On May 1, 2007, at 12:09 AM, R. Elliott M. wrote:
I’d use RMagick, but it’s overkill. I don’t need any complex
processing, just the width/height of the image. In my experience
RMagick adds a lot of overhead.
Well, I intend this to make it onto RubyForge one day, but since you
have a need 
Drop this into your RAILS_ROOT/lib and Enjoy! If
you want the test/unit/image_size_test.rb, send me an offline email.
-Rob
image_size.rb
Copyright (c) 2007 Rob B.
Rob_Biedenharn [at] alum.mit.edu
Permission is hereby granted, free of charge, to any person
obtaining a copy
of this software and associated documentation files (the
“Software”), to
deal in the Software without restriction, including without
limitation the
rights to use, copy, modify, merge, publish, distribute,
sublicense, and/or
sell copies of the Software, and to permit persons to whom the
Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be
included in
all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE
SOFTWARE.
require ‘stringio’ # strictly for the ::of_blob
Given a duck-typed +IO+ object (which could be from +StringIO+,
+File+,
+open-uri+, etc.) containing image data, attempt to grab the width and
height. As a convenience, the size is defined as “#{width}x#{height}”
The description of JPEG and GIF files was taken from the
/usr/share/file/magic file from Mac OS X. The PNG
description was
validated against the spec at
class ImageSize
Prepare a new IO source. Should support IO#pos=, IO#read, and
IO#gets.
Supported formats:
* JPEG
* PNG
* GIF
def initialize source
@image = source
@header = nil
@width = @height = nil
end
A convenience method to pull size information from a file
def self.of_file path
obj = nil
File.open(path) do |f|
obj = new(f)
obj.size
end
obj
end
A convenience method to pull size information from a blob (i.e.,
a +String+)
def self.of_blob blob
obj = nil
sio = ::StringIO.new(blob)
obj = self.new(sio)
obj.size
obj
end
def size
[self.width,self.height] * ‘x’
end
[ :width, :height ].each do |property|
meth = “def #{property}; @#{property} ||= case\n”
[ :jpeg, :png, :gif ].each do |format|
meth << " when #{format}? : #{format}_#{property}; "
end
meth << " else -1; end; end"
class_eval meth
end
protected
Generic header takes bytes from the front. Once a type is known, a
format-specific header is retrieved from the data.
def header
@header unless @header.nil?
@image.pos=0
@image.read(100)
end
==================================================
# GIF
0 string GIF8 GIF image data
>4 string 7a \b, version 8%s,
>4 string 9a \b, version 8%s,
def gif?
header[0…6] =~ /GIF8[79]a/
end
>6 leshort >0 %hd x
>8 leshort >0 %hd
def gif_width
gif_header[6…8].unpack(‘v’).first
end
def gif_height
gif_header[8…10].unpack(‘v’).first
end
def gif_header
@header ||= read_gif_header
end
def read_gif_header
@image.pos=0
@image.read(10)
end
==================================================
PNG
# 137 P N G \r \n ^Z \n [4-byte length] H E A D [HEAD data]
[HEAD crc] …
0 string \x89PNG PNG image data,
>4 belong !0x0d0a1a0a CORRUPTED,
>4 belong 0x0d0a1a0a
def png?
header[0…8] == “\x89PNG\r\n\x1a\n”
end
>>16 belong x %ld x
>>20 belong x %ld,
Chunks.html
4.1.1. IHDR Image header
The IHDR chunk must appear FIRST. It contains:
Width: 4 bytes
Height: 4 bytes
Bit depth: 1 byte
Color type: 1 byte
Compression method: 1 byte
Filter method: 1 byte
Interlace method: 1 byte
def png_width
png_header[16…20].unpack(‘N’).first
end
def png_height
png_header[20…24].unpack(‘N’).first
end
def png_header
@header ||= read_png_header
end
def read_png_header
@image.pos=0
@image.read(37) # through the end of the IHDR chunk
end
==================================================
JPEG
def jpeg?
# 0 beshort 0xffd8 JPEG image data
header[0…2] == “\xff\xd8” # ffd8 is the SOI - Start of Image
marker
end
def jpeg_header
@header ||= read_jpeg_header
end
The JFIF header is nasty in the sense that the size marker isn’t
in a
fixed place. Build the header up in pieces by reading the
markers and
data sizes until an “interesting” segment is found.
JPEG_SEG = /\xff[\xc0\xc1\xc2]/nm
def read_jpeg_header
@image.pos=0
buffer = ‘’
buffer << @image.read(4) # ffd8 ff__
found_segment = false # the first one has to be ffe_ which
is an
# APPx segment and not one that
would match
# our interesting JPEG_SEG pattern
while buffer[-2,2] != “\xff\xd9” # ffd9 is the EOI - End of
Image marker
buffer << @image.read(2)
datasize = buffer[-2,2].unpack(‘n’).first
# datasize includes the 2 bytes for itself so this gets the data
# (datasize-2 bytes) and the next marker (2 bytes), too
buffer << @image.read(datasize)
break if found_segment
found_segment = buffer[-2,2] =~ JPEG_SEG
end
buffer
end
Or, we can show the encoding type (I’ve included only the three
most common)
and image dimensions if we are lucky and the SOFn (image
segment) is here:
>(4.S+5) byte 0xC0 \b, baseline
>>(4.S+6) byte x \b, precision %d
>>(4.S+7) beshort x \b, %dx
>>(4.S+9) beshort x \b%d
>(4.S+5) byte 0xC1 \b, extended sequential
>>(4.S+6) byte x \b, precision %d
>>(4.S+7) beshort x \b, %dx
>>(4.S+9) beshort x \b%d
>(4.S+5) byte 0xC2 \b, progressive
>>(4.S+6) byte x \b, precision %d
>>(4.S+7) beshort x \b, %dx
>>(4.S+9) beshort x \b%d
DIM_RE = Regexp.new(%{#{JPEG_SEG}…(…)(…)}, Regexp::MULTILINE,
‘n’)
def jpeg_width
begin
jpeg_header[DIM_RE, 2].unpack(‘n’).first if jpeg?
rescue => e
raise ArgumentError, “Looking for #{DIM_RE} in JPEG data:\n#
{jpeg_header.unpack(‘H*’).first.scan(/.{2,64}/).join(”\n")}\n"
end
end
def jpeg_height
begin
jpeg_header[DIM_RE, 1].unpack(‘n’).first if jpeg?
rescue => e
raise ArgumentError, “Looking for #{DIM_RE} in JPEG data:\n#
{jpeg_header.unpack(‘H*’).first.scan(/.{2,64}/).join(”\n")}\n"
end
end
end
END
Rob B. http://agileconsultingllc.com
[email protected]
