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Format fix 2012/blakely/README.md

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Cody Boone Ferguson committed 2023-05-03 09:51:15 -07:00
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@@ -3,4 +3,6 @@ output.gif
paraboloid.gif
empty.gif
flat.gif
pic.gif
ripple.gif
saddle.gif
2012/blakely/README.md
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# Most GIFted expressions
Philip Blakely
Cambridge, UK
<pmblakely@gmail.com>
Philip Blakely
UK
<pmblakely@gmail.com>
## To build:
@@ -28,7 +28,11 @@ time ./blakely 0 250 > flat.gif
## Judges' remarks:
Several IOCCC themes (RPN calculator, animated GIF generator, ray tracing) in one package! Very impressive!
Several IOCCC themes ([RPN
calculator](https://en.wikipedia.org/wiki/Reverse_Polish_notation), animated
[GIF](https://en.wikipedia.org/wiki/GIF) generator, [ray
tracing](https://en.wikipedia.org/wiki/Ray_tracing_(graphics))) in one package!
Very impressive!
The checkerboard pattern in the output can be broken. Could you figure out how?
@@ -38,9 +42,13 @@ Larger GIF files confuse some image viewers.
### Remarks
This program renders a generic RPN expression in ray-traced animated GIF form.
The expression in spatial variables x and y is specified on the command-line with the required resolution,
and an animated GIF showing a fly-around of the resulting surface is output to stdout.
This program renders a generic
[RPN](https://en.wikipedia.org/wiki/Reverse_Polish_notation) expression in
[ray-traced](https://en.wikipedia.org/wiki/Ray_tracing_(graphics)) animated
[GIF](https://en.wikipedia.org/wiki/GIF) form. The expression in spatial
variables `x` and `y` is specified on the command-line with the required
resolution, and an animated GIF showing a fly-around of the resulting surface is
output to stdout.
### Usage
@@ -50,32 +58,46 @@ The program should be run as:
./blakely xy* 32 > pic.gif
```
where the first parameter is a RPN expression, and the second is the width of the required GIF, up to a maximum of 250.
where the first parameter is a
[RPN](https://en.wikipedia.org/wiki/Reverse_Polish_notation) expression, and the
second is the width of the required [GIF](https://en.wikipedia.org/wiki/GIF), up
to a maximum of 250.
The expressions are written in Reverse Polish Notation with the following characters recognised:
The expressions are written in [Reverse Polish
Notation](https://en.wikipedia.org/wiki/Reverse_Polish_notation) with the
following characters recognised:
* +,-,\*,/ - usual arithmetic operations
* ^ - Exponentiation - valid for constant exponent only
* s - sin
* c - cos
* 0-9 - single digit integers
* d - duplicate number on stack
* t - transpose top two elements on stack
* x - x coordinate value
* y - y coordinate value
```
+, -, *, / - usual arithmetic operations
^ - Exponentiation; valid for constant exponent only
s - sin
c - cos
0-9 - single digit integers
d - duplicate number on stack
t - transpose top two elements on stack
x - x coordinate value
y - y coordinate value
```
NB all arithmetic is done in double precision, so 23/ is two-thirds, rather than 0
NB all arithmetic is done in double precision, so `23/` is two-thirds, rather
than 0.
The surface plotted corresponds to -1 <= x,y <= 1 and for best results, the generated z value should lie between -1 and 1.
The surface plotted corresponds to `-1 <= x,y <= 1` and for best results, the
generated `z` value should lie between `-1` and `1`.
If there is a division by zero, square-root of a negative number, or similar operation, then the results are undefined.
If there is a division by zero, square-root of a negative number, or similar
operation, then the results are undefined.
### Examples
For test-runs, use a resolution of 32. For simple expressions, the GIF generation should take only
a few seconds with a modern processor and full optimization. Run-time scales as O(N^3) with resolution N,
and more complex expressions take longer to compute. For example, increasing the resolution of ripple.gif
to 250 took ~35 minutes to render on an i7 processor.
For test-runs, use a resolution of 32. For simple expressions, the
[GIF](https://en.wikipedia.org/wiki/GIF) generation should take only
a few seconds with a modern processor and full optimization. Runtime scales as
[O(N^3)](https://en.wikipedia.org/wiki/Big_O_notation) with resolution `N`,
and more complex expressions take longer to compute. For example, increasing the
resolution of `ripple.gif`
to 250 took ~35 minutes to render on an [i7
processor](https://en.wikipedia.org/wiki/Intel_Core#Core_i7).
```sh
./blakely xy* 32 > saddle.gif
@@ -86,33 +108,41 @@ to 250 took ~35 minutes to render on an i7 processor.
### Algorithm
A generic root-finding algorithm to find the point of intersection between a ray
and the surface is done using a simple forward search from the eye.
In order to make the rendering realistic, both the value of the expression, and its derivative with respect to x and y must be tracked.
This is done using an obvious stack-based approach.
and the surface is done using a simple forward search from the eye. In order to
make the rendering realistic, both the value of the expression, and its
derivative with respect to `x` and `y` must be tracked. This is done using an
obvious stack-based approach.
### Obfuscations
Much of the obfuscation is achieved through the use of single-letter variables and #defines.
The characters output for the GIF format are generated in myriad and wonderful ways, making use
of XOR, return values from putchar, and odd expressions.
The layout is in the form of X Y Z.
Much of the obfuscation is achieved through the use of single-letter variables
and `#define`s. The characters output for the [GIF
format](https://en.wikipedia.org/wiki/GIF#File_format) are generated in myriad
and wonderful ways, making use of
[XOR](https://en.wikipedia.org/wiki/Exclusive_or), return values from
`putchar()`, and odd expressions. The layout is in the form of `X Y Z`.
### Limitations
ASCII is assumed since many bytes for the GIF output are generated from characters given in the source code.
On compilation a few warnings about parentheses come up when using gcc 4.6.3, and one about no return value from main() (who uses exit codes anyway?),
but clang 3.1 emits no warnings.
ASCII is assumed since many bytes for the
[GIF](https://en.wikipedia.org/wiki/GIF) output are generated from characters
given in the source code. On compilation a few warnings about parentheses come
up when using gcc 4.6.3, and one about no return value from `main()` (who uses
exit codes anyway?), but clang 3.1 emits no warnings.
### Questions
* Apart from their appearance when computing the RPN expression, sin and cos are not used.
* Apart from their appearance when computing the [RPN
expression](https://en.wikipedia.org/wiki/Reverse_Polish_notation), [sin and
cos](https://en.wikipedia.org/wiki/Sine_and_cosine) are not used.
How do we then get the correct angles for rotation?
* Exactly how many times will the animation repeat?
* Why is there an upper limit of 250x250 for the resolution?
* What is the point of b[0]?
* Where are the words NETSCAPE and GIF hidden in the source?
* What is the point of `b[0]`?
* Where are the words `NETSCAPE` and `GIF` hidden in the source?
(Note that the author may or may not be able to answer all of the above questions.)
(Note that the author may or may not be able to answer all of the above
questions.)
## Copyright and CC BY-SA 4.0 License: