Provided by: libmath-planepath-perl_122-1_all 
NAME
Math::PlanePath::DragonMidpoint -- dragon curve midpoints
SYNOPSIS
use Math::PlanePath::DragonMidpoint;
my $path = Math::PlanePath::DragonMidpoint->new;
my ($x, $y) = $path->n_to_xy (123);
DESCRIPTION
This is the midpoint of each segment of the dragon curve of Heighway, Harter, et al, per
Math::PlanePath::DragonCurve.
17--16 9---8 5
| | | |
18 15 10 7 4
| | | |
19 14--13--12--11 6---5---4 3
| |
20--21--22 3 2
| |
33--32 25--24--23 2 1
| | | |
34 31 26 0---1 <- Y=0
| | |
35 30--29--28--27 -1
|
36--37--38 43--44--45--46 -2
| | |
39 42 49--48--47 -3
| | |
40--41 50 -4
|
51 -5
|
52--53--54 -6
|
..--64 57--56--55 -7
| |
63 58 -8
| |
62--61--60--59 -9
^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 X=0 1
The dragon curve begins as follows. The midpoints of each segment are numbered starting
from 0,
+--8--+ +--4--+
| | | |
9 7 5 3
| | | | |
+-10--+--6--+ +--2--+ rotate 45 degrees |
| | v
11 1
| |
+-12--+ *--0--+ * = Origin
|
...
These midpoints are on fractions X=0.5,Y=0, X=1,Y=0.5, etc. For this "DragonMidpoint"
path they're turned clockwise 45 degrees and shrunk by sqrt(2) to be integer X,Y values a
unit apart and initial direction to the right.
The midpoints are distinct X,Y positions because the dragon curve traverses each edge only
once.
The dragon curve is self-similar in 2^level sections due to its unfolding. This can be
seen in the midpoints too as for example above N=0 to N=16 is the same shape as N=16 to
N=32, with the latter rotated 90 degrees and in reverse.
For reference, Knuth in "Diamonds and Dragons" has a different numbering for segment
midpoints where the dragon orientation is unchanged and instead multiply by 2 to have
midpoints as integers. For example the first dragon midpoint at X=1/2,Y=0 is doubled out
to X=1,Y=0. That can be obtained from the path here by
KnuthX = X - Y + 1
KnuthY = X + Y
Arms
Like the "DragonCurve" the midpoints fill a quarter of the plane and four copies mesh
together perfectly when rotated by 90, 180 and 270 degrees. The "arms" parameter can
choose 1 to 4 curve arms, successively advancing.
For example "arms => 4" begins as follows, with N=0,4,8,12,etc being the first arm (the
same as the plain curve above), N=1,5,9,13 the second, N=2,6,10,14 the third and
N=3,7,11,15 the fourth.
arms => 4
...-107-103 83--79--75--71 6
| | |
68--64 36--32 99 87 59--63--67 5
| | | | | | |
72 60 40 28 95--91 55 4
| | | | |
76 56--52--48--44 24--20--16 51 3
| | |
80--84--88 17--13---9---5 12 47--43--39 ... 2
| | | | | |
100--96--92 21 6---2 1 8 27--31--35 106 1
| | | | | |
104 33--29--25 10 3 0---4 23 94--98-102 <- Y=0
| | | | | |
... 37--41--45 14 7--11--15--19 90--86--82 -1
| | |
49 18--22--26 46--50--54--58 78 -2
| | | | |
53 89--93 30 42 62 74 -3
| | | | | | |
65--61--57 85 97 34--38 66--70 -4
| | |
69--73--77--81 101-105-... -5
^
-6 -5 -4 -3 -2 -1 X=0 1 2 3 4 5
With four arms like this every X,Y point is visited exactly once, because four arms of the
"DragonCurve" traverse every edge exactly once.
Tiling
Taking pairs of adjacent points N=2k and N=2k+1 gives little rectangles with the following
tiling of the plane repeating in 4x4 blocks.
+---+---+---+-+-+---+-+-+---+
| | | | | | | | | | |
+---+ | +---+ | +---+ | +---+
| | | |9 8| | | | | | |
+-+-+---+-+-+-+-+-+-+-+-+-+-+
| | | | |7| | | | | | |
| | +---+ | +---+ | +---+ | |
| | | | |6|5 4| | | | | |
+---+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |3| | | | |
+---+ | +---+ | +---+ | +---+
| | | | | |2| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |0 1| | | | | | <- Y=0
| | +---+ | +---+ | +---+ | |
| | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | |
+---+ | +---+ | +---+ | +---+
| | | | | | | | | | |
+---+-+-+---+-+-+---+-+-+---+
^
X=0
The pairs follow this pattern both for the main curve N=0 etc shown, and also for the
rotated copies per "Arms" above. This tiling is in the tilingsearch database as
<http://tilingsearch.org/HTML/data24/K02A.html>
Taking pairs N=2k+1 and N=2k+2, being each odd N and its successor, gives a regular
pattern too, but this time repeating in blocks of 16x16.
|||--||||||--||--||--||||||--||||||--||||||--||||||--||||||--|||
|||--||||||--||--||--||||||--||||||--||||||--||||||--||||||--|||
-||------||------||------||------||------||------||------||-----
-||------||------||------||------||------||------||------||-----
|||--||||||||||||||--||||||||||||||--||||||||||||||--|||||||||||
|||--||||||||||||||--||||||||||||||--||||||||||||||--|||||||||||
-----||------||------||------||------||------||------||------||-
-----||------||------||------||------||------||------||------||-
-||--||--||--||--||--||||||--||--||--||--||--||--||--||||||--||-
-||--||--||--||--||--||||||--||--||--||--||--||--||--||||||--||-
-||------||------||------||------||------||------||------||-----
-||------||------||------||------||------||------||------||-----
|||||||||||--||||||||||||||--||||||||||||||--||||||||||||||--|||
|||||||||||--||||||||||||||--||||||||||||||--||||||||||||||--|||
-----||------||------||------||------||------||------||------||-
-----||------||------||------||------||------||------||------||-
|||--||||||--||--||--||||||--|| ||--||||||--||--||--||||||--|||
|||--||||||--||--||--||||||--|| ||--||||||--||--||--||||||--|||
-||------||------||------||------||------||------||------||-----
-||------||------||------||------||------||------||------||-----
|||--||||||||||||||--||||||||||||||--||||||||||||||--|||||||||||
|||--||||||||||||||--||||||||||||||--||||||||||||||--|||||||||||
-----||------||------||------||------||------||------||------||-
-----||------||------||------||------||------||------||------||-
-||--||||||--||--||--||--||--||--||--||||||--||--||--||--||--||-
-||--||||||--||--||--||--||--||--||--||||||--||--||--||--||--||-
-||------||------||------||------||------||------||------||-----
-||------||------||------||------||------||------||------||-----
|||||||||||--||||||||||||||--||||||||||||||--||||||||||||||--|||
|||||||||||--||||||||||||||--||||||||||||||--||||||||||||||--|||
-----||------||------||------||------||------||------||------||-
-----||------||------||------||------||------||------||------||-
Curve from Midpoints
Since the dragon curve always turns left or right, never straight ahead or reverse, its
segments are alternately horizontal and vertical. Rotated -45 degrees for the midpoints
here this means alternately "opposite diagonal" and "leading diagonal". They fall on X,Y
alternately even or odd. So the original dragon curve can be recovered from the midpoints
by choosing leading diagonal or opposite diagonal segment according to X,Y even or odd,
which is the same as N even or odd.
DragonMidpoint dragon segment
-------------- -----------------
"even" N==0 mod 2 opposite diagonal
which is X+Y==0 mod 2 too
"odd" N==1 mod 2 leading diagonal
which is X+Y==1 mod 2 too
/
3 0 at X=0,Y=0 "even", opposite diagonal
/ 1 at X=1,Y=0 "odd", leading diagonal
\ etc
2
\
\ /
0 1
\ /
FUNCTIONS
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
"$path = Math::PlanePath::DragonMidpoint->new ()"
Create and return a new path object.
"($x,$y) = $path->n_to_xy ($n)"
Return the X,Y coordinates of point number $n on the path. Points begin at 0 and if
"$n < 0" then the return is an empty list.
Fractional positions give an X,Y position along a straight line between the integer
positions.
"$n = $path->n_start()"
Return 0, the first N in the path.
Level Methods
"($n_lo, $n_hi) = $path->level_to_n_range($level)"
Return "(0, 2**$level - 1)", or for multiple arms return "(0, $arms * 2**$level - 1)".
There are 2^level segments comprising the dragon, or arms*2^level when multiple arms,
numbered starting from 0.
FORMULAS
X,Y to N
An X,Y point is turned into N by dividing out digits of a complex base i+1. This base is
per the doubling of the "DragonCurve" at each level. In midpoint coordinates an
adjustment subtracting 0 or 1 must be applied to move an X,Y which is either N=2k or
N=2k+1 to the position where dividing out i+1 gives the N=k X,Y.
The adjustment is in a repeating pattern of 4x4 blocks. Points N=2k and N=2k+1 both move
to the same place corresponding to N=k multiplied by i+1. The adjustment pattern is a
little like the pair tiling shown above, but for some pairs both the N=2k and N=2k+1
positions must move, it's not enough just to shift the N=2k+1 to the N=2k.
Xadj Yadj
Ymod4 Ymod4
3 | 0 1 1 0 3 | 1 1 0 0
2 | 1 0 0 1 2 | 1 1 0 0
1 | 1 0 0 1 1 | 0 0 1 1
0 | 0 1 1 0 0 | 0 0 1 1
+-------- +--------
0 1 2 3 0 1 2 3
Xmod4 Xmod4
The same tables work for both the main curve and for the rotated copies per "Arms" above.
until -1<=X<=0 and 0<=Y<=1
Xm = X - Xadj(X mod 4, Y mod 4)
Ym = Y - Yadj(X mod 4, Y mod 4)
new X,Y = (Xm+i*Ym) / (i+1)
= (Xm+i*Ym) * (1-i)/2
= (Xm+Ym)/2, (Ym-Xm)/2 # Xm+Ym and Ym-Xm are both even
Nbit = Xadj xor Yadj # bits of N low to high
The X,Y reduction stops at one of the start points for the four arms
X,Y endpoint Arm +---+---+
------------ --- | 2 | 1 | Y=1
0, 0 0 +---+---+
0, 1 1 | 3 | 0 | Y=0
-1, 1 2 +---+---+
-1, 0 3 X=-1 X=0
For arms 1 and 3 the N bits must be flipped 0<->1. The arm number and hence whether this
flip is needed is not known until reaching the endpoint.
For bignum calculations there's no need to apply the "/2" shift in newX=(Xm+Ym)/2 and
newY=(Ym-Xm)/2. Instead keep a bit position which is the logical low end and pick out two
bits from there for the Xadj,Yadj lookup. A whole word can be dropped when the bit
position becomes a multiple of 32 or 64 or whatever.
Boundary
Taking unit squares at each point, the boundary MB[k] of the resulting shape from 0 to
N=2^k-1 inclusive can be had from the boundary B[k] of the plain dragon curve. Taking
points N=0 to N=2^k-1 inclusive is the midpoints of the dragon curve line segments N=0 to
N=2^k inclusive.
MB[k] = B[k] + 2
= 4, 6, 10, 18, 30, 50, 86, 146, 246, 418, 710, 1202, ...
2 + x + 2*x^2
generating function 2 * -------------
1 - x - 2*x^3
A unit square at the midpoint is a diamond on a dragon line segment
/ \
/ \ midpoint m
*--m--* diamond on dragon curve line segment
\ /
\ /
A boundary segment of the dragon curve has two sides of the diamond which are boundary.
But when the boundary makes a right hand turn two such sides touch and are therefore not
midpoint boundary.
/^\
/ | \ right turn
\ | //\ two diamond sides touch
\|// \
*<----*
\ /
\ /
The dragon curve at N=0 points East and the last segment N=2^k-1 to N=2^k is North. Since
the curve never overlaps itself this means that when going around the right side of the
curve there is 1 more left turn than right turn,
lefts - rights = 1
The total line segments on the right is the dragon curve R[k] and there are R[k]-1 turns,
so the total turns lefts+rights is
lefts + rights + 1 = R[k]
So the lefts and rights are obtained separately
2*lefts = R[k] adding the two equations
2*rights = R[k] - 2 subtracting the two equations
The result is then
MR[k] = 2*R[k] - 2*rights
= 2*R[k] - 2*(R[k]-2)/2
= R[k] + 2
A similar calculation is made on the left side of the curve. The net turn is the same and
so the same lefts-rights=1 and thus from the dragon curve L[k] left boundary
ML[k] = 2*L[k] - 2*lefts
= 2*L[k] - 2*(L[k]/2)
= L[k]
The total is then
MB[k] = MR[k] + ML[k]
= R[k]+2 + L[k]
= B[k] + 2 since B[k]=R[k]+L[k]
The generating function can be had from the partial fractions form of the dragon curve
boundary. B[k]+2 means adding 2/(1-x) which cancels out the -2/(1-x) in gB(x).
OEIS
The "DragonMidpoint" is in Sloane's Online Encyclopedia of Integer Sequences as
<http://oeis.org/A073089> (etc)
A073089 abs(dY) of n-1 to n, so 0=horizontal,1=vertical
(extra initial 0)
A077860 Y at N=2^k, being Re(-(i+1)^k + i-1)
A090678 turn=1, straight=0 (extra initial 1,1)
A203175 boundary of unit squares N=0 to N=2^k-1, value 4 onwards
A073089
For A073089=abs(dY), the midpoint curve is vertical when the "DragonCurve" has a vertical
followed by a left turn, or horizontal followed by a right turn. "DragonCurve" verticals
are whenever N is odd, and the turn is the bit above the lowest 0 in N (per "Turn" in
Math::PlanePath::DragonCurve). So
abs(dY) = lowbit(N) XOR bit-above-lowest-zero(N)
The n in A073089 is offset by 2 from the N numbering of the path here, so n=N+2. The
initial value at n=1 in A073089 has no corresponding N (it would be N=-1).
The mod-16 definitions in A073089 express combinations of N odd/even and bit-above-low-0
which are the vertical midpoint segments. The recurrence a(8n+1)=a(4n+1) acts to strip
zeros above a low 1 bit,
n = 0b..uu0001
-> 0b...uu001
In terms of N=n-2 this reduces
N = 0b..vv1111
-> 0b...vv111
which has the effect of seeking a lowest 0 in the range of the mod-16 conditions.
SEE ALSO
Math::PlanePath, Math::PlanePath::DragonCurve, Math::PlanePath::DragonRounded
Math::PlanePath::AlternatePaperMidpoint, Math::PlanePath::R5DragonMidpoint,
Math::PlanePath::TerdragonMidpoint
HOME PAGE
<http://user42.tuxfamily.org/math-planepath/index.html>
LICENSE
Copyright 2011, 2012, 2013, 2014, 2015 Kevin Ryde
Math-PlanePath is free software; you can redistribute it and/or modify it under the terms
of the GNU General Public License as published by the Free Software Foundation; either
version 3, or (at your option) any later version.
Math-PlanePath is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with Math-
PlanePath. If not, see <http://www.gnu.org/licenses/>.