module ChunkyPNG::Canvas::Resampling

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 ChunkyPNG::Canvas

Public Instance Methods

resample(new_width, new_height)
Also aliased as: resize
resample_bilinear(new_width, new_height) click to toggle source
    # File lib/chunky_png/canvas/resampling.rb
139 def resample_bilinear(new_width, new_height)
140   dup.resample_bilinear!(new_width, new_height)
141 end
resample_bilinear!(new_width, new_height) click to toggle source

Resamples the canvas with bilinear interpolation. @param [Integer] new_width The width of the resampled canvas. @param [Integer] new_height The height of the resampled canvas. @return [ChunkyPNG::Canvas] A new canvas instance with the resampled pixels.

    # File lib/chunky_png/canvas/resampling.rb
103 def resample_bilinear!(new_width, new_height)
104   index_x, interp_x = steps_residues(width, new_width)
105   index_y, interp_y = steps_residues(height, new_height)
106 
107   pixels = Array.new(new_width*new_height)
108   i = 0
109   for y in 1..new_height
110     # Clamp the indicies to the edges of the image
111     y1 = [index_y[y-1], 0].max
112     y2 = [index_y[y-1] + 1, height - 1].min
113     y_residue = interp_y[y-1]
114 
115     for x in 1..new_width
116       # Clamp the indicies to the edges of the image
117       x1 = [index_x[x-1], 0].max
118       x2 = [index_x[x-1] + 1, width - 1].min
119       x_residue = interp_x[x-1]
120 
121       pixel_11 = get_pixel(x1, y1)
122       pixel_21 = get_pixel(x2, y1)
123       pixel_12 = get_pixel(x1, y2)
124       pixel_22 = get_pixel(x2, y2)
125 
126       # Interpolate by Row
127       pixel_top = ChunkyPNG::Color.interpolate_quick(pixel_21, pixel_11, x_residue)
128       pixel_bot = ChunkyPNG::Color.interpolate_quick(pixel_22, pixel_12, x_residue)
129 
130       # Interpolate by Column
131 
132       pixels[i] = ChunkyPNG::Color.interpolate_quick(pixel_bot, pixel_top, y_residue)
133       i += 1
134     end
135   end
136   replace_canvas!(new_width.to_i, new_height.to_i, pixels)
137 end
resample_nearest_neighbor(new_width, new_height) click to toggle source
   # File lib/chunky_png/canvas/resampling.rb
95 def resample_nearest_neighbor(new_width, new_height)
96   dup.resample_nearest_neighbor!(new_width, new_height)
97 end
Also aliased as: resample
resample_nearest_neighbor!(new_width, new_height) click to toggle source

Resamples the canvas using nearest neighbor interpolation. @param [Integer] new_width The width of the resampled canvas. @param [Integer] new_height The height of the resampled canvas. @return [ChunkyPNG::Canvas] A new canvas instance with the resampled pixels.

   # File lib/chunky_png/canvas/resampling.rb
78 def resample_nearest_neighbor!(new_width, new_height)
79   steps_x = steps(width, new_width)
80   steps_y = steps(height, new_height)
81 
82 
83   pixels = Array.new(new_width*new_height)
84   i = 0
85   for y in steps_y
86     for x in steps_x
87       pixels[i] = get_pixel(x, y)
88       i += 1
89     end
90   end
91   
92   replace_canvas!(new_width.to_i, new_height.to_i, pixels)
93 end
resize(new_width, new_height)
Alias for: resample
steps(width, new_width) click to toggle source

Integer Interpolation between two values

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

@param [Integer] width The width of the source @param [Integer] new_width The width of the destination @return [Array<Integer>] An Array of Integer indicies

   # File lib/chunky_png/canvas/resampling.rb
24 def steps(width, new_width)
25   indicies, residues = steps_residues(width, new_width)
26   
27   for i in 1..new_width
28     indicies[i-1] = (indicies[i-1] + (residues[i-1] + 127)/255)
29   end
30   return indicies
31 end
steps_residues(width, new_width) click to toggle source

Fractional Interpolation between two values

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

@param [Integer] width The width of the source @param [Integer] new_width The width of the destination @return [Array<Integer>, Array<Integer>] Two arrays of indicies and residues

   # File lib/chunky_png/canvas/resampling.rb
41 def steps_residues(width, new_width)
42   indicies = Array.new(new_width, obj=nil)
43   residues = Array.new(new_width, obj=nil)
44   
45   # This works by accumulating the fractional error and
46   # overflowing when necessary.
47 
48   # We use mixed number arithmetic with a denominator of
49   # 2 * new_width
50   base_step = width / new_width
51   err_step = (width % new_width) << 1
52   denominator = (new_width) << 1
53           
54   # Initial pixel
55   index = (width - new_width) / denominator
56   err = (width - new_width) % denominator
57 
58   for i in 1..new_width
59     indicies[i-1] = index
60     residues[i-1] = (255.0 * err.to_f / denominator.to_f).round
61 
62     index += base_step
63     err += err_step
64     if err >= denominator
65       index += 1
66       err -= denominator
67     end
68   end
69 
70   return indicies, residues
71 end