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 exactly as above, and the result gets combined before the compression step.
@see ChunkyPNG::Canvas::PNGDecoding @see www.w3.org/TR/PNG/ The W3C PNG format specification
Writes the canvas to a file, encoded as a PNG image. @param [String] filename The file to save the PNG image to. @param constraints (see ChunkyPNG::Canvas::PNGEncoding#to_datastream) @return [void]
# File lib/chunky_png/canvas/png_encoding.rb, line 42 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.
# File lib/chunky_png/canvas/png_encoding.rb, line 49 def to_blob(constraints = {}) to_datastream(constraints).to_blob end
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.
@option constraints [Fixnum] :bit_depth The bit depth to use. This option is only used
for indexed images, in which case it overrides the determined minimal bit depth. For all the other color modes, a bit depth of 8 is used.
@return [ChunkyPNG::Datastream] The PNG datastream containing the encoded canvas. @see ChunkyPNG::Canvas::PNGEncoding#determine_png_encoding
# File lib/chunky_png/canvas/png_encoding.rb, line 74 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], :depth => encoding[:bit_depth], :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[:bit_depth], encoding[:interlace], encoding[:filtering]) ds.data_chunks = Chunk::ImageData.split_in_chunks(data, encoding[:compression]) ds.end_chunk = Chunk::End.new return ds end
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) @return [void]
# File lib/chunky_png/canvas/png_encoding.rb, line 34 def write(io, constraints = {}) to_datastream(constraints).write(io) end
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}
# File lib/chunky_png/canvas/png_encoding.rb, line 102 def determine_png_encoding(constraints = {}) encoding = case constraints when :fast_rgb; { :color_mode => ChunkyPNG::COLOR_TRUECOLOR, :compression => Zlib::BEST_SPEED } when :fast_rgba; { :color_mode => ChunkyPNG::COLOR_TRUECOLOR_ALPHA, :compression => Zlib::BEST_SPEED } when :best_compression; { :compression => Zlib::BEST_COMPRESSION, :filtering => ChunkyPNG::FILTER_PAETH } when :good_compression; { :compression => Zlib::BEST_COMPRESSION, :filtering => ChunkyPNG::FILTER_NONE } when :no_compression; { :compression => Zlib::NO_COMPRESSION } when :black_and_white; { :color_mode => ChunkyPNG::COLOR_GRAYSCALE, :bit_depth => 1 } when Hash; constraints else raise ChunkyPNG::Exception, "Unknown encoding preset: #{constraints.inspect}" end # Do not create a palette 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 encoding[:bit_depth] ||= self.encoding_palette.determine_bit_depth else encoding[:bit_depth] ||= 8 end else self.encoding_palette = self.palette suggested_color_mode, suggested_bit_depth = encoding_palette.best_color_settings encoding[:color_mode] ||= suggested_color_mode encoding[:bit_depth] ||= suggested_bit_depth 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 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] bit_depth The bit depth of the image. @param [Integer] filtering The filtering method to use.
# File lib/chunky_png/canvas/png_encoding.rb, line 203 def encode_png_image_pass_to_stream(stream, color_mode, bit_depth, filtering) start_pos = stream.bytesize pixel_size = Color.pixel_bytesize(color_mode) line_width = Color.scanline_bytesize(color_mode, bit_depth, width) # Determine the filter method encode_method = encode_png_pixels_to_scanline_method(color_mode, bit_depth) 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 0.upto(height - 1) do |y| stream << send(encode_method, row(y)) 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 the canvas according to the PNG format specification with a given color mode and Adam7 interlacing.
This method will split the original canvas 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] bit_depth The bit depth of the image. @param [Integer] filtering The filtering method to use. @return [String] The PNG encoded canvas as string.
# File lib/chunky_png/canvas/png_encoding.rb, line 188 def encode_png_image_with_interlacing(color_mode, bit_depth = 8, filtering = ChunkyPNG::FILTER_NONE) stream = ChunkyPNG::Datastream.empty_bytearray 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, bit_depth, filtering) end stream 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] bit_depth The bit depth of the image. @param [Integer] filtering The filtering method to use. @return [String] The PNG encoded canvas as string.
# File lib/chunky_png/canvas/png_encoding.rb, line 172 def encode_png_image_without_interlacing(color_mode, bit_depth = 8, filtering = ChunkyPNG::FILTER_NONE) stream = ChunkyPNG::Datastream.empty_bytearray encode_png_image_pass_to_stream(stream, color_mode, bit_depth, filtering) stream end
Encodes a line of pixels using 1-bit grayscale mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 300 def encode_png_pixels_to_scanline_grayscale_1bit(pixels) chars = [] pixels.each_slice(8) do |p1, p2, p3, p4, p5, p6, p7, p8| chars << ((p1.nil? ? 0 : (p1 & 0x0000ffff) >> 15 << 7) | (p2.nil? ? 0 : (p2 & 0x0000ffff) >> 15 << 6) | (p3.nil? ? 0 : (p3 & 0x0000ffff) >> 15 << 5) | (p4.nil? ? 0 : (p4 & 0x0000ffff) >> 15 << 4) | (p5.nil? ? 0 : (p5 & 0x0000ffff) >> 15 << 3) | (p6.nil? ? 0 : (p6 & 0x0000ffff) >> 15 << 2) | (p7.nil? ? 0 : (p7 & 0x0000ffff) >> 15 << 1) | (p8.nil? ? 0 : (p8 & 0x0000ffff) >> 15)) end chars.pack('xC*') end
Encodes a line of pixels using 2-bit grayscale mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 318 def encode_png_pixels_to_scanline_grayscale_2bit(pixels) chars = [] pixels.each_slice(4) do |p1, p2, p3, p4| chars << ((p1.nil? ? 0 : (p1 & 0x0000ffff) >> 14 << 6) | (p2.nil? ? 0 : (p2 & 0x0000ffff) >> 14 << 4) | (p3.nil? ? 0 : (p3 & 0x0000ffff) >> 14 << 2) | (p4.nil? ? 0 : (p4 & 0x0000ffff) >> 14)) end chars.pack('xC*') end
Encodes a line of pixels using 2-bit grayscale mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 332 def encode_png_pixels_to_scanline_grayscale_4bit(pixels) chars = [] pixels.each_slice(2) do |p1, p2| chars << ((p1.nil? ? 0 : ((p1 & 0x0000ffff) >> 12) << 4) | (p2.nil? ? 0 : ((p2 & 0x0000ffff) >> 12))) end chars.pack('xC*') end
Encodes a line of pixels using 8-bit grayscale mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 343 def encode_png_pixels_to_scanline_grayscale_8bit(pixels) pixels.map { |p| p >> 8 }.pack("xC#{width}") end
Encodes a line of pixels using 8-bit grayscale alpha mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 350 def encode_png_pixels_to_scanline_grayscale_alpha_8bit(pixels) pixels.pack("xn#{width}") end
Encodes a line of pixels using 1-bit indexed mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 250 def encode_png_pixels_to_scanline_indexed_1bit(pixels) chars = [] pixels.each_slice(8) do |p1, p2, p3, p4, p5, p6, p7, p8| chars << ((encoding_palette.index(p1) << 7) | (encoding_palette.index(p2) << 6) | (encoding_palette.index(p3) << 5) | (encoding_palette.index(p4) << 4) | (encoding_palette.index(p5) << 3) | (encoding_palette.index(p6) << 2) | (encoding_palette.index(p7) << 1) | (encoding_palette.index(p8))) end chars.pack('xC*') end
Encodes a line of pixels using 2-bit indexed mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 268 def encode_png_pixels_to_scanline_indexed_2bit(pixels) chars = [] pixels.each_slice(4) do |p1, p2, p3, p4| chars << ((encoding_palette.index(p1) << 6) | (encoding_palette.index(p2) << 4) | (encoding_palette.index(p3) << 2) | (encoding_palette.index(p4))) end chars.pack('xC*') end
Encodes a line of pixels using 4-bit indexed mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 282 def encode_png_pixels_to_scanline_indexed_4bit(pixels) chars = [] pixels.each_slice(2) do |p1, p2| chars << ((encoding_palette.index(p1) << 4) | (encoding_palette.index(p2))) end chars.pack('xC*') end
Encodes a line of pixels using 8-bit indexed mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 293 def encode_png_pixels_to_scanline_indexed_8bit(pixels) pixels.map { |p| encoding_palette.index(p) }.pack("xC#{width}") end
Returns the method name to use to decode scanlines into pixels. @param [Integer] color_mode The color mode of the image. @param [Integer] depth The bit depth of the image. @return [Symbol] The method name to use for decoding, to be called on the canvas class. @raise [ChunkyPNG::NotSupported] when the color_mode and/or bit depth is not supported.
# File lib/chunky_png/canvas/png_encoding.rb, line 360 def encode_png_pixels_to_scanline_method(color_mode, depth) encoder_method = case color_mode when ChunkyPNG::COLOR_TRUECOLOR; :"encode_png_pixels_to_scanline_truecolor_#{depth}bit" when ChunkyPNG::COLOR_TRUECOLOR_ALPHA; :"encode_png_pixels_to_scanline_truecolor_alpha_#{depth}bit" when ChunkyPNG::COLOR_INDEXED; :"encode_png_pixels_to_scanline_indexed_#{depth}bit" when ChunkyPNG::COLOR_GRAYSCALE; :"encode_png_pixels_to_scanline_grayscale_#{depth}bit" when ChunkyPNG::COLOR_GRAYSCALE_ALPHA; :"encode_png_pixels_to_scanline_grayscale_alpha_#{depth}bit" else nil end raise ChunkyPNG::NotSupported, "No encoder found for color mode #{color_mode} and #{depth}-bit depth!" unless respond_to?(encoder_method) encoder_method end
Encodes a line of pixels using 8-bit truecolor mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 236 def encode_png_pixels_to_scanline_truecolor_8bit(pixels) pixels.pack('x' + ('NX' * width)) end
Encodes a line of pixels using 8-bit truecolor alpha mode. @param [Array<Integer>] pixels A row of pixels of the original image. @return [String] The encoded scanline as binary string
# File lib/chunky_png/canvas/png_encoding.rb, line 243 def encode_png_pixels_to_scanline_truecolor_alpha_8bit(pixels) pixels.pack("xN#{width}") 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] bit_depth The bit depth of the image. @param [Integer] interlace The interlacing method to use. @return [String] The PNG encoded canvas as string.
# File lib/chunky_png/canvas/png_encoding.rb, line 153 def encode_png_pixelstream(color_mode = ChunkyPNG::COLOR_TRUECOLOR, bit_depth = 8, interlace = ChunkyPNG::INTERLACING_NONE, filtering = ChunkyPNG::FILTER_NONE) if color_mode == ChunkyPNG::COLOR_INDEXED raise ChunkyPNG::ExpectationFailed, "This palette is not suitable for encoding!" if encoding_palette.nil? || !encoding_palette.can_encode? raise ChunkyPNG::ExpectationFailed, "This palette has too many colors!" if encoding_palette.size > (1 << bit_depth) end case interlace when ChunkyPNG::INTERLACING_NONE; encode_png_image_without_interlacing(color_mode, bit_depth, filtering) when ChunkyPNG::INTERLACING_ADAM7; encode_png_image_with_interlacing(color_mode, bit_depth, filtering) else raise ChunkyPNG::NotSupported, "Unknown interlacing method: #{interlace}!" end end
Encodes a scanline of a pixelstream using AVERAGE filtering. This will modify the stream. @param (see encode_png_str_scanline_none) @return [void]
# File lib/chunky_png/canvas/png_encoding.rb, line 414 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 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. nil 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 [void]
# File lib/chunky_png/canvas/png_encoding.rb, line 385 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 PAETH filtering. This will modify the stream. @param (see encode_png_str_scanline_none) @return [void]
# File lib/chunky_png/canvas/png_encoding.rb, line 426 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
Encodes a scanline of a pixelstream using SUB filtering. This will modify the stream. @param (see encode_png_str_scanline_none) @return [void]
# File lib/chunky_png/canvas/png_encoding.rb, line 392 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 [void]
# File lib/chunky_png/canvas/png_encoding.rb, line 403 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
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