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

#resample_bilinear(new_width, new_height) ⇒ Object 



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# File 'lib/chunky_png/canvas/resampling.rb', line 134

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

#resample_bilinear!(new_width, new_height) ⇒ ChunkyPNG::Canvas

Resamples the canvas with bilinear interpolation.

Parameters:

  • new_width (Integer) —

    The width of the resampled canvas.

  • new_height (Integer) —

    The height of the resampled canvas.

Returns:



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# File 'lib/chunky_png/canvas/resampling.rb', line 98

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

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



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# File 'lib/chunky_png/canvas/resampling.rb', line 90

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

#resample_nearest_neighbor!(new_width, new_height) ⇒ ChunkyPNG::Canvas

Resamples the canvas using nearest neighbor interpolation.

Parameters:

  • new_width (Integer) —

    The width of the resampled canvas.

  • new_height (Integer) —

    The height of the resampled canvas.

Returns:



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# File 'lib/chunky_png/canvas/resampling.rb', line 74

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

#steps(width, new_width) ⇒ Array<Integer>

Integer Interpolation between two values

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

Parameters:

  • width (Integer) —

    The width of the source

  • new_width (Integer) —

    The width of the destination

Returns:

  • (Array<Integer>) —

    An Array of Integer indicies



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# File 'lib/chunky_png/canvas/resampling.rb', line 21

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
  indicies
end

#steps_residues(width, new_width) ⇒ Array<Integer>

Fractional Interpolation between two values

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

Parameters:

  • width (Integer) —

    The width of the source

  • new_width (Integer) —

    The width of the destination

Returns:

  • (Array<Integer>, Array<Integer>) —

    Two arrays of indicies and residues



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# File 'lib/chunky_png/canvas/resampling.rb', line 38

def steps_residues(width, new_width)
  indicies = Array.new(new_width, nil)
  residues = Array.new(new_width, 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

  [indicies, residues]
end