calculator.coffee

Jump To … +

src/algorithms.coffee src/calculator.coffee src/chroma.coffee src/fpcalc.coffee src/silenceRemover.coffee

calculator.coffee
¶
```
@chromaprint ?= {}
```
¶

Creating images comes down to choosing a ‘shade of grey’ from here.
```
GRAYCODE = [ 0, 1, 3, 2 ]
@chromaprint.GRAYCODE = GRAYCODE
```
¶

Images

Chromaprint does its work on “images” - 2 dimensional arrays which are calculated from an input stream.

integral = (image) ->
  result = []
  image.forEach (row, i) ->
    result[i] ?= []
    row.forEach (n, j) ->
      result[i][j] = n          +
        (result[i  ]?[j-1] ? 0) +
        (result[i-1]?[j  ] ? 0) -
        (result[i-1]?[j-1] ? 0)
  result

area = (image, x1, y1, x2, y2) ->
  x1 ?= 0
  y1 ?= 0
  x2 ?= image.length
  y2 ?= image[x2]?.length
  result = image[x2]?[y2]
  return 0 if x2 < x1 or y2 < y1
  if x1 > 0
    result -= image[x1-1][y2]
    result += image[x1-1][y1-1] if y1 > 0
  result -= image[x2][y1-1] if y1 > 0
  result

measure = (image) ->
  h: image.length
  w: image[0].length ? 0

@chromaprint.Image = { integral, area, measure }

¶

Filters

A filter in chromaprint modifies a portion of an image. There are 5 different filters available. The filter function takes as arguments:

the type of filter (0-4)
the y offset from where to apply the comparison
the x offset from where to apply the comparison
the width upon which to run the comparison
the height upon which to run the comparison
the actual comparison function (currently always subtraction, but log subtraction is also available.

filter = (type, y, w, h, cmp) ->
  cmp ?= filter.compare.subtractLog
  f = (img, x) -> filter[type](img, x, y, w, h, cmp)
  f.w = w
  f.h = h
  f.y = y
  f

filter[0] = (img, x, y, w, h, cmp) ->                       # oooooooooooo
  a = area(img, x, y, x + w - 1, y + h - 1)                 # oooooooooooo
  b = 0                                                     # oooooooooooo
  cmp(a, b)                                                 # oooooooooooo

oooooooooooo oooooooooooo

filter[1] = (img, x, y, w, h, cmp) ->                       # ............
  h2 = h / 2                                                # ............
  a = area(img, x, y + h2, x + w - 1, y + h  - 1)           # ............
  b = area(img, x, y     , x + w - 1, y + h2 - 1)           # oooooooooooo
  cmp(a, b)                                                 # oooooooooooo

oooooooooooo

filter[2] = (img, x, y, w, h, cmp) ->                       # ......oooooo
  w2 = w / 2                                                # ......oooooo
  a = area(img, x + w2, y,  x + w - 1,  y + h - 1)          # ......oooooo
  b = area(img, x     , y, x + w2 - 1,  y + y - 1)          # ......oooooo
  cmp(a, b)                                                 # ......oooooo

……oooooo

filter[3] = (img, x, y, w, h, cmp) ->                       # ......oooooo
  [w2, h2] = [w / 2, h / 2]                                 # ......oooooo
  a = area(img, x     , y + h2, x + w2 - 1, y + h  - 1) +   # ......oooooo
      area(img, x + w2, y     , x + w  - 1, y + h2 - 1)     # oooooo......
  b = area(img, x     , y     , x + w2 - 1, y + h2 - 1) +   # oooooo......
      area(img, x + w2, y + h2, x + w  - 1, y + h  - 1)     # oooooo......
  cmp(a, b)

filter[4] = (img, x, y, w, h, cmp) ->                       # ............
  h3 = h / 3                                                # ............
  a = area(img, x, y +     h3, x + w - 1, y + 2 * h3 - 1)   # oooooooooooo
  b = area(img, x, y         , x + w - 1, y +     h3 - 1) + # oooooooooooo
      area(img, x, y + 2 * h3, x + w - 1, y +     h  - 1)   # ............
  cmp(a, b)                                                 # ............
  
filter[5] = (img, x, y, w, h, cmp) ->                       # ....oooo....
  w3 = w / 3                                                # ....oooo....
  a = area(img, x +     w3, y, x + 2 * w3 - 1, y + h - 1)   # ....oooo....
  b = area(img, x         , y, x +     w3 - 1, y + h - 1) + # ....oooo....
      area(img, x + 2 * w3, y, x +     w  - 1, y + h -1)    # ....oooo....
  cmp(a, b)                                                 # ....oooo....

filter.compare =
  subtract: (a, b) -> a - b
  subtractLog: (a, b) -> Math.log(1 + a) - Math.log(1 + b)

@chromaprint.filter = filter

¶

Quantizer

Makes a very simple function to return 0, 1, 2, or 3 depending on where the value falls within the three values given.

quantizer = (t0, t1, t2) ->
  (value) ->
    if value < t1
      return 0 if value < t0
      return 1
    return 2 if value < t2
    3
@chromaprint.quantizer = quantizer

¶

Classifiers

classifier is a higher order function that makes a filter and a quantizer work together on an image.

classifier = (coefficients) ->
  f = filter(coefficients.f...)
  q = quantizer(coefficients.q...)
  fn = (image, offset) -> q(f(image, offset))
  fn.filterWidth = f.w
  fn
@chromaprint.classifier = classifier

¶

Calculator

The calculator actually calculates the fingerprint of an image, by creating the right classifier, and passing the image into the resulting function in chunks.

calculator = (coefficients...) ->
  classifiers = coefficients.map (cc) ->
    classifier(cc)

  max = classifiers.reduce ((w, c) -> Math.max(w, c.filterWidth)), 0

  subfingerprint = (image, offset) ->
    classify = (b, classifier) -> (b << 2) | GRAYCODE[classifier(image, offset)]
    classifiers.reduce classify, 0
  (image) ->
    length = measure(image).h - max + 1
    img = integral(image)
    subfingerprint(img, offset) for offset in [0...length]

@chromaprint.calculator = calculator

calculator.coffee

Images

Filters

Quantizer

Classifiers

Calculator