fix: Vertical flip

Fix the difference in (0,0) assumption of jpg and computer graphics.

main.cpp

@@ -87,7 +87,9 @@ int main(int argc, char * argv[])
 
       // Compute viewing ray
       Ray ray;
-      viewing_ray(camera,i,j,width,height,ray);
+      /* NOTE I flipped the vertical axis of the pixels, because in computer graphics we assume the bottom left pixel
+       * is (0,0), while traditional jpg assumes (0,0) to be the upper left pixel. */
+      viewing_ray(camera,height - i,j,width,height,ray);
 
       // Find first visible object hit by ray and its surface normal n
       double t;