[magick-users] Crop a large image into multiple smaller ones

[RobBellJr at aol.com]

In a message  dated 5/12/2008 8:26:46 A.M. Central Daylight Time, 
sundar at cumulus-systems.com  writes:

I was  trying to crop an image into multiple images so as to load them in  a
parallel fashion,
(so that the user can start seeing the fragments  quicker, when the bandwidth
available is low etc, over the internet), as  per the topic
"Tile Cropping, sub-dividing one image into multiple  images"
in  HYPERLINK
"http://www.imagemagick.org/Usage/crop/#crop"http://www.imagemagick.org/Usag
e/crop/#crop.

I  used the command :
convert -crop 120x120 big_file  small_file.jpeg

The original file size is 28K, it got cropped into 4  files each of size
around 14K, with the result that the total size becomes  much more than the
original.
Is there a way to crop an image into  smaller chunks which will be
proportionately smaller in size too, as  otherwise it defeats my purpose ?
(assuming loss of quality can be  tolerated ?)

Thanks in advance,
Sundar
Sundar,
 
Chopping up an image into multiple parts for display in a web page has  
potential for improving perceived loading speed even if the resulting  individual 
part file sizes exceed the original.  The idea is to give the  visitor a more 
immediate sense of progress by displaying at least a portion of  the overall 
image more quickly.  The alternative, displaying the  original image without 
chopping may result in a delay before any  significant portion of the image 
appears.
 
Your specific example of a 28K image hardly warrants the effort, but there  
is a chance that much larger file sized images may time out while loading over  
narrow bandwidth connections.  However, that same image displayed in  chunks 
might instead be successfully displayed as a result of each  individual part 
completing more quickly.
 
Although a web browser may initiate the process of loading subsequent  images 
before earlier encountered images have finished, my  observational experience 
is that images are loaded sequentially top to  bottom, left to right.  The 
process does not seem to  be purely parallel in the sense that you can depend on 
all the images  in the page loading at the same rate of speed (so don't plan 
based on  that).
 
All that said, I don't know how it would be done on the command line,  but 
the individual resulting pieces of the image would need to have assigned to  
them a higher amount of jpeg compression to produce smaller file  sizes.  
Hopefully others will pitch in with instructions  for that.
 
I hope this helps,
 
Rob



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