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Jpg: Small Size is Increased when Opened

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  #1  
Old 09-11-2009, 06:17 AM
ylwdog ylwdog is offline
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Question Jpg: Small Size is Increased when Opened

I have a Photoshop file that I saved out from a psd format to 194K in a jpg format file.
Reading the properties of the file show 194k, when I open it in Photoshop it blows it up to 34.5mb.
A) Curious what is going on
B) Is it possible to have Photoshop open the jpg file as the current file size is

Thanks,

Doug
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Old 09-11-2009, 12:14 PM
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mistermonday mistermonday is offline
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Re: Jpg: Small Size is Increased when Opened

Doug, welcome to Retouch Pro. JPG is a format which compresses file size for more compact storage. Everytime you open the file it expands back to its native size. JPG is what is called a lossy compression because in order to compress the file, the algorithm treats colors which are close to each other as equal. The destruction is minimal and not easily noticed by the human eye IF you have used a high quality setting (0 is the lowest quality, smallest file size, while 12 is the highest quality largest setting). If you have a 34MB file that has been compressed to 194K, it must have been on setting 0 or 1 in which case your images are likely severely damaged. You can tell if this is the case by zooming in where you will notice big rectangular blocks - a phenomenon called Pixelation.
If you have sufficient memory in your computer, and a current speed processor, then working with 34MB files should be no issue.
Regards, Murray
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Old 09-12-2009, 05:14 AM
ylwdog ylwdog is offline
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Re: Jpg: Small Size is Increased when Opened

Hello Murray:
Thanks for your assistance on this and your thoughtful answer. I am struggling w/ this concept you have presented w/ how a jpg file is processed, though.
The file was originally a .NEF file opened in Photoshop and then processed/saved for Web to allow me to reduce the size of the file small enough to upload it here to the board. So it was optimized to a particular size.

My thinking on it is once a file has been instructed to get rid of certain information, it only would add info back in if it was instructed to do so, e.g. increase resolution or image size.

If you are correct that would mean each time I opened a jpg from any other source it would restore it to its native size if it originally was a Photoshop file, correct?

Thanks, I look forward to hearing more about this, and again thanks for the help.

Doug
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Old 09-12-2009, 10:10 AM
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mistermonday mistermonday is offline
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Re: Jpg: Small Size is Increased when Opened

Doug,
When you save a file in jpg file format, the information is only compressed for storage. When the file is reconstructed upon opening, it still needs to have the same number of pixels proportioned in the same dimensions as before it was compressed.
The JPG compression algorithm examines a fixed area of pixels (for example 8 x 8). When it sees nearby pixels which are close in color, it say "yep, these are close so lets say they are the same". The more you compress, the more far apart colors can be before they are treated as the same color. As the approach the 0 quality setting, the compression tends to take these 8x8 areas and treat then as solid colors and hence you end up with pixelation blocks.
I have attached 3 images to show you an example. The 1st is the overall autumn scene for a reference point. The 2nd is a zoomed in view of the original saved at jpg setting 0. The 3rd is the original saved at the highest quality 12. Place the 2nd and 3rd side by side and zoom in so you have a close look at the difference.
The damage is done during the compression process. When the file is reconstructed it must have the same number of pixels it had before it was compressed. What has been lost/degraded in the process is the quality of the data (color information).
Rather than compressing a 34MB file down to 194KB you would have been much better of downsizing (downsampling) the file in Photoshop and then saving it at a high quality jpg setting.
I hope this helps.
Regards, Murray
Attached Images
File Type: jpg _DSC0657.jpg (166.3 KB, 11 views)
File Type: jpg JPG Low.jpg (24.3 KB, 12 views)
File Type: jpg JPG High.jpg (52.2 KB, 12 views)
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Old 09-12-2009, 11:06 AM
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mistermonday mistermonday is offline
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Re: Jpg: Small Size is Increased when Opened

John, you are corrct about the severe degradation but incorrect regarding the size when reopening. It will be ~34 MB once again. Assuming the file was from a 12 megapixel DSLR and was 4200 x 2800 pixels, in 8 bit RGB, it means the actual DRAM required by PS when the file is opened is 4200 x 2800 x 3 (1 byte per channel) = ~35MB. Please check your status bar at the bottom of the PS window.
When you re-save the file, if you do not change the jpg compression factor, it will re-compress down to the 194K.
While the open file will blow back up to 34MB in memory, most of the information will be garbage because the color info has been severely damaged.
Regards, Murray
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Old 09-12-2009, 07:55 PM
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Re: Jpg: Small Size is Increased when Opened

Murray is correct. Yet, I would think that any 34MB file that compresses to 194K and still looks good is likely to have a lot of pixels that are very very similar, such as graphic images would. This particular ratio is a bit unusual.

Ylwdog,
The whole process is much like using a "zip" utility to compress files, or PDF formats for documents. The more similar the data, the better the compression ratios. The algorithm for JPG's is a bit different, but the concept is the same.... why repeat data millions of times in a file, when you can describe its data once, then mark the locations of the occurrences.

Your question... "If you are correct that would mean each time I opened a jpg from any other source it would restore it to its native size if it originally was a Photoshop file, correct?" Yes, and it does not have to have been a Photoshop file. Any JPG does when opened. And they do this when opened in any editing application, not just Photoshop. Now you get an understanding of why so much RAM is important for Photoshop users, even non-professional users. You open a JPG, it expands. You duplicate the background layer, it doubles in size, and so on. When you save it as a PSD, there is no compression, so you start seeing really large files. Everyone hesitates to save as a JPG again, because of the data loss associated with the compression algorithm. Not the file size loss, but the loss of color information between adjacent pixels... the ones that the JPG algorithm considered "close enough", treated as equal, and discarded the differences forever.

Photoshop's "save for web" option also has a few additional tweaks to make the files a bit smaller than normal jpg compression algorithms.
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