RetouchPRO

Go Back   RetouchPRO > Community > Salon
Register Blogs FAQ Site Nav Search Today's Posts Mark Forums Read


Salon Just hanging around...
(Social area, where non-retouching talk is encouraged)

Puzzling file size

Reply
 
Thread Tools
  #1  
Old 10-03-2003, 07:45 PM
winwintoo's Avatar
winwintoo winwintoo is offline
Senior Member
 
Join Date: Feb 2002
Location: Regina, Saskatchewan
Posts: 942
Puzzling file size

I'm working on my presentation to the computer group and came across some interesting information.

I want to give them an overview of file size, resolution etc.

BUT - just when I thought I understood it........

I have this image, it's saved for web, 48 kb on my machine. I put it on a web site, went to the web site, right-clicked and copied it to the clipboard and then immediately pasted it into PhotoShop Elements. PSE says the image is now 287 kb

What is happening?? Can anyone explain??

Thanks, Margaret
Reply With Quote top
  #2  
Old 10-03-2003, 09:05 PM
Doug Nelson's Avatar
Doug Nelson Doug Nelson is offline
Janitor
 
Join Date: Aug 2001
Posts: 7,068
Blog Entries: 21
Photoshop files are a lot larger than JPG files, not even counting the compression offered by JPG. JPG saves space by more than just compression. It also strips out a lot of other information. Plus, PS seems to always inflate the file size of open files.
Reply With Quote top
  #3  
Old 10-03-2003, 09:32 PM
catia's Avatar
catia catia is offline
Senior Member
 
Join Date: Jul 2003
Posts: 255
Perhaps the same size gremlins that take a 200K file and "convert" it to 287K when I attach it to a Yahoo email. Then we don't want to even talk about AOL.



Catia
Reply With Quote top
  #4  
Old 10-03-2003, 09:38 PM
Doug Nelson's Avatar
Doug Nelson Doug Nelson is offline
Janitor
 
Join Date: Aug 2001
Posts: 7,068
Blog Entries: 21
Actually, it's very similar. JPG files save size by storing the command "8 black pixels" instead of storing 8 actual black pixels (as an example, the actual mechanism is much more sophisticated than this). Plus, they apply LZW compression after that (sort of like a ZIP file, but a different algorythm). Uncompressing these files intails uncompressing the LZW, then actually painting in each pixel. At this point it would be similar to a TIF file with no compression, or a BMP file (of which the AOL ART file is a version). Similar, but not interchangeable, since (again) this is a gross oversimplification. Photoshop also adds its own information, plus there's room for other information such as EXIF data, etc.
Reply With Quote top
  #5  
Old 10-03-2003, 09:56 PM
catia's Avatar
catia catia is offline
Senior Member
 
Join Date: Jul 2003
Posts: 255
Thanks Doug,

That was very informative. I guess a pixel is not a pixel is not a pixel.

Or something like that.

Catia
Reply With Quote top
  #6  
Old 10-03-2003, 10:15 PM
Doug Nelson's Avatar
Doug Nelson Doug Nelson is offline
Janitor
 
Join Date: Aug 2001
Posts: 7,068
Blog Entries: 21
While I'm on a roll I might as well go on and explain why it's not good to save a JPG file as another JPG file. As I mentioned, JPG will look at repeating patterns as a source of compression, but you can control how intently it looks for patterns using the "quality" or "amount" setting. At high compression levels it will see, for example, 8 black pixels, then 1 white pixel, then another 8 pixels and say essentially "screw it, that 1 white pixel isn't that important" and record the data as just 17 black pixels. Then the next time it will record those 17 black pixels as a single command and look around for other black pixels and decide how important the information inbetween them is. This is called "lossy compression", and naturally it does it for every color, not just black. So, just hitting "save" on a JPG file can actually destroy data, as it does this analysis each time the file is saved. Gradually it will toss more and more data.

On an interesting sidenote (I love throwing tidbits like this in), if you happen to know the precise compression setting the JPG had when it was originally saved, and replicate that exact setting, virtually no data will be lost when re-saving it as a JPG. That's more in the realm of interesting trivia than actual workflow advice, however.
Reply With Quote top
  #7  
Old 10-03-2003, 10:36 PM
winwintoo's Avatar
winwintoo winwintoo is offline
Senior Member
 
Join Date: Feb 2002
Location: Regina, Saskatchewan
Posts: 942
Doug, this presentation is on October 15 from 1 - 3:30 pm at the Seniors Education Center just down the street. If you can be there to explain file sizes, I'll buy you lunch

Thank you so much for the information. I never even thought that interpreting the "8 black pixels" line into actual black pixels would make the size different.

Margaret
Reply With Quote top
  #8  
Old 10-03-2003, 10:41 PM
catia's Avatar
catia catia is offline
Senior Member
 
Join Date: Jul 2003
Posts: 255
Hmmm,

I thought Jpeg was based on 8 pixel by 8 pixel block encoding. Each 8x8 block is cosine transformed ( this is a linear transformation that packs all of the energy into the low spatial frequency terms). Then bits are assigned to the terms with the most energy. So, high spatial frequency stuff gets a small number of bits. To be able to recontruct the image, one needs the "bitmap." The cosine coefficients and the bit map are "packed" into the file. Now, that was the "source" coding part. The fun begins with the "channel" coding. We have to add back in enough redundancy to insure that we can recover the bitmap and the cosine transform coefficients. That is where the variability arises. By the way, the DC term of each transform block (the zero spatial frequency term) is basically the average intensity of that block. Any error in this term produces the dreaded blocking error.

Hmmmmm. Sorry, I guess I am getting carried away. Information theory was my minor and every once in a while it slips out.

Catia

Last edited by catia; 10-03-2003 at 10:47 PM.
Reply With Quote top
  #9  
Old 10-03-2003, 10:59 PM
Doug Nelson's Avatar
Doug Nelson Doug Nelson is offline
Janitor
 
Join Date: Aug 2001
Posts: 7,068
Blog Entries: 21
Like I said, "gross oversimplification"
Reply With Quote top
  #10  
Old 10-03-2003, 11:07 PM
catia's Avatar
catia catia is offline
Senior Member
 
Join Date: Jul 2003
Posts: 255
You are a sweetheart Doug and we all love you. We really do. Keep up the good work.



Catia
Reply With Quote top
Reply

  RetouchPRO > Community > Salon


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Resizing images for RetouchPRO jeaniesa Photo Retouching 19 11-05-2016 09:28 PM
Impressionist plugin: Troubleshooting Cheryl H Photo-Art Resources 34 06-15-2009 11:13 PM
File size too big? mama alligator Image Help 7 10-08-2002 02:23 PM
Small vs. Large File Size For Restorations Doug Nelson Photo Restoration 8 04-29-2002 10:02 AM
help - compression problem cendres Photoshop Help 15 02-11-2002 10:42 AM


All times are GMT -6. The time now is 10:13 PM.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
SEO by vBSEO ©2011, Crawlability, Inc.
Copyright © 2016 Doug Nelson. All Rights Reserved