From: John Joyce <dangerwillrobinsondanger@...>
Date: 2007-07-19T08:36:39+09:00
Subject: Re: Validating an Image file is an image file


On Jul 18, 2007, at 4:13 PM, Daniel Berger wrote:

> On Jul 18, 10:20 am, John Joyce <dangerwillrobinsondan...@gmail.com>
> wrote:
>
> <snip>
>
>> The file command and bindings to it are OK, but results are not
>> consistent across common image file types.  What's worse is that the
>> code would be unportable. Ideally, the solution would rely simply on
>> the file format internally and thus be portable.
>
> How's this for a portable version? I based this on a 5 minute overview
> of the Wikipedia content at http://en.wikipedia.org/wiki/ 
> Magic_number_(programming),
> combined with some trial and error.
>
> class File
>    def self.image?(file)
>       bmp?(file) || jpg?(file) || png?(file) || gif?(file)
>    end
>
>    def self.bmp?(file)
>       IO.read(file, 3) == "BM6" && File.extname(file).downcase ==
> '.bmp'
>    end
>
>    def self.jpg?(file)
>       IO.read(file, 10) == "\377\330\377\340\000\020JFIF" &&
> File.extname(file).downcase == '.jpg'
>    end
>
>    def self.png?(file)
>       IO.read(file, 4) == "\211PNG" && File.extname(file).downcase ==
> '.png'
>    end
>
>    def self.gif?(file)
>       ['GIF89a', 'GIF97a'].include?(IO.read(file, 6)) &&
> File.extname(file).downcase == '.gif'
>    end
> end
>
> Regards,
>
> Dan
>
>
Cool, Dan, that looks pretty slick. I certainly couldn't have done  
that so quickly, but I knew what basically was needed. That looks  
like about what I was thinking of conceptually. Very nice and clean  
code! Nice touch with downcasing file extensions too! I hate that  
cameras all like to upcase filenames... (> <)!
That will probably run a lot lighter than RMagick methods, I'll try  
it out later. I've been fiddling with the RMagick API all afternoon.  
There's a lot of documentation but some of it is lacking in clear  
examples.
At this point I know how to scale images and keep the aspect ratio  
within a maximum new size, but I'd like to have the final output be  
square with matting on the sides for portrait orientation, matting on  
the top and bottom for landscape orientation, either one with the  
scaled image centered.
Will this require subsequent compositing after scaling? Or did I miss  
a method somewhere in the docs that does all of this at once?

John Joyce