Cfrgtkky

I have a big challenge, I'm developing a software to calculate the cost of an impression 3d. For that, I need to get the volume of the STL piece to calculate the weight. With the grams I could calculate the price easily.
The problem is that I want to get the volume of the walls and the infill's volume, the walls have approximately 1mm and the infill the rest of the volume. The walls are solid and the infill have a percentage of infill (e.x. 30%). There are figures what have more walls that infill and vice versa.
The volume of both is calculated in Three.js. Now this is what I'm trying. I calculated the total volume of the piece and the volume of the piece scaled in a lower scale without walls(scaling in x, y and z to have 1mm minus) and subtract both.

//x, y and z are the size of mesh

//scale is the percentaje of scaling(uniform on all axes)

function calculateVolume(mesh,scale,x,y,z){

    //Scale the mesh

    x= x*scale;

    y= y*scale;

    z= z*scale;

    var volume = 0;

    var volume_wall = 0;

    var volume_infill = 0;

    var volume_reduced = 0;

    //Calculate the proportion to be reduced with walls of 1mm of each side

    var proportion_x = (x-10)/x;

    var proportion_y = (y-10)/y;

    var proportion_z = (z-10)/z;



    //I travel array of mesh triangles

    mesh.traverse(function (child){

    if (child instance of THREE.Mesh){

        //I get array of vector positions

        var positions = child.geometry.getAttribute("position").array;

        for(var i=0;i<positions.length; i+=9){

            //I calculate volume total of triangle

            volume += volumeOfVectors(positions,i,scale,scale,scale);

            //I calculate volume of triangle minus the walls

            volume_reduced += volumeOfVectors(positions,i,scale*proportion_x,

                scale*proportion_y,scale*proportion_z);



            }

        }

    });

    //I convert volume in mm^3 to cm^3 

    volume = volume / 1000;

    volume_reduced = volume_reduced / 1000;



    //I calculate the wall volume

    volume_wall = volume - volume_reduced;

    //I set the fill volume as 30%

    volume_infill = volume_reduced *0.30;

    // 1.24 is the density

    console.log("Volume 100% infill:"+volume*1.24); 

    console.log("Volume whitout wall:"+volume_reduced*1.24);        

    console.log("Volume wall:"+volume_wall*1.24);   

    console.log("Volume infill:"+volume_infill*1.24);



    return [volume_wall,volume_infill];

}



//Calculating volume with the vectors of the triangle

function volumeOfVectors(positions,i,scale_x,scale_y,scale_z){

    //Point 1

    var t1 = {};

    t1.x = positions[i+0]*scale_x;

    t1.y = positions[i+1]*scale_y;

    t1.z = positions[i+2]*scale_z;

    //Point 2

    var t2 = {};

    t2.x = positions[i+3]*scale_x;

    t2.y = positions[i+4]*scale_y;

    t2.z = positions[i+5]*scale_z;

    //Point 3

    var t3 = {};

    t3.x = positions[i+6]*scale_x;

    t3.y = positions[i+7]*scale_y;

    t3.z = positions[i+8]*scale_z;

    //Volumen calcule

    return  signedVolumeOfTriangle(t1,t2,t3);   

}





function signedVolumeOfTriangle(p1,p2,p3){

    var v321 = p3.x*p2.y*p1.z;

    var v231 = p2.x*p3.y*p1.z;

    var v312 = p3.x*p1.y*p2.z;

    var v132 = p1.x*p3.y*p2.z;

    var v213 = p2.x*p1.y*p3.z;

    var v123 = p1.x*p2.y*p3.z;

    return (-v321 + v231 + v312 - v132 - v213 + v123)/6;

}

But this way is not accurate. Because the object is scaled in all directions, getting a volume as the first image. The 3d red object have been scaled to remove walls and the transparent is the original 3d object. As you see the 3d object isn't adapt to the borders.

image problem 1
image problem 2

The volume should be is as the next image. This way keep 1mm of wall in all directions.
image good 1
image good 2

Can you think of an idea or a way forward?