Ole-Morten Duesund
fc752cfa0c
'Path tracing' in graphics literature means Monte Carlo solution of the rendering equation, with random hemisphere sampling at every diffuse surface and indirect light transport. card.cc has none of that — the floor branch in S() returns immediately with a flat ambient term and casts no new rays. What it actually does is Whitted-style recursive specular reflection (mirrors only), with distribution-style stochastic sampling layered on top for soft shadows, depth of field, and AA. Group the stochastic features (soft shadows, DOF, AA) under that qualifier; keep mirror reflections, checker floor, and sky gradient as separate non-stochastic features.
101 lines
3.7 KiB
Markdown
101 lines
3.7 KiB
Markdown
# card-raytracer
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A study of [Andrew Kensler's "business card raytracer"][aek] — a complete
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**Whitted-style raytracer with stochastic sampling** for soft shadows,
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depth of field, and anti-aliasing, plus mirror reflections, a checker
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floor, and a sky gradient, all crammed into a single C++ source file
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small enough to print on the back of a business card.
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> **`card.cc` is exactly 1337 bytes.** Yes, **leet**. Verify it yourself:
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>
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> ```sh
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> $ wc -c card.cc
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> 1337 card.cc
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> ```
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>
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> That is suspiciously round — hand-golfed C++ rarely lands on a famous
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> leet number by accident. The compression tricks all show their work:
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> typedefs (`i` for `int`, `f` for `float`), operator overloads on
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> `struct v` that turn `+` / `%` / `^` / `!` into vector math, and not
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> a wasted space anywhere.
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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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