Ole-Morten Duesund 2af36e2ac0 README: soften 'Kensler designed for 1337' claim to an observation

The previous wording asserted design intent ('Kensler golfed [...] until
it hit the magic number', 'every typedef exists to land on 1337') without
a primary source. CLAUDE.md requires factual claims in committed artifacts
to be checked. The byte count is verified; intent is not. Rephrased to
note the observation ('suspiciously round', 'rarely lands by accident')
without claiming insight into the design process, and the obfuscation
breakdown is kept since the techniques are visible in the code itself.

2026-05-28 14:01:33 +02:00

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card-raytracer

A study of Andrew Kensler's "business card raytracer" — a complete recursive path-tracer with soft shadows, depth of field, anti-aliasing, reflections, a checker floor, and a sky gradient, all crammed into a single C++ source file small enough to print on the back of a business card.

card.cc is exactly 1337 bytes. Yes, leet. Verify it yourself:
$ wc -c card.cc
1337 card.cc
That is suspiciously round — hand-golfed C++ rarely lands on a famous leet number by accident. The compression tricks all show their work: typedefs (i for int, f for float), operator overloads on struct v that turn + / % / ^ / ! into vector math, and not a wasted space anywhere.

The original code is preserved verbatim as card.cc. A de-obfuscated, heavily-annotated rewrite lives in card_explained.cc. Both produce the same image:

The spheres spell a · e · k, Andrew Kensler's initials, encoded as a 9-column bitmap in the G[] array of the original source.

Quick start

Requires a C++14 compiler, CMake ≥ 3.16, and (optionally) netpbm for PPM → PNG conversion.

cmake -S . -B build
cmake --build build

# Render with the original (PPM, 786 KiB, ~6 s on a modern laptop)
./build/card > aek.ppm

# Or via the convenience target:
cmake --build build --target image            # → build/aek.ppm
cmake --build build --target image_explained  # → build/aek_explained.ppm

# View it (PPM is supported by most image viewers; or convert):
pnmtopng aek.ppm > aek.png

What's interesting about it

The whole renderer is built on operator overloading: a single struct v provides + (add), * (scale), % (dot product), ^ (cross product), and unary ! (normalise). Once those are in place, the entire rendering pipeline — sphere intersections, reflection vectors, Phong shading, checker-pattern tile lookup, camera basis construction — collapses into just a handful of expressions.

Other tricks worth admiring:

Soft shadows for free: the light position is jittered each call, so averaging 64 samples per pixel produces a real penumbra without any area-light formalism.
Depth of field for free: the camera origin is also jittered across a 99-unit "lens" each sample, so points off the focal plane go blurry.
Anti-aliasing for free: sub-pixel jitter on the ray direction.

The same 64-sample loop pays for all three at once.

A line-by-line walkthrough of the algorithm is the long-form comments inside card_explained.cc.

Layout

.
├── card.cc            # original, untouched — do not edit
├── card_explained.cc  # de-obfuscated, documented rewrite
├── CMakeLists.txt
├── docs/
│   └── aek.png        # published render
├── README.md
└── CLAUDE.md          # working notes for AI assistance

Credits

Original raytracer: Andrew Kensler ("aek").
Excellent walkthrough that inspired this study: Fabien Sanglard, Decyphering The Business Card Raytracer.

Licence

The original card.cc is reproduced as-is from publicly published material for educational purposes; credit and copyright belong to Andrew Kensler. The supporting files in this repository (CMake, README, de-obfuscated rewrite) are placed in the public domain by the repository author.

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