Object
A convenience class that wraps the logic for extracting the parts of a PNG image that we need to embed them in a PDF
Process a new PNG image
data |
A binary string of PNG data |
# File lib/prawn/images/png.rb, line 29 def initialize(data) data = StringIO.new(data.dup) data.read(8) # Skip the default header @palette = "" @img_data = "" @transparency = {} loop do chunk_size = data.read(4).unpack("N")[0] section = data.read(4) case section when 'IHDR' # we can grab other interesting values from here (like width, # height, etc) values = data.read(chunk_size).unpack("NNCCCCC") @width = values[0] @height = values[1] @bits = values[2] @color_type = values[3] @compression_method = values[4] @filter_method = values[5] @interlace_method = values[6] when 'PLTE' @palette << data.read(chunk_size) when 'IDAT' @img_data << data.read(chunk_size) when 'tRNS' # This chunk can only occur once and it must occur after the # PLTE chunk and before the IDAT chunk @transparency = {} case @color_type when 3 # Indexed colour, RGB. Each byte in this chunk is an alpha for # the palette index in the PLTE ("palette") chunk up until the # last non-opaque entry. Set up an array, stretching over all # palette entries which will be 0 (opaque) or 1 (transparent). @transparency[:indexed] = data.read(chunk_size).unpack("C*") short = 255 - @transparency[:indexed].size @transparency[:indexed] += ([255] * short) if short > 0 when 0 # Greyscale. Corresponding to entries in the PLTE chunk. # Grey is two bytes, range 0 .. (2 ^ bit-depth) - 1 grayval = data.read(chunk_size).unpack("n").first @transparency[:grayscale] = grayval when 2 # True colour with proper alpha channel. @transparency[:rgb] = data.read(chunk_size).unpack("nnn") end when 'IEND' # we've got everything we need, exit the loop break else # unknown (or un-important) section, skip over it data.seek(data.pos + chunk_size) end data.read(4) # Skip the CRC end end
# File lib/prawn/images/png.rb, line 120 def alpha_channel? @color_type == 4 || @color_type == 6 end
Adobe Reader can’t handle 16-bit png channels – chop off the second byte (least significant)
# File lib/prawn/images/png.rb, line 127 def alpha_channel_bits 8 end
Build a PDF object representing this image in document, and return a Reference to it.
# File lib/prawn/images/png.rb, line 134 def build_pdf_object(document) if compression_method != 0 raise Errors::UnsupportedImageType, 'PNG uses an unsupported compression method' end if filter_method != 0 raise Errors::UnsupportedImageType, 'PNG uses an unsupported filter method' end if interlace_method != 0 raise Errors::UnsupportedImageType, 'PNG uses unsupported interlace method' end # some PNG types store the colour and alpha channel data together, # which the PDF spec doesn't like, so split it out. split_alpha_channel! case colors when 1 color = :DeviceGray when 3 color = :DeviceRGB else raise Errors::UnsupportedImageType, "PNG uses an unsupported number of colors (#{png.colors})" end # build the image dict obj = document.ref!( :Type => :XObject, :Subtype => :Image, :Height => height, :Width => width, :BitsPerComponent => bits, :Length => img_data.size, :Filter => :FlateDecode ) unless alpha_channel obj.data[:DecodeParms] = {:Predictor => 15, :Colors => colors, :BitsPerComponent => bits, :Columns => width} end # append the actual image data to the object as a stream obj << img_data # sort out the colours of the image if palette.empty? obj.data[:ColorSpace] = color else # embed the colour palette in the PDF as a object stream palette_obj = document.ref!(:Length => palette.size) palette_obj << palette # build the color space array for the image obj.data[:ColorSpace] = [:Indexed, :DeviceRGB, (palette.size / 3) -1, palette_obj] end # ************************************* # add transparency data if necessary # ************************************* # For PNG color types 0, 2 and 3, the transparency data is stored in # a dedicated PNG chunk, and is exposed via the transparency attribute # of the PNG class. if transparency[:grayscale] # Use Color Key Masking (spec section 4.8.5) # - An array with N elements, where N is two times the number of color # components. val = transparency[:grayscale] obj.data[:Mask] = [val, val] elsif transparency[:rgb] # Use Color Key Masking (spec section 4.8.5) # - An array with N elements, where N is two times the number of color # components. rgb = transparency[:rgb] obj.data[:Mask] = rgb.collect { |x| [x,x] }.flatten elsif transparency[:indexed] # TODO: broken. I was attempting to us Color Key Masking, but I think # we need to construct an SMask i think. Maybe do it inside # the PNG class, and store it in alpha_channel #obj.data[:Mask] = transparency[:indexed] end # For PNG color types 4 and 6, the transparency data is stored as a alpha # channel mixed in with the main image data. The PNG class seperates # it out for us and makes it available via the alpha_channel attribute if alpha_channel? smask_obj = document.ref!( :Type => :XObject, :Subtype => :Image, :Height => height, :Width => width, :BitsPerComponent => alpha_channel_bits, :Length => alpha_channel.size, :Filter => :FlateDecode, :ColorSpace => :DeviceGray, :Decode => [0, 1] ) smask_obj << alpha_channel obj.data[:SMask] = smask_obj end obj end
number of color components to each pixel
# File lib/prawn/images/png.rb, line 94 def colors case self.color_type when 0, 3, 4 return 1 when 2, 6 return 3 end end
Returns the minimum PDF version required to support this image.
# File lib/prawn/images/png.rb, line 249 def min_pdf_version if bits > 8 # 16-bit color only supported in 1.5+ (ISO 32000-1:2008 8.9.5.1) 1.5 elsif alpha_channel? # Need transparency for SMask 1.4 else 1.0 end end
Generated with the Darkfish Rdoc Generator 2.