part. That made it pretty trivial to debug.

-me

treeway

May 1, 2007 at 1:43 PM

the normal of a triangle is the cross product of any two of its vertices (it doesnt matter which ones).

sipickles

May 1, 2007 at 1:57 PM

Are you using DirectX?

The D3DX library has functions to do this. See:

D3DXLoadMeshFromX
D3DXComputeNormals
D3DXComputeTangents

etc

:)

Simon

sonka

May 1, 2007 at 2:30 PM

Thank you for your replies!
I'm using OpenGL, so I cannot enjoy the DirectX integrated functions :)

I modified my function to this:

void Mesh3ds::CreateNormals( void ) {	if (!VertexCount || !TriangleCount) return;	crVec3 normal, v1, v2;	Normals = new crVec3[VertexCount];	normal = crVec3( 0, 0, 0 );	for(unsigned long i=0; i<VertexCount; i++) {		for(unsigned long f=0; f<TriangleCount; f++) {			if (Triangles[f].a == i ) {					v1 = Vertices[Triangles.b] - Vertices[Triangles.a];					v2 = Vertices[Triangles.c] - Vertices[Triangles.a];			}			if (Triangles[f].b == i ) {					v1 = Vertices[Triangles.c] - Vertices[Triangles.b];					v2 = Vertices[Triangles.a] - Vertices[Triangles.b];			}			if (Triangles[f].c == i ) {					v1 = Vertices[Triangles.a] - Vertices[Triangles.c];					v2 = Vertices[Triangles.b] - Vertices[Triangles.c];			}				normal += cross( v1, v2 );		}		Normals = normalize (normal);	}}

But it also gives straaaaange results :S
Of course, I'm drawing the normals, that is from I know they aren't pointing to the right way. And somewhere I've got three normals per vertex !!

noisecrime

May 1, 2007 at 5:22 PM

Looks like you're simply forgetting to reset the normal at the start of each vertex loop. At the moment it gets initialised once outside the vertex loop, and from then on each consecutive vertex checked will continue to add its face normals to the normal vector, instead of just those faces that include the vertex index.

ThrustGoblin

May 1, 2007 at 6:04 PM

When you are assigning v1 and v2, you're using i as an index into Triangles[] when you should be using f.

Cheers :)

Vorpy

May 1, 2007 at 7:35 PM

If you see three normals coming out of one vertex, then either there is something wrong with your normal rendering or there are actually multiple vertexes in the same spot (which can be used to get sharper corners and edges).

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