Module: ChunkyPNG::Canvas::Resampling

Included in:
ChunkyPNG::Canvas
Defined in:
lib/chunky_png/canvas/resampling.rb

Overview

The ChunkyPNG::Canvas::Resampling module defines methods to perform image resampling to a ChunkyPNG::Canvas.

Currently, only the nearest neighbor algorithm is implemented. Bilinear and cubic algorithms may be added later on.

See Also:

Instance Method Summary (collapse)

Instance Method Details

- (Object) resample_bilinear(new_width, new_height)



139
140
141
# File 'lib/chunky_png/canvas/resampling.rb', line 139

def resample_bilinear(new_width, new_height)
  dup.resample_bilinear!(new_width, new_height)
end

- (ChunkyPNG::Canvas) resample_bilinear!(new_width, new_height)

Resamples the canvas with bilinear interpolation.



103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
# File 'lib/chunky_png/canvas/resampling.rb', line 103

def resample_bilinear!(new_width, new_height)
  index_x, interp_x = steps_residues(width, new_width)
  index_y, interp_y = steps_residues(height, new_height)

  pixels = Array.new(new_width*new_height)
  i = 0
  for y in 1..new_height
    # Clamp the indicies to the edges of the image
    y1 = [index_y[y-1], 0].max
    y2 = [index_y[y-1] + 1, height - 1].min
    y_residue = interp_y[y-1]

    for x in 1..new_width
      # Clamp the indicies to the edges of the image
      x1 = [index_x[x-1], 0].max
      x2 = [index_x[x-1] + 1, width - 1].min
      x_residue = interp_x[x-1]

      pixel_11 = get_pixel(x1, y1)
      pixel_21 = get_pixel(x2, y1)
      pixel_12 = get_pixel(x1, y2)
      pixel_22 = get_pixel(x2, y2)

      # Interpolate by Row
      pixel_top = ChunkyPNG::Color.interpolate_quick(pixel_21, pixel_11, x_residue)
      pixel_bot = ChunkyPNG::Color.interpolate_quick(pixel_22, pixel_12, x_residue)

      # Interpolate by Column

      pixels[i] = ChunkyPNG::Color.interpolate_quick(pixel_bot, pixel_top, y_residue)
      i += 1
    end
  end
  replace_canvas!(new_width.to_i, new_height.to_i, pixels)
end

- (Object) resample_nearest_neighbor(new_width, new_height) Also known as: resample



95
96
97
# File 'lib/chunky_png/canvas/resampling.rb', line 95

def resample_nearest_neighbor(new_width, new_height)
  dup.resample_nearest_neighbor!(new_width, new_height)
end

- (ChunkyPNG::Canvas) resample_nearest_neighbor!(new_width, new_height)

Resamples the canvas using nearest neighbor interpolation.



78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
# File 'lib/chunky_png/canvas/resampling.rb', line 78

def resample_nearest_neighbor!(new_width, new_height)
  steps_x = steps(width, new_width)
  steps_y = steps(height, new_height)


  pixels = Array.new(new_width*new_height)
  i = 0
  for y in steps_y
    for x in steps_x
      pixels[i] = get_pixel(x, y)
      i += 1
    end
  end
  
  replace_canvas!(new_width.to_i, new_height.to_i, pixels)
end

- (Array<Integer>) steps(width, new_width)

Integer Interpolation between two values

Used for generating indicies for interpolation (eg, nearest neighbour).



24
25
26
27
28
29
30
31
# File 'lib/chunky_png/canvas/resampling.rb', line 24

def steps(width, new_width)
  indicies, residues = steps_residues(width, new_width)
  
  for i in 1..new_width
    indicies[i-1] = (indicies[i-1] + (residues[i-1] + 127)/255)
  end
  return indicies
end

- (Array<Integer>) steps_residues(width, new_width)

Fractional Interpolation between two values

Used for generating values for interpolation (eg, bilinear). Produces both the indices and the interpolation factors (residues).



41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
# File 'lib/chunky_png/canvas/resampling.rb', line 41

def steps_residues(width, new_width)
  indicies = Array.new(new_width, obj=nil)
  residues = Array.new(new_width, obj=nil)
  
  # This works by accumulating the fractional error and
  # overflowing when necessary.

  # We use mixed number arithmetic with a denominator of
  # 2 * new_width
  base_step = width / new_width
  err_step = (width % new_width) << 1
  denominator = (new_width) << 1
          
  # Initial pixel
  index = (width - new_width) / denominator
  err = (width - new_width) % denominator

  for i in 1..new_width
    indicies[i-1] = index
    residues[i-1] = (255.0 * err.to_f / denominator.to_f).round

    index += base_step
    err += err_step
    if err >= denominator
      index += 1
      err -= denominator
    end
  end

  return indicies, residues
end