Face normals and vertex normals are flipped

edapx · August 28, 2017, 10:35am

I’m doing a generative geometry setting vertex position and vertex normals.
It works correctly, as you see when I open the generated mesh with mesh lab and the light is correctly calculated, if the selected view is the vertex view.

When I select the faces view, the normals are flipped, they are pointing inside the tube

As this tube is an instance of of3DPrimitive, i can use drawNormals to debug them.
But in both cases, when drawing the normals per vertex:

tube.drawNormals(2, false);

Or when drawing the normals per face

tube.drawNormals(2, true);

The normals are always pointing out in the right direction, from inside the tube to outside, it is just in meshlab that they look wrong in the “faces” view.

I think it is a problem in my code because I’ve downloaded a couple of meshes and I’ve opened them in meshlab, and they look good
Is there something that I’m not taking into consideration?

hamoid · August 28, 2017, 1:27pm

Hi Davide! Do you have a minimal code example that shows this behavior?

edapx · August 28, 2017, 1:52pm

Hello Abe, it is an addon, this is a simple example with a “save mesh” option.

The mesh looks correct in OF but wrong in mesh lab.

This is the method that generate the cylinder mesh

github.com

edap/ofxBranchesPrimitive/blob/master/src/ofxBranchCylinder.h#L40


};


template <typename BRANCH>
static void putIntoMesh(shared_ptr<BRANCH> branch, ofMesh& mesh, ofxBranchCylinderOptions options){
    add(branch, mesh, options);
};


private:


template <typename BRANCH>
static void add(shared_ptr<BRANCH> branch, ofMesh& mesh, ofxBranchCylinderOptions opt){
    glm::vec4 startPos = branch->getStartPos();
    glm::vec4 endPos;
    // add an empty space between the branches, if padding > 0
    if (opt.padding > 0) {
        endPos = branch->getEndPos() - glm::vec4((branch->getEndDirection() * opt.padding), 1.0);
    } else {
        endPos = branch->getEndPos();
    }


    glm::quat startOrientation = branch->getStartOrientation();

This is the part where I’m adding the vertices in the mesh, as you see I’m adding the normals as supposed

github.com

edap/ofxBranchesPrimitive/blob/master/src/ofxBranchCylinder.h#L136


    }


    //top
    tcoord.y = textureRepeat;
    mesh.addVertex(glm::vec3(circleTop));
    mesh.addNormal(glm::vec3(normalTop));
    mesh.addTexCoord(tcoord);
    if (addVertexColor) {
        mesh.addColor(colorTop);
    }
}


// Cylinder cap
if (cap) {
    int topMiddlePoint = mesh.getNumVertices();
    glm::vec3 topDir = endDirection;


    mesh.addVertex(glm::vec3(endPos));
    mesh.addNormal(topDir);
    mesh.addTexCoord(glm::vec2(wrapLimitCap/2,wrapLimitCap/2));
    if (addVertexColor) {

and this is working, just face normals are wrong.

zach · August 28, 2017, 1:53pm

could this be left hand vs right hand coordinate system issue?

https://www.evl.uic.edu/ralph/508S98/coordinates.html

arturo · August 28, 2017, 2:58pm

yes this is probably related to the winding of the polygons, usually you should create the faces in counter clockwise order

edapx · August 28, 2017, 5:46pm

Ok, I will have a look in this direction and let you know. Thanks!

edapx · August 30, 2017, 6:22am

I can confirm, i was adding the indexes in the mesh in clockwise order, thank you guys for the support