Cody Boone Ferguson
3deb7ae603
Amongst the changes here is that we do not need to have the hex or html
code (numbers) for open and close parentheses (in links) as there are
names. '(' is & followed by lpar; and the ')' is the same but 'rpar;'
instead. This makes it easier to remember and type and it generates the
same html as well (they are all valid). Now the markdown files are
consistent in this (in some cases hex was used and in others the html
code).
Although many markdown files were updated no html files needed to be
updated as it generates the same html. Even so I ran the appropriate
rules to do so just to verify this.
170 lines
5.0 KiB
Markdown
170 lines
5.0 KiB
Markdown
## To build:
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``` <!---sh-->
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make
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```
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### Bugs and (Mis)features:
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The current status of this entry is:
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> **STATUS: INABIAF - please DO NOT fix**
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For more detailed information see [2012/blakely in bugs.html](../../bugs.html#2012_blakely).
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## To use:
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``` <!---sh-->
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./blakely "RPN formula" resolution > output.gif
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```
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## Try:
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``` <!---sh-->
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./try.sh
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```
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## Judges' remarks:
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Several IOCCC themes ([RPN
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calculator](https://en.wikipedia.org/wiki/Reverse_Polish_notation), animated
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[GIF](https://en.wikipedia.org/wiki/GIF) generator, [ray
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tracing](https://en.wikipedia.org/wiki/Ray_tracing_(graphics)) graphics) in one package!
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Very impressive!
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The checkerboard pattern in the output can be broken. Could you figure out how?
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Larger GIF files confuse some image viewers.
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## Author's remarks:
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### Remarks
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This program renders a generic
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[RPN](https://en.wikipedia.org/wiki/Reverse_Polish_notation) expression in
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[ray-traced](https://en.wikipedia.org/wiki/Ray_tracing_(graphics)) animated
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[GIF](https://en.wikipedia.org/wiki/GIF) form. The expression in spatial
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variables `x` and `y` is specified on the command-line with the required
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resolution, and an animated GIF showing a fly-around of the resulting surface is
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output to `stdout`.
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### Usage
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The program should be run as:
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``` <!---sh-->
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./blakely xy* 32 > pic.gif
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```
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where the first parameter is a
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[RPN](https://en.wikipedia.org/wiki/Reverse_Polish_notation) expression, and the
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second is the width of the required [GIF](https://en.wikipedia.org/wiki/GIF), up
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to a maximum of 250.
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The expressions are written in [Reverse Polish
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Notation](https://en.wikipedia.org/wiki/Reverse_Polish_notation) with the
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following characters recognised:
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```
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+, -, *, / - usual arithmetic operations
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^ - Exponentiation; valid for constant exponent only
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s - sin
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c - cos
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0-9 - single digit integers
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d - duplicate number on stack
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t - transpose top two elements on stack
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x - x coordinate value
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y - y coordinate value
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```
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NB: all arithmetic is done in double precision, so `23/` is two-thirds, rather
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than 0.
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The surface plotted corresponds to `-1 <= x,y <= 1` and for best results, the
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generated `z` value should lie between `-1` and `1`.
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If there is a division by zero, square-root of a negative number, or similar
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operation, then the results are undefined.
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### Examples
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For test-runs, use a resolution of 32. For simple expressions, the
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[GIF](https://en.wikipedia.org/wiki/GIF) generation should take only
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a few seconds with a modern processor and full optimization. Runtime scales as
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O^(N^3), (see [Big O notation](https://en.wikipedia.org/wiki/Big_O_notation) web page)
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with resolution `N`,
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and more complex expressions take longer to compute. For example, increasing the
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resolution of `ripple.gif`
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to 250 took ~35 minutes to render on an [i7
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processor](https://en.wikipedia.org/wiki/Intel_Core#Core_i7).
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``` <!---sh-->
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./blakely xy* 32 > saddle.gif
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./blakely xx*yy*1++d5*ct/ 64 > ripple.gif
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```
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### Algorithm
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A generic root-finding algorithm to find the point of intersection between a ray
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and the surface is done using a simple forward search from the eye. In order to
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make the rendering realistic, both the value of the expression, and its
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derivative with respect to `x` and `y` must be tracked. This is done using an
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obvious stack-based approach.
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### Obfuscations
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Much of the obfuscation is achieved through the use of single-letter variables
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and `#define`s. The characters output for the [GIF
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format](https://en.wikipedia.org/wiki/GIF#File_format) are generated in myriad
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and wonderful ways, making use of
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[XOR](https://en.wikipedia.org/wiki/Exclusive_or), return values from
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`putchar()`, and odd expressions. The layout is in the form of `X Y Z`.
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### Limitations
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ASCII is assumed since many bytes for the
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[GIF](https://en.wikipedia.org/wiki/GIF) output are generated from characters
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given in the source code. On compilation a few warnings about parentheses come
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up when using gcc 4.6.3, and one about no return value from `main()` (who uses
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exit codes anyway?), but clang 3.1 emits no warnings.
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### Questions
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* Apart from their appearance when computing the [RPN
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expression](https://en.wikipedia.org/wiki/Reverse_Polish_notation), [sin and
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cos](https://en.wikipedia.org/wiki/Sine_and_cosine) are not used.
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How do we then get the correct angles for rotation?
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* Exactly how many times will the animation repeat?
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* Why is there an upper limit of 250x250 for the resolution?
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* What is the point of `b[0]`?
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* Where are the words `NETSCAPE` and `GIF` hidden in the source?
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(Note that the author may or may not be able to answer all of the above
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questions.)
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<!--
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Copyright © 1984-2024 by Landon Curt Noll. All Rights Reserved.
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You are free to share and adapt this file under the terms of this license:
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Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
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For more information, see:
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https://creativecommons.org/licenses/by-sa/4.0/
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-->
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