module ChunkyPNG
class Canvas
# Methods for encoding a Canvas instance into a PNG datastream.
#
# Overview of the encoding process:
#
# * The image is split up in scanlines (i.e. rows of pixels);
# * All pixels are encoded as a pixelstream, based on the color mode.
# * All the pixel bytes in the pixelstream are adjusted using a filtering
# method if one is specified.
# * Compress the resulting string using deflate compression.
# * Split compressed data over one or more PNG chunks.
# * These chunks should be embedded in a datastream with at least a IHDR and
# IEND chunk and possibly a PLTE chunk.
#
# For interlaced images, the initial image is first split into 7 subimages.
# These images get encoded exectly as above, and the result gets combined
# before the compression step.
#
# @see ChunkyPNG::Canvas::PNGDecoding
# @see http://www.w3.org/TR/PNG/ The W3C PNG format specification
module PNGEncoding
# The palette used for encoding the image.This is only in used for images
# that get encoded using indexed colors.
# @return [ChunkyPNG::Palette]
attr_accessor :encoding_palette
# Writes the canvas to an IO stream, encoded as a PNG image.
# @param [IO] io The output stream to write to.
# @param constraints (see ChunkyPNG::Canvas::PNGEncoding#to_datastream)
def write(io, constraints = {})
to_datastream(constraints).write(io)
end
# Writes the canvas to a file, encoded as a PNG image.
# @param [String] filname The file to save the PNG image to.
# @param constraints (see ChunkyPNG::Canvas::PNGEncoding#to_datastream)
def save(filename, constraints = {})
File.open(filename, 'wb') { |io| write(io, constraints) }
end
# Encoded the canvas to a PNG formatted string.
# @param constraints (see ChunkyPNG::Canvas::PNGEncoding#to_datastream)
# @return [String] The PNG encoded canvas as string.
def to_blob(constraints = {})
to_datastream(constraints).to_blob
end
alias_method :to_string, :to_blob
alias_method :to_s, :to_blob
# Converts this Canvas to a datastream, so that it can be saved as a PNG image.
# @param [Hash, Symbol] constraints The constraints to use when encoding the canvas.
# This can either be a hash with different constraints, or a symbol which acts as a
# preset for some constraints. If no constraints are given, ChunkyPNG will decide
# for itself how to best create the PNG datastream.
# Supported presets are <tt>:fast_rgba</tt> for quickly saving images with transparency,
# <tt>:fast_rgb</tt> for quickly saving opaque images, and <tt>:best_compression</tt> to
# obtain the smallest possible filesize.
# @option constraints [Fixnum] :color_mode The color mode to use. Use one of the
# ChunkyPNG::COLOR_* constants.
# @option constraints [true, false] :interlace Whether to use interlacing.
# @option constraints [Fixnum] :compression The compression level for Zlib. This can be a
# value between 0 and 9, or a Zlib constant like Zlib::BEST_COMPRESSION.
# @return [ChunkyPNG::Datastream] The PNG datastream containing the encoded canvas.
# @see ChunkyPNG::Canvas::PNGEncoding#determine_png_encoding
def to_datastream(constraints = {})
encoding = determine_png_encoding(constraints)
ds = Datastream.new
ds.header_chunk = Chunk::Header.new(:width => width, :height => height,
:color => encoding[:color_mode], :interlace => encoding[:interlace])
if encoding[:color_mode] == ChunkyPNG::COLOR_INDEXED
ds.palette_chunk = encoding_palette.to_plte_chunk
ds.transparency_chunk = encoding_palette.to_trns_chunk unless encoding_palette.opaque?
end
data = encode_png_pixelstream(encoding[:color_mode], encoding[:interlace], encoding[:filtering])
ds.data_chunks = Chunk::ImageData.split_in_chunks(data, encoding[:compression])
ds.end_chunk = Chunk::End.new
return ds
end
protected
# Determines the best possible PNG encoding variables for this image, by analyzing
# the colors used for the image.
#
# You can provide constraints for the encoding variables by passing a hash with
# encoding variables to this method.
#
# @param [Hash, Symbol] constraints The constraints for the encoding. This can be a
# Hash or a preset symbol.
# @return [Hash] A hash with encoding options for {ChunkyPNG::Canvas::PNGEncoding#to_datastream}
def determine_png_encoding(constraints = {})
if constraints == :fast_rgb
encoding = { :color_mode => ChunkyPNG::COLOR_TRUECOLOR, :compression => Zlib::BEST_SPEED }
elsif constraints == :fast_rgba
encoding = { :color_mode => ChunkyPNG::COLOR_TRUECOLOR_ALPHA, :compression => Zlib::BEST_SPEED }
elsif constraints == :best_compression
encoding = { :compression => Zlib::BEST_COMPRESSION, :filtering => ChunkyPNG::FILTER_PAETH }
elsif constraints == :good_compression
encoding = { :compression => Zlib::BEST_COMPRESSION, :filtering => ChunkyPNG::FILTER_NONE }
elsif constraints == :no_compression
encoding = { :compression => Zlib::NO_COMPRESSION }
else
encoding = constraints
end
# Do not create a pallete when the encoding is given and does not require a palette.
if encoding[:color_mode]
if encoding[:color_mode] == ChunkyPNG::COLOR_INDEXED
self.encoding_palette = self.palette
end
else
self.encoding_palette = self.palette
encoding[:color_mode] ||= encoding_palette.best_colormode
end
# Use Zlib's default for compression unless otherwise provided.
encoding[:compression] ||= Zlib::DEFAULT_COMPRESSION
encoding[:interlace] = case encoding[:interlace]
when nil, false, ChunkyPNG::INTERLACING_NONE; ChunkyPNG::INTERLACING_NONE
when true, ChunkyPNG::INTERLACING_ADAM7; ChunkyPNG::INTERLACING_ADAM7
else encoding[:interlace]
end
encoding[:filtering] ||= case encoding[:compression]
when Zlib::BEST_COMPRESSION; ChunkyPNG::FILTER_PAETH
when Zlib::NO_COMPRESSION..Zlib::BEST_SPEED; ChunkyPNG::FILTER_NONE
else ChunkyPNG::FILTER_UP
end
return encoding
end
# Encodes the canvas according to the PNG format specification with a given color
# mode, possibly with interlacing.
# @param [Integer] color_mode The color mode to use for encoding.
# @param [Integer] interlace The interlacing method to use.
# @return [String] The PNG encoded canvas as string.
def encode_png_pixelstream(color_mode = ChunkyPNG::COLOR_TRUECOLOR, interlace = ChunkyPNG::INTERLACING_NONE, filtering = ChunkyPNG::FILTER_NONE)
if color_mode == ChunkyPNG::COLOR_INDEXED && (encoding_palette.nil? || !encoding_palette.can_encode?)
raise ChunkyPNG::ExpectationFailed, "This palette is not suitable for encoding!"
end
case interlace
when ChunkyPNG::INTERLACING_NONE then encode_png_image_without_interlacing(color_mode, filtering)
when ChunkyPNG::INTERLACING_ADAM7 then encode_png_image_with_interlacing(color_mode, filtering)
else raise ChunkyPNG::NotSupported, "Unknown interlacing method: #{interlace}!"
end
end
# Encodes the canvas according to the PNG format specification with a given color mode.
# @param [Integer] color_mode The color mode to use for encoding.
# @param [Integer] filtering The filtering method to use.
# @return [String] The PNG encoded canvas as string.
def encode_png_image_without_interlacing(color_mode, filtering = ChunkyPNG::FILTER_NONE)
stream = ""
encode_png_image_pass_to_stream(stream, color_mode, filtering)
stream
end
# Encodes the canvas according to the PNG format specification with a given color
# mode and Adam7 interlacing.
#
# This method will split the original canva in 7 smaller canvases and encode them
# one by one, concatenating the resulting strings.
#
# @param [Integer] color_mode The color mode to use for encoding.
# @param [Integer] filtering The filtering method to use.
# @return [String] The PNG encoded canvas as string.
def encode_png_image_with_interlacing(color_mode, filtering = ChunkyPNG::FILTER_NONE)
stream = ""
0.upto(6) do |pass|
subcanvas = self.class.adam7_extract_pass(pass, self)
subcanvas.encoding_palette = encoding_palette
subcanvas.encode_png_image_pass_to_stream(stream, color_mode, filtering)
end
stream
end
# Encodes the canvas to a stream, in a given color mode.
# @param [String] stream The stream to write to.
# @param [Integer] color_mode The color mode to use for encoding.
# @param [Integer] filtering The filtering method to use.
def encode_png_image_pass_to_stream(stream, color_mode, filtering)
start_pos = stream.bytesize
pixel_size = Color.bytesize(color_mode)
line_width = pixel_size * width
# Encode the whole image without filtering
stream << case color_mode
when ChunkyPNG::COLOR_TRUECOLOR; pixels.pack(('x' + ('NX' * width)) * height)
when ChunkyPNG::COLOR_TRUECOLOR_ALPHA; pixels.pack("xN#{width}" * height)
when ChunkyPNG::COLOR_INDEXED; pixels.map { |p| encoding_palette.index(p) }.pack("xC#{width}" * height)
when ChunkyPNG::COLOR_GRAYSCALE; pixels.map { |p| p >> 8 }.pack("xC#{width}" * height)
when ChunkyPNG::COLOR_GRAYSCALE_ALPHA; pixels.pack("xn#{width}" * height)
else raise ChunkyPNG::NotSupported, "Cannot encode pixels for this mode: #{color_mode}!"
end
# Determine the filter method
filter_method = case filtering
when ChunkyPNG::FILTER_SUB; :encode_png_str_scanline_sub
when ChunkyPNG::FILTER_UP; :encode_png_str_scanline_up
when ChunkyPNG::FILTER_AVERAGE; :encode_png_str_scanline_average
when ChunkyPNG::FILTER_PAETH; :encode_png_str_scanline_paeth
else nil
end
# Now, apply filtering if any
if filter_method
(height - 1).downto(0) do |y|
pos = start_pos + y * (line_width + 1)
prev_pos = (y == 0) ? nil : pos - (line_width + 1)
send(filter_method, stream, pos, prev_pos, line_width, pixel_size)
end
end
end
# Encodes a scanline of a pixelstream without filtering. This is a no-op.
# @param [String] stream The pixelstream to work on. This string will be modified.
# @param [Integer] pos The starting position of the scanline.
# @param [Integer, nil] prev_pos The starting position of the previous scanline. <tt>nil</tt> if
# this is the first line.
# @param [Integer] line_width The number of bytes in this scanline, without counting the filtering
# method byte.
# @param [Integer] pixel_size The number of bytes used per pixel.
# @return [nil]
def encode_png_str_scanline_none(stream, pos, prev_pos, line_width, pixel_size)
# noop - this method shouldn't get called at all.
end
# Encodes a scanline of a pixelstream using SUB filtering. This will modify the stream.
# @param (see #encode_png_str_scanline_none)
# @return [nil]
def encode_png_str_scanline_sub(stream, pos, prev_pos, line_width, pixel_size)
line_width.downto(1) do |i|
a = (i > pixel_size) ? stream.getbyte(pos + i - pixel_size) : 0
stream.setbyte(pos + i, (stream.getbyte(pos + i) - a) & 0xff)
end
stream.setbyte(pos, ChunkyPNG::FILTER_SUB)
end
# Encodes a scanline of a pixelstream using UP filtering. This will modify the stream.
# @param (see #encode_png_str_scanline_none)
# @return [nil]
def encode_png_str_scanline_up(stream, pos, prev_pos, line_width, pixel_size)
line_width.downto(1) do |i|
b = prev_pos ? stream.getbyte(prev_pos + i) : 0
stream.setbyte(pos + i, (stream.getbyte(pos + i) - b) & 0xff)
end
stream.setbyte(pos, ChunkyPNG::FILTER_UP)
end
# Encodes a scanline of a pixelstream using AVERAGE filtering. This will modify the stream.
# @param (see #encode_png_str_scanline_none)
# @return [nil]
def encode_png_str_scanline_average(stream, pos, prev_pos, line_width, pixel_size)
line_width.downto(1) do |i|
a = (i > pixel_size) ? stream.getbyte(pos + i - pixel_size) : 0
b = prev_pos ? stream.getbyte(prev_pos + i) : 0
stream.setbyte(pos + i, (stream.getbyte(pos + i) - ((a + b) >> 1)) & 0xff)
end
stream.setbyte(pos, ChunkyPNG::FILTER_AVERAGE)
end
# Encodes a scanline of a pixelstream using PAETH filtering. This will modify the stream.
# @param (see #encode_png_str_scanline_none)
# @return [nil]
def encode_png_str_scanline_paeth(stream, pos, prev_pos, line_width, pixel_size)
line_width.downto(1) do |i|
a = (i > pixel_size) ? stream.getbyte(pos + i - pixel_size) : 0
b = (prev_pos) ? stream.getbyte(prev_pos + i) : 0
c = (prev_pos && i > pixel_size) ? stream.getbyte(prev_pos + i - pixel_size) : 0
p = a + b - c
pa = (p - a).abs
pb = (p - b).abs
pc = (p - c).abs
pr = (pa <= pb && pa <= pc) ? a : (pb <= pc ? b : c)
stream.setbyte(pos + i, (stream.getbyte(pos + i) - pr) & 0xff)
end
stream.setbyte(pos, ChunkyPNG::FILTER_PAETH)
end
end
end
end