88 lines
3.2 KiB
Markdown
88 lines
3.2 KiB
Markdown
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# card-raytracer
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A study of [Andrew Kensler's "business card raytracer"][aek] — a complete
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recursive path-tracer with soft shadows, depth of field, anti-aliasing,
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reflections, a checker floor, and a sky gradient, all crammed into ~1337
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characters of obfuscated C++ small enough to print on the back of a
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business card.
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The original code is preserved verbatim as [`card.cc`](card.cc). A
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de-obfuscated, heavily-annotated rewrite lives in
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[`card_explained.cc`](card_explained.cc). Both produce the same image:
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The spheres spell **a · e · k**, Andrew Kensler's initials, encoded as a
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9-column bitmap in the `G[]` array of the original source.
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## Quick start
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Requires a C++14 compiler, CMake ≥ 3.16, and (optionally) `netpbm` for
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PPM → PNG conversion.
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```sh
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cmake -S . -B build
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cmake --build build
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# Render with the original (PPM, 786 KiB, ~6 s on a modern laptop)
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./build/card > aek.ppm
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# Or via the convenience target:
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cmake --build build --target image # → build/aek.ppm
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cmake --build build --target image_explained # → build/aek_explained.ppm
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# View it (PPM is supported by most image viewers; or convert):
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pnmtopng aek.ppm > aek.png
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```
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## What's interesting about it
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The whole renderer is built on **operator overloading**: a single `struct v`
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provides `+` (add), `*` (scale), `%` (dot product), `^` (cross product), and
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unary `!` (normalise). Once those are in place, the entire rendering
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pipeline — sphere intersections, reflection vectors, Phong shading,
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checker-pattern tile lookup, camera basis construction — collapses into
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just a handful of expressions.
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Other tricks worth admiring:
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- **Soft shadows for free**: the light position is jittered each call, so
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averaging 64 samples per pixel produces a real penumbra without any
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area-light formalism.
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- **Depth of field for free**: the camera origin is also jittered across
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a 99-unit "lens" each sample, so points off the focal plane go blurry.
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- **Anti-aliasing for free**: sub-pixel jitter on the ray direction.
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The same 64-sample loop pays for all three at once.
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A line-by-line walkthrough of the algorithm is the long-form
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[comments inside `card_explained.cc`](card_explained.cc).
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## Layout
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```
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.
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├── card.cc # original, untouched — do not edit
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├── card_explained.cc # de-obfuscated, documented rewrite
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├── CMakeLists.txt
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├── docs/
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│ └── aek.png # published render
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├── README.md
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└── CLAUDE.md # working notes for AI assistance
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```
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## Credits
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- Original raytracer: [Andrew Kensler][aek-home] ("aek").
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- Excellent walkthrough that inspired this study:
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Fabien Sanglard, [_Decyphering The Business Card Raytracer_][aek].
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## Licence
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The original `card.cc` is reproduced as-is from publicly published material
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for educational purposes; credit and copyright belong to Andrew Kensler.
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The supporting files in this repository (CMake, README, de-obfuscated
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rewrite) are placed in the public domain by the repository author.
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[aek]: https://fabiensanglard.net/rayTracing_back_of_business_card/
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[aek-home]: https://www.cs.utah.edu/~aek/
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