Merge pull request #771 from RayTracing/negative_t

Brief explanation about negative t values

Author: hollasch

CHANGELOG.md

@@ -11,6 +11,7 @@ Change Log -- Ray Tracing in One Weekend
   - Fix: Synchronize copies of `hittable_list.h`, `material.h`, `sphere.h`
 
 ### In One Weekend
+  - Change: Wrote brief explanation waving away negative t values in initial normal sphere
   - Fix: Catch cases where `lambertian::scatter()` yields degenerate scatter rays (#619)
 
 ### The Next Week

books/RayTracingInOneWeekend.html

@@ -725,9 +725,10 @@
 visualize the normals with a color map. A common trick used for visualizing normals (because it’s
 easy and somewhat intuitive to assume $\mathbf{n}$ is a unit length vector -- so each
 component is between -1 and 1) is to map each component to the interval from 0 to 1, and then map
-x/y/z to r/g/b. For the normal, we need the hit point, not just whether we hit or not. Let’s assume
-the closest hit point (smallest $t$). These changes in the code let us compute and visualize
-$\mathbf{n}$:
+x/y/z to r/g/b. For the normal, we need the hit point, not just whether we hit or not. We only have
+one sphere in the scene, and it's directly in front of the camera, so we won't worry about negative
+values of $t$ yet. We'll just assume the closest hit point (smallest $t$). These changes in the code
+let us compute and visualize $\mathbf{n}$:
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
     double hit_sphere(const point3& center, double radius, const ray& r) {