v4.0.0-alpha.2 (2024-04-07)

This alpha wraps up most of the major changes we expect to make to book 2 for the impending v4.0.0
release, along with a bunch of updates to the other two books. Since the alpha.1 release last
August, we've been lucky to have onboarded two new contributors: Arman Uguray and Nate Rupsis.
They've been helping out a ton with this release, and Arman is also developing his GPU Ray Tracing
book at the same time!

This release is a bit faster, thanks to some new BVH optimizations. We've eliminated the negative
radius sphere hack to model hollow spheres, instead accomplishing this with refraction indices. This
eliminates a bunch of places in the code where we had to accomodate this, and probably a bunch of
bugs we still haven't found. We now load texture images in linear color space, fixing a long-running
bug where we were gamma-correcting our textures twice -- you'll notice object texture maps look a
bit darker and less washed out. Refraction text has gotten a bit of an overhaul, and a better
example of total internal reflection. Of course, this also includes a load of small fixes, tweaks,
and improvements.

Our current plan is to get the final v4.0.0 release out the door by SIGGRAPH 2024, targeting July
28. With that, here are the latest changes since our alpha.1 release:

Common

• Delete -- Removed rtw_stb_image.h header from book 1 source, as it's unused there.
• Change -- Increase compile warning levels for MSVC, and corrected newly-flagged code.
• Change -- Default to using post-increment everywhere
• Change -- We've removed the few cases where we used C++ default constructors. Instead, we either
  require all parameters, or use operator overloading to use default values.
• Change -- For clarity across audiences with broad programming backgrounds, we now use
  double(x) instead of static_cast<double>(x), and similarly for other types, for
  easier readability for non-C++ programmers.
• Change -- The ray class constructors no longer use C++ default parameter values
• Change -- Remove pixel sampling knowledge from write_color(). This simplifies write_color()
  to take only the desired output color, and made each phase in color computation easier
  to understand.
• Change -- ray::origin() and ray::direction() getters now return const references, avoiding
  unnecessary copies.
• Change -- Cleaned up the use of the hit_record class in material.h
• Change -- All books now point to the project wiki instead of the in1weekend blog for further
  reading links.
• Change -- New BVH optimization splits the bounds according to the longest bounding box
  dimension, yielding a 15-20% speedup (#1007)
• Change -- Reversed the ray-sphere direction and calculations throughout equations and code for
  all books. This ended up simplifying equations and code in several places (#1191)
• Change -- Pass vec3, point3, ray, and color parameters by const reference where
  possible (#1250)
• Change -- Changed BVH construction (removed const qualifer for objects vector) so sorting is
  done in place, and copying of vector is avoided, yielding better BVH build performance
  (#1327, #1388, #1391)
• Change -- Implement hollow spheres using refraction index instead of negative radii.
  Additionally, we now block negative radius spheres. This fixes a bunch of corner
  cases with inverted spheres (#1420)
• Change -- Refactor pixel subsampling to make the sampling functions simpler and better focused
  in scope (#1421)
• Change -- All constructor parameter names now match their member names if assigned directly. C++
  can handle this without ambiguity, and it means we don't have to come up with
  alternate names for everything (#1427)
• Change -- material::scatter() gets a trivial default implementation (#1455)
• Fix -- Fixed section describing total internal reflection. It turns out that spheres with
  refraction index greater than the surrounding atmosphere cannot exhibit total internal
  reflection. Changed example to instead model a bubble of air in water, and updated the
  rendered images to match (#900)
• Fix -- Fix references from random_in_hemisphere() to random_on_hemisphere() (#1198)
• Fix -- The linear_to_gamma() function has been hardened against negative inputs (#1202)
• Fix -- Fixed default camera look-from and look-at values (#1341)
• Fix -- The quad bounding box now considers all four vertices instead of erroneously only
  using two (#1402)
• New -- Added PRINTING.md to give information about how to print these books to PDF or paper.
  We will also be including PDFs of each book with each new GitHub release going
  forward.

In One Weekend

• Change -- Update reference to "Fundamentals of Interactive Computer Graphics" to "Computer
  Graphics: Principles and Practice". This is the name used by newer editions of the
  book.
• Change -- Updated the "Next Steps" section at the end of book 1 (#1209)
• Change -- Update rtweekend.h header introduction and use (#1473)
• Fix -- Fix code listing ordering bug with lambertian texture support (#1258)
• New -- Improved help for the very first build and run.
• New -- Define albedo prior to first use (#1430)

The Next Week

• Change -- Lots of miscellaneous edits and clarifications to book two as we encountered them.
  This also includes various improvements to code listings to provide better context and
  address discrepancies between the listings and the actual source code.
• Change -- perlin::turb() no longer defaults the value for the depth parameter.
• Change -- AABB automatically pads to mininmum size for any dimension; no longer requires
  primitives to call aabb::pad() function.
• Change -- Switch from ray = A + tb to ray = Q + td in AABB text.
• Change -- Update checker scale to 0.32
• Change -- Refactor AABB class. Renamed aabb::axis() to aabb::axis_interval(). Minor
  refactoring of aabb::hit() function. (#927, #1270)
• Change -- Reworked the AABB chapter. Created skippable sections for planar coordinates
  derivation (#1236)
• Fix -- Updated book 2 images to match the latest code.
• Fix -- Images loaded for texture mapping are now converted from their original gamma to
  linear color space for use. Rendered images are still gamma corrected to 2.0 on
  output (#842)
• Fix -- Fix regression in calls to Perlin turb() functions with scaled points (these should
  be unscaled). (#1286)
• New -- Add section on alternative 2D primitives such as triangle, ellipse and annulus (#1204,
  #1205)

The Rest of Your Life

• Fix -- Add missing backslash for LaTeX operatorname (#1311)
• Fix -- Fix LaTeX functions with underscore (#1330)