class ChunkyPNG::Palette

A palette describes the set of colors that is being used for an image.

A PNG image can contain an explicit palette which defines the colors of that image, but can also use an implicit palette, e.g. all truecolor colors or all grayscale colors.

This palette supports decoding colors from a palette if an explicit palette is provided in a PNG datastream, and it supports encoding colors to an explicit palette (stores as PLTE & tRNS chunks in a PNG file).

@see ChunkyPNG::Color

Public Class Methods

from_canvas(canvas) click to toggle source

Builds a palette instance from a given canvas. @param canvas [ChunkyPNG::Canvas] The canvas to create a palette for. @return [ChunkyPNG::Palette] The palette instance.

   # File lib/chunky_png/palette.rb
59 def self.from_canvas(canvas)
60   # Although we don't need to call .uniq.sort before initializing, because
61   # Palette subclasses SortedSet, we get significantly better performance
62   # by doing so.
63   new(canvas.pixels.uniq.sort)
64 end
from_chunks(palette_chunk, transparency_chunk = nil) click to toggle source

Builds a palette instance from a PLTE chunk and optionally a tRNS chunk from a PNG datastream.

This method will cerate a palette that is suitable for decoding an image.

@param palette_chunk [ChunkyPNG::Chunk::Palette] The palette chunk to

load from

@param transparency_chunk [ChunkyPNG::Chunk::Transparency, nil] The

optional transparency chunk.

@return [ChunkyPNG::Palette] The loaded palette instance. @see ChunkyPNG::Palette#can_decode?

   # File lib/chunky_png/palette.rb
37 def self.from_chunks(palette_chunk, transparency_chunk = nil)
38   return nil if palette_chunk.nil?
39 
40   decoding_map = []
41   index = 0
42 
43   palatte_bytes = palette_chunk.content.unpack("C*")
44   alpha_channel = transparency_chunk ? transparency_chunk.content.unpack("C*") : []
45 
46   index = 0
47   palatte_bytes.each_slice(3) do |bytes|
48     bytes << alpha_channel.fetch(index, ChunkyPNG::Color::MAX)
49     decoding_map << ChunkyPNG::Color.rgba(*bytes)
50     index += 1
51   end
52 
53   new(decoding_map, decoding_map)
54 end
from_pixels(pixels) click to toggle source

Builds a palette instance from a given set of pixels. @param pixels [Enumerable<Integer>] An enumeration of pixels to create a

palette for

@return [ChunkyPNG::Palette] The palette instance.

   # File lib/chunky_png/palette.rb
70 def self.from_pixels(pixels)
71   new(pixels)
72 end
new(enum, decoding_map = nil) click to toggle source

Builds a new palette given a set (Enumerable instance) of colors.

@param enum [Enumerable<Integer>] The set of colors to include in this

palette.This Enumerable can contain duplicates.

@param decoding_map [Array] An array of colors in the exact order at

which they appeared in the palette chunk, so that this array can be
used for decoding.
Calls superclass method
   # File lib/chunky_png/palette.rb
21 def initialize(enum, decoding_map = nil)
22   super(enum)
23   @decoding_map = decoding_map if decoding_map
24 end

Public Instance Methods

[](index) click to toggle source

Returns a color, given the position in the original palette chunk. @param index [Integer] The 0-based position of the color in the palette. @return [ChunkyPNG::Color] The color that is stored in the palette under

the given index

@see ChunkyPNG::Palette#can_decode?

    # File lib/chunky_png/palette.rb
144 def [](index)
145   @decoding_map[index]
146 end
best_color_settings() click to toggle source

Determines the most suitable colormode for this palette. @return [Integer] The colormode which would create the smallest possible

file for images that use this exact palette.
    # File lib/chunky_png/palette.rb
191 def best_color_settings
192   if black_and_white?
193     [ChunkyPNG::COLOR_GRAYSCALE, 1]
194   elsif grayscale?
195     if opaque?
196       [ChunkyPNG::COLOR_GRAYSCALE, 8]
197     else
198       [ChunkyPNG::COLOR_GRAYSCALE_ALPHA, 8]
199     end
200   elsif indexable?
201     [ChunkyPNG::COLOR_INDEXED, determine_bit_depth]
202   elsif opaque?
203     [ChunkyPNG::COLOR_TRUECOLOR, 8]
204   else
205     [ChunkyPNG::COLOR_TRUECOLOR_ALPHA, 8]
206   end
207 end
black_and_white?() click to toggle source

Check whether this palette only contains bacl and white. @return [true, false] True if all colors in this palette are grayscale

teints.

@see ChunkyPNG::Color#grayscale??

    # File lib/chunky_png/palette.rb
100 def black_and_white?
101   entries == [ChunkyPNG::Color::BLACK, ChunkyPNG::Color::WHITE]
102 end
can_decode?() click to toggle source

Checks whether this palette is suitable for decoding an image from a datastream.

This requires that the positions of the colors in the original palette chunk is known, which is stored as an array in the +@decoding_map+ instance variable.

@return [true, false] True if a decoding map was built when this palette

was loaded.
    # File lib/chunky_png/palette.rb
122 def can_decode?
123   !@decoding_map.nil?
124 end
can_encode?() click to toggle source

Checks whether this palette is suitable for encoding an image from to datastream.

This requires that the position of the color in the future palette chunk is known, which is stored as a hash in the +@encoding_map+ instance variable.

@return [true, false] True if a encoding map was built when this palette

was loaded.
    # File lib/chunky_png/palette.rb
135 def can_encode?
136   !@encoding_map.nil?
137 end
determine_bit_depth() click to toggle source

Determines the minimal bit depth required for an indexed image @return [Integer] Number of bits per pixel, i.e. 1, 2, 4 or 8, or nil if

this image cannot be saved as an indexed image.
    # File lib/chunky_png/palette.rb
212 def determine_bit_depth
213   case size
214   when 1..2 then 1
215   when 3..4 then 2
216   when 5..16 then 4
217   when 17..256 then 8
218   end
219 end
grayscale?() click to toggle source

Check whether this palette only contains grayscale colors. @return [true, false] True if all colors in this palette are grayscale

teints.

@see ChunkyPNG::Color#grayscale??

   # File lib/chunky_png/palette.rb
92 def grayscale?
93   all? { |color| Color.grayscale?(color) }
94 end
index(color) click to toggle source

Returns the position of a color in the palette @param color [ChunkyPNG::Color] The color for which to look up the index. @return [Integer] The 0-based position of the color in the palette. @see ChunkyPNG::Palette#can_encode?

    # File lib/chunky_png/palette.rb
152 def index(color)
153   color.nil? ? 0 : @encoding_map[color]
154 end
indexable?() click to toggle source

Checks whether the size of this palette is suitable for indexed storage. @return [true, false] True if the number of colors in this palette is at

most 256
   # File lib/chunky_png/palette.rb
77 def indexable?
78   size <= 256
79 end
opaque?() click to toggle source

Check whether this palette only contains opaque colors. @return [true, false] True if all colors in this palette are opaque. @see ChunkyPNG::Color#opaque?

   # File lib/chunky_png/palette.rb
84 def opaque?
85   all? { |color| Color.opaque?(color) }
86 end
opaque_palette() click to toggle source

Returns a palette with all the opaque variants of the colors in this palette. @return [ChunkyPNG::Palette] A new Palette instance with only opaque

colors.

@see ChunkyPNG::Color#opaque!

    # File lib/chunky_png/palette.rb
109 def opaque_palette
110   self.class.new(map { |c| ChunkyPNG::Color.opaque!(c) })
111 end
to_plte_chunk() click to toggle source

Creates a PLTE chunk that corresponds with this palette to store the r, g, and b channels of all colors.

@note A PLTE chunk should only be included if the image is encoded using

index colors. After this chunk has been built, the palette becomes
suitable for encoding an image.

@return [ChunkyPNG::Chunk::Palette] The PLTE chunk. @see ChunkyPNG::Palette#can_encode?

    # File lib/chunky_png/palette.rb
176 def to_plte_chunk
177   @encoding_map = {}
178   colors        = []
179 
180   each_with_index do |color, index|
181     @encoding_map[color] = index
182     colors += ChunkyPNG::Color.to_truecolor_bytes(color)
183   end
184 
185   ChunkyPNG::Chunk::Palette.new("PLTE", colors.pack("C*"))
186 end
to_trns_chunk() click to toggle source

Creates a tRNS chunk that corresponds with this palette to store the alpha channel of all colors.

Note that this chunk can be left out of every color in the palette is opaque, and the image is encoded using indexed colors.

@return [ChunkyPNG::Chunk::Transparency] The tRNS chunk.

    # File lib/chunky_png/palette.rb
163 def to_trns_chunk
164   ChunkyPNG::Chunk::Transparency.new("tRNS", map { |c| ChunkyPNG::Color.a(c) }.pack("C*"))
165 end