Find determinant

html/images/space.svg

@@ -149,9 +149,14 @@
 
       triangles.forEach(triangle => {
         const attr = triangle.getAttribute('points').split(delim)
+        const [[xa, ya], [xb, yb], [xc, yc]] = attr.map(t => t.split(',').map(n => +n));
+        const det = (xb - xa) * (yc - ya) - (xc - xa) * (yb - ya);
         attr.pop();
         attr.push(`${positionX},${positionY}`);
         triangle.setAttribute('points', attr.join(delim));
+        triangle.style.fill = det < 0 ? "white" : "none";
+        triangle.style.stroke = det < 0 ? "red" : "none";
+        // i think i can also discard obtuse triangles?
       });
     }