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