Create linear interpolations from key points and values
Ruby
Latest commit 4f22c02 Sep 3, 2012 @m104 committed with 0.3.0 release:
* Interpolation class moved to Interpolate::Points
* Binary search to find the correct interpolation interval
* Optional blending function block can be passed to Interpolate::Points
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README.md

Interpolate

Author

Adam Collins [adam@m104.us]

Description

Interpolate is a library for generic linear interpolation objects. Useful for such things as calculating linear motion between locations (or arrays of locations), multi-channel color gradients, piecewise functions, or even just placing values within intervals.

General Usage

Interpolation generators can be created with the Interpolate::Points class, given a Hash of "key points" and associated key values.

By default, the key values should be able to calculate their own blending function (by defining an +interpolate+ instance method). Alternatively, the Interpolate::Points object can be passed a block that takes three arguments: the lower value, the higher value, and the balance ratio between the two.

Here's an example for placing values within one of seven buckets, accomplished with the help of a floor blending function:

require 'rubygems'
require 'interpolate'

# min_value => bucket
buckets = {
  0.000 => 1,
  0.500 => 2,
  1.250 => 3,
  7.725 => 4,
  28.85 => 5,
  50.00 => 6,
  127.5 => 7
}

values = [
  -20.2,
  0.234,
  65.24,
  9.234,
  398.4,
  4000
]

# using Interpolate::Points to place values within discrete intervals
bucketizer = Interpolate::Points.new(buckets)
# the blending function will mimic the mathematical floor function
bucketizer.blend_with {|low, high, balance| low }

values.each do |value|
  bucket = bucketizer.at(value).floor
  puts "A value of #{value} falls into bucket #{bucket}"
end

Array-based Interpolate::Points

Aside from single value gradient points, you can interpolate over uniformly sized arrays. Between two interpolation points, let's say a and b, the final result will be c where c[0] is the interpolation of a[0] and b[0] and c[1] is interpolated between a[1] and b[1] and so on up to c[n].

Here is an example:

require 'rubygems'
require 'interpolate'
require 'pp'

# a non-linear set of multi-dimensional points;
# perhaps the location of some actor in relation to time
time_frames = {
  0 => [0, 0, 0],
  1 => [1, 0, 0],
  2 => [0, 1, 0],
  3 => [0, 0, 2],
  4 => [3, 0, 1],
  5 => [1, 2, 3],
  6 => [0, 0, 0]
}

path = Interpolate::Points.new(time_frames)

# play the actor's positions in time increments of 0.25
(0).step(6, 0.25) do |time|
  position = path.at(time)
  puts ">> At #{time}s, actor is at:"
  p position
end

Nested Array Interpolate::Points

As long as each top level array is uniformly sized in the first dimension and each nested array is uniformly sized in the second dimension (and so on...), multidimensional interpolation point values will just work.

Here's an example of a set of 2D points being morphed:

require 'rubygems'
require 'interpolate'
require 'pp'


# a number of sets 2D vertices, each set corresponding to a particular
# shape on the grid
time_frames = {
  0 => [[0, 0], [1, 0], [2, 0], [3, 0], [4, 0]], # a horizontal line
  1 => [[0, 0], [1, 0], [3, 0], [0, 4], [0, 0]], # a triangle
  2 => [[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]], # a square
  3 => [[0, 0], [1, 0], [2, 0], [3, 0], [4, 0]], # a horizontal line, again
  4 => [[0, 0], [0, 1], [0, 2], [0, 3], [0, 4]]  # a vertical line
}


paths = Interpolate::Points.new(time_frames)

# show the vertex positions in time increments of 0.25
(0).step(4, 0.25) do |time|
  points = paths.at(time)
  puts ">> At #{time}s, points are:"
  p points
end

Other Interpolations

For other classes of value objects, you'll need to implement a blending function. Here's an example using an RGB color gradient with the help of the 'color' gem:

require 'rubygems'
require 'interpolate'
require 'color'

# a nice weathermap-style color gradient
points = {
  1 => Color::RGB::Cyan,
  2 => Color::RGB::Lime,
# 3 => ? (between Lime and Yellow; Interpolate will figure it out)
  4 => Color::RGB::Yellow,
  5 => Color::RGB::Orange,
  6 => Color::RGB::Red,
  7 => Color::RGB::Magenta,
  8 => Color::RGB::White,
}

# we need to implement a blending function in order for Interpolate::Points to
# work properly
#
# fortunately, Color::RGB includes +mix_with+, which is almost functionally
# identical to what we need

gradient = Interpolate::Points.new(points)
gradient.blend_with {|color, other, balance|
  color.mix_with(other, balance * 100.0)
}

# what are the colors of the gradient from 1 to 8
# in increments of 0.2?
(1).step(7, 0.2) do |value|
  color = gradient.at(value)
  puts "A value of #{value.round(3)} means #{color.html}"
end

License

(The MIT License)

Copyright (c) 2008-2012 Adam Collins [adam@m104.us]

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.