From: Timothy Hunter <TimHunter@...>
Date: 2006-09-17T01:21:57+09:00
Subject: Re: RMagick problem: Image::read works well but Image::from_blob fails

Mike Meng wrote:
> Hi all,
>     I want to determine the geometry of an image. The image file may be
> in local harddisk or on web. So I use open-uri and
> Magick::Image::from_blob to do that work, here is the code:
>
> open('<path or URI of image file>') { |f|
>     image = Magick::Image::from_blob(f.read).first
>     puts "Width: #{image.columns}; Height: #{image.rows}"
> }
>
> For most of the cases, this works well. However, we I tried to read an
> JPEG generated by my Ricoh Caplio digital camera, it failed and emitted
> these message:
>
> ---------- code ----------
> E:/workspace/ruby/idapted/testrmg.rb:6:in `from_blob': JPEG datastream
> contains
> no image `' (Magick::ImageMagickError)
>   ....
> -----------------------------
>
>   Surprisingly enough, if I use Magick::Image::read method to read the
> same image file directly from harddisk, everything is fine, here is the
> code:
> ---------- code ----------
> g = Magick::Image::read('<local file only>').first
> puts "Width: #{g.columns}; Height: #{g.rows}"
> -----------------------------
>
>   Why do Image::read and Image::from_blob make differeces? How to
> determine the geometry of  a remote image?
>
>   Any comment is appreciated. Thanks!
>
>
>   
Under the covers, #read and #from_blob share most of their code. If
Image.read can read the file then Image.from_blob can handle it, too.
The argument to Image.from_blob must be a string containing the entire
image. I'm guessing that the argument you're giving it is only part of
the image. Assign f.read to a variable and display its length. Is it the
same as the size of the image file on disk?