Implement diffuse scattering

src/camera.c

@@ -1,23 +1,46 @@
 #include "camera.h"
 
 #include "ray.h"
+#include "rng.h"
 
 #include <float.h>
 #include <stdio.h>
 
+#define MAX_ITER 10
+#define MIN_T 1e-6
+
 static const vec3_t lightblue = { 0.4, 0.6, 1.0 };
 static const vec3_t white = { 1.0, 1.0, 1.0 };
+static const vec3_t black = { 0.0, 0.0, 0.0 };
+
+static ray_t scatter(hit_t hit)
+{
+	const vec3_t rand = rng_vec3();
+	const vec3_t dir
+	    = vec3_dot(rand, hit.normal) > 0.0 ? rand : vec3_scale(rand, -1.0);
+	return (ray_t) { .orig = hit.point, .dir = dir };
+}
 
 static vec3_t trace(ray_t ray, const obj_t *scene, unsigned scene_count)
 {
-	hit_t hit = { .t = DBL_MAX };
-	for (unsigned i = 0; i < scene_count; ++i)
-		scene[i].intersect(scene[i].params, ray, &hit, 0.0, hit.t);
-	if (hit.t != DBL_MAX)
-		return vec3_scale(vec3_add(hit.normal, white), 0.5);
+	double coeff = 1.0;
+	for (unsigned i = 0; i < MAX_ITER; ++i) {
+		hit_t hit = { .t = DBL_MAX };
+		for (unsigned j = 0; j < scene_count; ++j)
+			scene[j].intersect(scene[j].params, ray, &hit, MIN_T, hit.t);
 
-	const double a = (ray.dir.y + 1.0) / 2.0;
-	return vec3_add(vec3_scale(lightblue, a), vec3_scale(white, 1 - a));
+		if (hit.t == DBL_MAX) {
+			const double a = (ray.dir.y + 1.0) / 2.0;
+			return vec3_scale(
+			    vec3_add(vec3_scale(lightblue, a), vec3_scale(white, 1 - a)),
+			    coeff);
+		}
+
+		ray = scatter(hit);
+		coeff *= 0.5;
+	}
+
+	return black;
 }
 
 static void setpix(vec3_t col, pix_t *out)