Create README.md

Author: mikew1

README.md

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+# Simple ray tracer
+
+This is the code from Paul Graham's ANSI Common Lisp, tidied with my comments plus a few tweaks.
+
+In particular, I added the ability to move the light source.
+
+# Output
+
+This is the basic output with the world as per the book, at 4x scale to be more impressive.
+
+![screen shot canvas](/output-png/spheres-4.png?raw=true "")
+
+Emphasis of this code is on simplicity and correctness, not speed.
+
+Note though, [Ray tracing](https://en.wikipedia.org/wiki/Ray_tracing_(graphics)) in general is very amenable to parallelisation because the calculation
+of each ray can be considered computationally distinct. This code is many thousands of times slower than it could be done on your GPU.
+
+We can pan the eye position left which is quite trivial:
+
+![screen shot canvas](/output-png/spheres-eye-left-10.png?raw=true "")
+![screen shot canvas](/output-png/spheres-eye-left-20.png?raw=true "")
+![screen shot canvas](/output-png/spheres-eye-left-30.png?raw=true "")
+![screen shot canvas](/output-png/spheres-eye-left-40.png?raw=true "")
+
+But moving the light source is quite nice (this is the bit I added to the code in the book).
+
+Here the light source is moved left by log scale increments:
+
+![screen shot canvas](/output-png/spheres-light-source-left-100.png?raw=true "")
+![screen shot canvas](/output-png/spheres-light-source-left-200.png?raw=true "")
+![screen shot canvas](/output-png/spheres-light-source-left-400.png?raw=true "")
+![screen shot canvas](/output-png/spheres-light-source-left-800.png?raw=true "")
+
+Personal next steps? I plan to check back in on this code after reading 
+[7 Concurrency Models](https://pragprog.com/book/pb7con/seven-concurrency-models-in-seven-weeks).
+
+---
+#### Running the code
+
+Obviously there's no system definition (`.asd` file) here, all I was doing is evaluating
+each source file directly into a REPL. Be warned it does take about a minute on my 2015 macbook
+pro to generate an image even at the smallest size. 
+
+There is more info in comments in the source files.