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README for [stream]
===================
This README file will talk about the internals of [stream], that is,
how it knows which colour to pick.
The difficulty resides with the number of different possible cases. If
we were to allow the check for the 8 surrounding chunks, that would
give a 2^8 (256) possibilities and thus 256 different colours which is
too much since the image is indexed and thus already has a limit of
256 colours. So we'll check for only the 4 cardinal directions (N, E,
S, W), which gives 2^4 (16) possibilities.
The goal is to find who around the chunk we want to transform is of
same nature, that is, has the same colour as the chunk under
consideration, this way we'll know if we should transform it into a
straight line, a right corner angle, a dead-end or a 4-way crossing
for instance.
Let's give arbitrary values of:
- 1 for "same chunk on north side" (binary: 0001)
- 2 for "same chunk on east side" (binary: 0010)
- 4 for "same chunk on south side" (binary: 0100)
- 8 for "same chunk on west side" (binary: 1000)
We can image 4 functions that will return either 1 or 0 if a chunk
exists around the one being transformed on each side. Let's call those
is_north, is_east, is_south and is_west.
To know what identical chunk is around, we just make the sum of the 4
tests multiplied by the arbitrary value of each:
1 x is_north() + 2 x is_east() + 4 x is_south() + 8 x is_west().
This will return a value between 0 and 15 (16 different cases) which
cover all possible cases. All we have to do is to retrieve a colour
value for this newly calculated index.
In a word, we have, is the chunk is for a 4-way crossing, we'd have:
1111
WSEN
A T-crossing, would be:
1110
WSE-
This is why we store in the config file the colour in the following order:
0 1-n 1-e 2-ne 1-s 2-ns 2-es 3-nes 1-w 2-nw 2-ew 3-new 2-sww 3-nsw 3-esw 4
If you were to calculate the corresponding index in the same order
you'd find it goes from 0 to 15 by step of 1, so we can store the
appropriate colour at the right index.
Written by Artaxerxes2 at iname dot com
=======================================
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