BSP surface unwinding for surface area calculation

r_bsp_model.c

/*
 * @brief Unwinds the surface, iterating all non-collinear vertices.
 *
 * @return The next winding point, or NULL if the face is completely unwound.
 */
static const vec_t *R_UnwindBspSurface(const r_bsp_model_t *bsp, const r_bsp_surface_t *surf,
		const vec_t *last) {

	const int32_t *se = &bsp->surface_edges[surf->first_edge];
	const vec_t *v0 = R_BSP_VERTEX(bsp, *se)->position;

	if (!last)
		return v0;

	uint16_t i;

	for (i = 0; i < surf->num_edges; i++) {
		const vec_t *v = R_BSP_VERTEX(bsp, se[i])->position;

		if (VectorCompare(last, v)) {
			break;
		}
	}

	while (i < surf->num_edges - 1) {
		const vec_t *v1 = R_BSP_VERTEX(bsp, se[(i + 1) % surf->num_edges])->position;
		const vec_t *v2 = R_BSP_VERTEX(bsp, se[(i + 2) % surf->num_edges])->position;

		vec3_t delta1, delta2;

		VectorSubtract(v1, v0, delta1);
		VectorSubtract(v2, v0, delta1);

		VectorNormalize(delta1);
		VectorNormalize(delta2);

		if (DotProduct(delta1, delta2) < 1.0 - SIDE_EPSILON) {
			return v1;
		}

		i++;
	}

	return NULL;
}

/*
 * @brief Resolves the surface bounding box and lightmap texture coordinates.
 */
static void R_SetupBspSurface(r_bsp_model_t *bsp, r_bsp_surface_t *surf) {
	
	// ..
	
	if (surf->texinfo->light) { // resolve surface area

		const vec_t *v0 = R_UnwindBspSurface(bsp, surf, NULL);
		const vec_t *v1 = R_UnwindBspSurface(bsp, surf, v0);
		const vec_t *v2 = R_UnwindBspSurface(bsp, surf, v1);

		while (v1 && v2) {
			vec3_t delta1, delta2, cross;

			VectorSubtract(v1, v0, delta1);
			VectorSubtract(v2, v0, delta2);

			CrossProduct(delta1, delta2, cross);
			surf->area += 0.5 * VectorLength(cross);

			v1 = R_UnwindBspSurface(bsp, surf, v2);
			v2 = R_UnwindBspSurface(bsp, surf, v1);
		}
	}

	// ..
}