Module: ChunkyPNG::Canvas::PNGEncoding

### Home Classes Methods

### In Files

- lib/chunky_png/canvas/png_encoding.rb

### Methods

- #determine_png_encoding
- #encode_png_image_pass_to_stream
- #encode_png_image_with_interlacing
- #encode_png_image_without_interlacing
- #encode_png_pixels_to_scanline_grayscale_1bit
- #encode_png_pixels_to_scanline_grayscale_2bit
- #encode_png_pixels_to_scanline_grayscale_4bit
- #encode_png_pixels_to_scanline_grayscale_8bit
- #encode_png_pixels_to_scanline_grayscale_alpha_8bit
- #encode_png_pixels_to_scanline_indexed_1bit
- #encode_png_pixels_to_scanline_indexed_2bit
- #encode_png_pixels_to_scanline_indexed_4bit
- #encode_png_pixels_to_scanline_indexed_8bit
- #encode_png_pixels_to_scanline_method
- #encode_png_pixels_to_scanline_truecolor_8bit
- #encode_png_pixels_to_scanline_truecolor_alpha_8bit
- #encode_png_pixelstream
- #encode_png_str_scanline_average
- #encode_png_str_scanline_none
- #encode_png_str_scanline_paeth
- #encode_png_str_scanline_sub
- #encode_png_str_scanline_up
- #save
- #to_blob
- #to_datastream
- #to_s
- #to_string
- #write

### Files

- BENCHMARKING.rdoc
- CHANGELOG.rdoc
- CONTRIBUTING.rdoc
- README.rdoc

### Class/Module Index

Quicksearch

- ChunkyPNG
- ChunkyPNG::CRCMismatch
- ChunkyPNG::Canvas
- ChunkyPNG::Canvas::Adam7Interlacing
- ChunkyPNG::Canvas::DataUrlExporting
- ChunkyPNG::Canvas::DataUrlImporting
- ChunkyPNG::Canvas::Drawing
- ChunkyPNG::Canvas::Masking
- ChunkyPNG::Canvas::Operations
- ChunkyPNG::Canvas::PNGDecoding
- ChunkyPNG::Canvas::PNGEncoding
- ChunkyPNG::Canvas::Resampling
- ChunkyPNG::Canvas::StreamExporting
- ChunkyPNG::Canvas::StreamImporting
- ChunkyPNG::Chunk
- ChunkyPNG::Chunk::Base
- ChunkyPNG::Chunk::CompressedText
- ChunkyPNG::Chunk::End
- ChunkyPNG::Chunk::Generic
- ChunkyPNG::Chunk::Header
- ChunkyPNG::Chunk::ImageData
- ChunkyPNG::Chunk::InternationalText
- ChunkyPNG::Chunk::Palette
- ChunkyPNG::Chunk::Text
- ChunkyPNG::Chunk::Transparency
- ChunkyPNG::Color
- ChunkyPNG::Datastream
- ChunkyPNG::Dimension
- ChunkyPNG::Exception
- ChunkyPNG::ExpectationFailed
- ChunkyPNG::Image
- ChunkyPNG::NotSupported
- ChunkyPNG::OutOfBounds
- ChunkyPNG::Palette
- ChunkyPNG::Point
- ChunkyPNG::RMagick
- ChunkyPNG::SignatureMismatch
- ChunkyPNG::Vector
- Enumerable
- String

No matching classes.

# ChunkyPNG::Canvas::PNGEncoding

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

### Attributes

encoding_palette[RW]

The palette used for encoding the image.This is only in used for images that get encoded using indexed colors. @return [ChunkyPNG::Palette]

### Public Instance Methods

save(filename, constraints = {}) click to toggle source

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

to_blob(constraints = {}) click to toggle source

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

Also aliased as: to_string, to_s
to_datastream(constraints = {}) click to toggle source

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

to_s(constraints = {}) click to toggle source
Alias for: to_blob
to_string(constraints = {}) click to toggle source
Alias for: to_blob
write(io, constraints = {}) click to toggle source

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

### Protected Instance Methods

determine_png_encoding(constraints = {}) click to toggle source

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

encode_png_image_pass_to_stream(stream, color_mode, bit_depth, filtering) click to toggle source

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

encode_png_image_with_interlacing(color_mode, bit_depth = 8, filtering = ChunkyPNG::FILTER_NONE) click to toggle source

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

encode_png_image_without_interlacing(color_mode, bit_depth = 8, filtering = ChunkyPNG::FILTER_NONE) click to toggle source

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

encode_png_pixels_to_scanline_grayscale_1bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_grayscale_2bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_grayscale_4bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_grayscale_8bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_grayscale_alpha_8bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_indexed_1bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_indexed_2bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_indexed_4bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_indexed_8bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_method(color_mode, depth) click to toggle source

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, true)
      encoder_method
    end

encode_png_pixels_to_scanline_truecolor_8bit(pixels) click to toggle source

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

encode_png_pixels_to_scanline_truecolor_alpha_8bit(pixels) click to toggle source

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

encode_png_pixelstream(color_mode = ChunkyPNG::COLOR_TRUECOLOR, bit_depth = 8, interlace = ChunkyPNG::INTERLACING_NONE, filtering = ChunkyPNG::FILTER_NONE) click to toggle source

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

encode_png_str_scanline_average(stream, pos, prev_pos, line_width, pixel_size) click to toggle source

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

encode_png_str_scanline_none(stream, pos, prev_pos, line_width, pixel_size) click to toggle source

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

encode_png_str_scanline_paeth(stream, pos, prev_pos, line_width, pixel_size) click to toggle source

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

encode_png_str_scanline_sub(stream, pos, prev_pos, line_width, pixel_size) click to toggle source

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

encode_png_str_scanline_up(stream, pos, prev_pos, line_width, pixel_size) click to toggle source

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

[Validate]

Generated with the Darkfish Rdoc Generator 2.