Merge pull request #764 from RayTracing/fix-degenerate-scatter

lambertian::scatter degenerate direction vec test

Author: hollasch

CHANGELOG.md

@@ -11,8 +11,10 @@ Change Log -- Ray Tracing in One Weekend
   - Fix: Synchronize copies of `hittable_list.h`, `material.h`, `sphere.h`
 
 ### In One Weekend
+  - Fix: Catch cases where `lambertian::scatter()` yields degenerate scatter rays (#619)
 
 ### The Next Week
+  - Fix: Catch cases where `lambertian::scatter()` yields degenerate scatter rays (#619)
 
 ### The Rest of Your Life
   - Fix: Missing `override` keyword for `xz_rect::pdf_value()` and `xz_rect::random()` methods

books/RayTracingInOneWeekend.html

@@ -2065,7 +2065,7 @@
             virtual bool scatter(
                 const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
             ) const override {
-                vec3 scatter_direction = rec.normal + random_unit_vector();
+                auto scatter_direction = rec.normal + random_unit_vector();
                 scattered = ray(rec.p, scatter_direction);
                 attenuation = albedo;
                 return true;
@@ -2081,6 +2081,57 @@
 Note we could just as well only scatter with some probability $p$ and have attenuation be
 $albedo/p$. Your choice.
 
+If you read the code above carefully, you'll notice a small chance of mischief. If the random unit
+vector we generate is exactly opposite the normal vector, the two will sum to zero, which will
+result in a zero scatter direction vector. This leads to bad scenarios later on (infinities and
+NaNs), so we need to intercept the condition before we pass it on.
+
+<div class='together'>
+In service of this, we'll create a new vector method -- `vec3::near_zero()` -- that returns true if
+the vector is very close to zero in all dimensions.
+
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
+    class vec3 {
+        ...
+        bool near_zero() const {
+            // Return true if the vector is close to zero in all dimensions.
+            const auto s = 1e-8;
+            return (fabs(e[0]) < s) && (fabs(e[1]) < s) && (fabs(e[2]) < s);
+        }
+        ...
+    };
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    [Listing [vec3-near-zero]: <kbd>[vec3.h]</kbd> The vec3::near_zero() method]
+
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
+    class lambertian : public material {
+        public:
+            lambertian(const color& a) : albedo(a) {}
+
+            virtual bool scatter(
+                const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
+            ) const override {
+                auto scatter_direction = rec.normal + random_unit_vector();
+
+
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
+                // Catch degenerate scatter direction
+                if (scatter_direction.near_zero())
+                    scatter_direction = rec.normal;
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
+
+                scattered = ray(rec.p, scatter_direction);
+                attenuation = albedo;
+                return true;
+            }
+
+        public:
+            color albedo;
+    };
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    [Listing [lambertian-catch-zero]: <kbd>[material.h]</kbd> Lambertian scatter, bullet-proof]
+</div>
+
 
 Mirrored Light Reflection
 --------------------------

books/RayTracingTheNextWeek.html

@@ -280,7 +280,13 @@
             virtual bool scatter(
                 const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
             ) const override {
-                vec3 scatter_direction = rec.normal + random_unit_vector();
+                auto scatter_direction = rec.normal + random_unit_vector();
+
+                // Catch degenerate scatter direction
+                if (scatter_direction.near_zero())
+                    scatter_direction = rec.normal;
+
+
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                 scattered = ray(rec.p, scatter_direction, r_in.time());
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
@@ -1252,7 +1258,12 @@
             virtual bool scatter(
                 const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
             ) const override {
-                vec3 scatter_direction = rec.normal + random_unit_vector();
+                auto scatter_direction = rec.normal + random_unit_vector();
+
+                // Catch degenerate scatter direction
+                if (scatter_direction.near_zero())
+                    scatter_direction = rec.normal;
+
                 scattered = ray(rec.p, scatter_direction, r_in.time());
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                 attenuation = albedo->value(rec.u, rec.v, rec.p);

books/RayTracingTheRestOfYourLife.html

@@ -858,15 +858,23 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     class lambertian : public material {
         public:
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
             lambertian(const color& a) : albedo(make_shared<solid_color>(a)) {}
             lambertian(shared_ptr<texture> a) : albedo(a) {}
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
 
-
-    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
             virtual bool scatter(
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                 const ray& r_in, const hit_record& rec, color& alb, ray& scattered, double& pdf
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
             ) const override {
-                auto direction = rec.normal + random_unit_vector();
+                auto scatter_direction = rec.normal + random_unit_vector();
+
+                // Catch degenerate scatter direction
+                if (scatter_direction.near_zero())
+                    scatter_direction = rec.normal;
+
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                 scattered = ray(rec.p, unit_vector(direction), r_in.time());
                 alb = albedo->value(rec.u, rec.v, rec.p);
                 pdf = dot(rec.normal, scattered.direction()) / pi;

src/InOneWeekend/material.h

@@ -32,7 +32,12 @@ class lambertian : public material {
         virtual bool scatter(
             const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
         ) const override {
-            vec3 scatter_direction = rec.normal + random_unit_vector();
+            auto scatter_direction = rec.normal + random_unit_vector();
+
+            // Catch degenerate scatter direction
+            if (scatter_direction.near_zero())
+                scatter_direction = rec.normal;
+
             scattered = ray(rec.p, scatter_direction);
             attenuation = albedo;
             return true;

src/TheNextWeek/material.h

@@ -37,7 +37,12 @@ class lambertian : public material {
         virtual bool scatter(
             const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
         ) const override {
-            vec3 scatter_direction = rec.normal + random_unit_vector();
+            auto scatter_direction = rec.normal + random_unit_vector();
+
+            // Catch degenerate scatter direction
+            if (scatter_direction.near_zero())
+                scatter_direction = rec.normal;
+
             scattered = ray(rec.p, scatter_direction, r_in.time());
             attenuation = albedo->value(rec.u, rec.v, rec.p);
             return true;

src/common/vec3.h

@@ -15,6 +15,7 @@
 #include <iostream>
 
 using std::sqrt;
+using std::fabs;
 
 class vec3 {
     public:
@@ -55,6 +56,12 @@ class vec3 {
             return e[0]*e[0] + e[1]*e[1] + e[2]*e[2];
         }
 
+        bool near_zero() const {
+            // Return true if the vector is close to zero in all dimensions.
+            const auto s = 1e-8;
+            return (fabs(e[0]) < s) && (fabs(e[1]) < s) && (fabs(e[2]) < s);
+        }
+
         inline static vec3 random() {
             return vec3(random_double(), random_double(), random_double());
         }