triangle.js

class MyTriangle extends MyPrimitive {
	constructor (scene, id, x1, y1, z1, x2, y2, z2, x3, y3, z3) {
		super (scene);

        this.x1 = x1;
        this.y1 = y1;
        this.z1 = z1;

        this.x2 = x2;
        this.y2 = y2;
        this.z2 = z2;

        this.x3 = x3;
        this.y3 = y3;
        this.z3 = z3;

        this.origCoords = [];

		this.initBuffers();
	};

	initBuffers() {
		this.vertices = [
            this.x1, this.y1, this.z1,
            this.x2, this.y2, this.z2,
            this.x3, this.y3, this.z3
        ];

        // https://www.khronos.org/opengl/wiki/Calculating_a_Surface_Normal
        let u = [this.x2-this.x1, this.y2-this.y1, this.z2-this.z1];    // a p1p2
        let v = [this.x3-this.x1, this.y3-this.y1, this.z3-this.z1];    // c p3p1
        let w = [this.x3-this.x2, this.y3-this.y2, this.z3-this.z2];    // b p2p3

        let nx = u[1]*v[2] - u[2]*v[1];
        let ny = u[2]*v[0] - u[0]*v[2];
        let nz = u[0]*v[1] - u[1]*v[0];

        let vec = [nx, ny, nz];
        let norm = this.norm(vec);

        this.normals = [
            nx/norm, ny/norm, nz/norm,
            nx/norm, ny/norm, nz/norm,
            nx/norm, ny/norm, nz/norm
        ];

		this.indices = [
            0, 1, 2
        ];

        let a = this.norm(u);
        let b = this.norm(w);
        let c = this.norm(v);

        let cosAlfa = (Math.pow(a, 2) - Math.pow(b, 2) + Math.pow(c, 2)) / (2 * a *c);
        let sinAlfa = Math.sqrt(1 - Math.pow (cosAlfa, 2));

        this.tex2 = a;
        this.tex3 = [c*cosAlfa, c*sinAlfa];

        this.texCoords = [
            0,0,
            a,0,
            this.tex3[0], this.tex3[1]
        ];

        this.texCoords = [...this.origCoords];

    	this.primitiveType=this.scene.gl.TRIANGLES;
		this.initGLBuffers();
    }

    norm(vector){
        let norm = 0;
        vector.forEach(v => {
            norm += Math.pow(v, 2);
        });
        norm = Math.sqrt(norm);

        return norm;
    }

	updateTexCoords(length_s, length_t) {
		this.texCoords = [
			0, 0,
			this.tex2/length_s, 0,
			this.tex3[0]/length_s, this.tex3[1]/length_t
		];

		this.updateTexCoordsGLBuffers();

	}
};