module ChunkyPNG::Canvas::Adam7Interlacing

Methods for decoding and encoding Adam7 interlacing.

Adam7 interlacing extracts 7 pass images out of a single image, that can be encoded to a stream separately so the image can be built up progressively. The module is included into ChunkyPNG canvas and is used to extract the pass images from the original image, or to reconstruct an original image from separate pass images.

Public Instance Methods

adam7_extract_pass(pass, canvas) click to toggle source

Extracts a pass from a complete image @param [Integer] pass The pass number, should be in 0..6. @param [ChunkyPNG::Canvas] canvas The image that is being deconstructed. @return [ChunkyPNG::Canvas] The extracted pass image.

   # File lib/chunky_png/canvas/adam7_interlacing.rb
63 def adam7_extract_pass(pass, canvas)
64   x_shift, x_offset, y_shift, y_offset = adam7_multiplier_offset(pass)
65   sm_pixels = []
66 
67   y_offset.step(canvas.height - 1, 1 << y_shift) do |y|
68     x_offset.step(canvas.width - 1, 1 << x_shift) do |x|
69       sm_pixels << canvas[x, y]
70     end
71   end
72 
73   new_canvas_args = adam7_pass_size(pass, canvas.width, canvas.height) + [sm_pixels]
74   ChunkyPNG::Canvas.new(*new_canvas_args)
75 end
adam7_merge_pass(pass, canvas, subcanvas) click to toggle source

Merges a pass image into a total image that is being constructed. @param [Integer] pass The pass number, should be in 0..6. @param [ChunkyPNG::Canvas] canvas The image that is being constructed. @param [ChunkyPNG::Canvas] subcanvas The pass image that should be merged

   # File lib/chunky_png/canvas/adam7_interlacing.rb
48 def adam7_merge_pass(pass, canvas, subcanvas)
49   x_shift, x_offset, y_shift, y_offset = adam7_multiplier_offset(pass)
50   for y in 0...subcanvas.height do
51     for x in 0...subcanvas.width do
52       new_x = (x << x_shift) | x_offset
53       new_y = (y << y_shift) | y_offset
54       canvas[new_x, new_y] = subcanvas[x, y]
55     end
56   end
57 end
adam7_multiplier_offset(pass) click to toggle source

Returns an array with the x-shift, x-offset, y-shift and y-offset for the requested pass. @param [Integer] pass The pass number, should be in 0..6.

   # File lib/chunky_png/canvas/adam7_interlacing.rb
14 def adam7_multiplier_offset(pass)
15   [
16     3 - (pass >> 1),
17     pass & 1 == 0 ? 0 : 8 >> ((pass + 1) >> 1),
18     pass == 0 ? 3 : 3 - ((pass - 1) >> 1),
19     pass == 0 || pass & 1 == 1 ? 0 : 8 >> (pass >> 1),
20   ]
21 end
adam7_pass_size(pass, original_width, original_height) click to toggle source

Returns the pixel dimensions of the requested pass. @param [Integer] pass The pass number, should be in 0..6. @param [Integer] original_width The width of the original image. @param [Integer] original_height The height of the original image.

   # File lib/chunky_png/canvas/adam7_interlacing.rb
27 def adam7_pass_size(pass, original_width, original_height)
28   x_shift, x_offset, y_shift, y_offset = adam7_multiplier_offset(pass)
29   [
30     (original_width  - x_offset + (1 << x_shift) - 1) >> x_shift,
31     (original_height - y_offset + (1 << y_shift) - 1) >> y_shift,
32   ]
33 end
adam7_pass_sizes(original_width, original_height) click to toggle source

Returns an array of the dimension of all the pass images. @param [Integer] original_width The width of the original image. @param [Integer] original_height The height of the original image. @return [Array<Array<Integer>>] Returns an array with 7 pairs of dimensions. @see adam7_pass_size

   # File lib/chunky_png/canvas/adam7_interlacing.rb
40 def adam7_pass_sizes(original_width, original_height)
41   (0...7).map { |pass| adam7_pass_size(pass, original_width, original_height) }
42 end