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Why do my JPEG file sizes shrink so dramatically?

Explorer ,
Oct 03, 2013 Oct 03, 2013

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This has got me completely and utterly baffled.

I have an image that I saved using Photoshop as JPEG at max quality and on disk it is 12.3mb.

After placing it at 100% in InDesign that spread when exported as a PDF with images set to max quality is a mere 2.9mb. (The spread is two pages, the image on one and the month of a calendar on the other)

The image in Photoshop is 3900px by 2650px and at 300ppi it's 33.02cm by 22.44. It is converted to CMYK and saved as JPEG at max 12

In InDesign it's frame is 330.2mm x 224.367mm on a document of 318mm x 212mm with 6mm bleed all round.

When InDesign is exporting as a PDF it must be recompressing the JPEG? Yet it seems to be throwing away 10mb even at max quality (see below) Or is it that Photoshop is saving CMYK JPEGs wrongly and producing files way too big - I've always thought PS produces huge JPEGs once you convert them to CMYK

What tests can I do? Have tried various output settings, these are current

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Guide ,
Oct 04, 2013 Oct 04, 2013

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Have you verified that the exported PDF contains CMYK images?

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Explorer ,
Oct 04, 2013 Oct 04, 2013

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Hi Doc

I haven't. I did try but I don't know how to do it. I have Adobe Reader and I can examine details about the document's properties like if any fonts are embedded but I can't see any info on the images at all.

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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http://forums.adobe.com/thread/1068393

There's a very real difference between "jpeg" and "automatic jpeg".

It's most likely with Automatic JPEG that the information is being zipped rather than lossy.

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Explorer ,
Oct 04, 2013 Oct 04, 2013

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Thanks Eugene but if it was being zipped then I would expect file size to increase rather than losing 10mb - clearly something lossy is going on and it doesn't matter whether I choose "jpeg" or "automatic jpeg" I lose about 10mb

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Choosing JPEG again might be adding another compression level again.

Choosing Automatic JPEG the application will decide what's best, lossy compression (losing actual data), and zipping.

And it's probably zipping the JPEG which would make it smaller, not bigger.

If you think you're losing data then choose no compression at all. You'll see a much larger file than what you actually started with.

You can always set your compression setting to compress images of 301 (instead of 450) - and then the image shouldn't be hit by the compression settings at all.

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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If you had Acrobat Pro you would be able to tell if the image was compressed again.

If you're not sure - feel free to send me a private message and you can email me the PDF and I will check out the image to it.

If it's not sensitive, and can be shared openly - feel free to upload to file sharing site, like dropbox, google drive etc. and share a public link.

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Explorer ,
Oct 04, 2013 Oct 04, 2013

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I didn't think it was possible to shave 80% off a JPEG file using zip

Anyway I have just done some tests

The CMYK jpeg from Photoshop is 12.3mb

When that singple page is to PDF with compression set to Automatic (JPEG) and image quality set to Maximum the resulting file is 2.93mb

When the setting is changed to JPEG with image quality set to Maximum the resulting file is also 2.93mb

I'm pretty sure that for that page InDesign chose max quality JPEG in either case rather than using zip or any other compression.

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Amy2014 wrote:

I'm pretty sure that for that page InDesign chose max quality JPEG in either case rather than using zip or any other compression.

Well you're not really "pretty sure" - that's a guess.

Get a trial of Acrobat Pro.

You will be able to use Edit to open the image in photoshop to see the real result.

http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/products/acrobat/pdfs/adobe-acrobat-xi...

Plus you could be converting to a different colour profile, like RGB which would add to the data loss (1 less channel than CMYK)

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Or converting to a colour profile of sRGB -

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Explorer ,
Oct 04, 2013 Oct 04, 2013

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Whether I chose Automatic or chose JPEG I got the same result so that surely rules out zip compression.

I've just tried compressing my 12.3mb CMYK JPEG with zip software and the best I can get it down to is 12mb using 7zip's .7z format set to ultra.

I'm now confident that InDesign isn't zipping the jpeg and , like you, I do suspect that the images are being converted back to sRGB at some point

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Well you need to go to the Colour portion of the PDF settings and choose "No Colour Conversion"

Don't select anything for compression in the compression portion of the PDF settings.

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Explorer ,
Oct 04, 2013 Oct 04, 2013

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Last time I did prepress work was two or three years ago with CS4 and the PDF files were reassuringly large with JPEG compression turned on.

If I don't select anything for compression I get 40mb files, my email limit is 20mb, I might go down the route of using Google Drive / Dropbox but I want to solve this puzzle.

Current settings are "No Colour Conversion". I've actually tried specifying "Coated F0GRA39 (ISO 12647-2:2004)" as Destination and selecting "Convert to Destination" in Colour Conversion drop down.

Nothing works, from starting out with default [Press Quality] then tinkering. I've been turning of things like "Crop Image Data to Frames" despite knowing that my image fills the frame 100% and there's none outside anyway.  Baffled.

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Is your JPEG image set to Coated Fogra 39 in the Colour Profiles in Photoshop?

What are your colour management policies in InDesign?

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Guide ,
Oct 04, 2013 Oct 04, 2013

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Is it possible to have one page of the exported PDF for analysis? I got Acrobat Pro here.

Just attach it to a reply.

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Doc Maik wrote:

Just attach it to a reply.

Sorry, attachemnts are not enabled on the forums. You would need to link to a server someplace....

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Explorer ,
Oct 04, 2013 Oct 04, 2013

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The image is of a semi-naked girl for a charity calendar, I'm going to duplicate all the documents with a censored image and will post a link here shortly.

InDesign Colour Settings are Europe General Purpose 3 same in PS

04-10-2013 13-20-41.jpg

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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What tests can I do?

Print it.

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Explorer ,
Oct 04, 2013 Oct 04, 2013

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My files are here: https://drive.google.com/folderview?id=0B5tpebpwHlb0aFU4VUV1R2M1amM&usp=sharing

There's the indd file cut down to two pages, the PDF output which is under 3mb (which I suspect is somehow sRGB) and the original files are in the folder created with Package.  I've pixelated the image which has knocked it down to 11.1mb when saved in Photoshop. Press PDF is coming out at 2.92mb.

I know I've misspelt censored, it's been a long night and it's now daytime in UK

04-10-2013 13-35-54.jpg

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Hey Gang, just in case you ever wondered about JPG compression:

The JPEG algorithms performs its compression in four phases:

  1. The JPEG algorithms first cuts up an image in blocks of 8x8 pixels. It converts image data to a luminance/chrominance color space, such as YUV. The algorithm retains more of the luminance in the compressed file.
  2. Next, apply a Discrete Cosine Transform (DCT) for the block. It replaces actual pixel color data for values that are relative to the average of the entire matrix. This replaces 8x8 pixel values by an 8x8 matrix of DCT coefficients.
  3. Two tables of quantization constants are calculated; luminance and chrominance. The constants from the two tables are used to quantize the DCT coefficients. Each DCT coefficient is divided by its corresponding constant in the quantization table and rounded off. The result of quantizing the DCT coefficients is that smaller coefficients will be replaced by zeros and larger coefficients will lose precision. This rounding-off causes a loss in quality. The resulting data are a list of streamlined DCT coefficients.
  4. Last, compress these coefficients using either a Huffman or arithmetic encoding scheme. Usually Huffman encoding is used. This is a second (lossless) compression that is applied.

By putting 2 compression algorithms on top of each other, JPEG achieves remarkable compression ratios, up to 20-to-1 can be achieved. JPEG decompression is supported in PostScript level 2 and 3 RIPs. This means that smaller files can be sent across the network to the RIP which frees the sending station faster, minimizes overhead on the print server and speeds up the RIP. The downside of JPEG compression is that the algorithm is only designed for continuous tone images. JPEG not does not lend itself for images with sharp changes in tone.

Mike Witherell

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Explorer ,
Oct 04, 2013 Oct 04, 2013

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What I've always wondered about is why so many web sites default to jpg when gif would be much better for flat art.

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Makes sense.

To put the OP at rest - I opened the image from the PDF in Photoshop - by using the Edit options in Acrobat.

The image is opening at 49mb at the original specs noted above.

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Thanks, Eugene.

The ironic thing is most people complain that their PDFs are too large.

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Explorer ,
Oct 04, 2013 Oct 04, 2013

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Eugene are you saying the image in the PDF is CMYK?

"The image is opening at 49mb"

I'm still losing data though. Even a blank image of that depth and size would be that sort of size when uncompressed.

In the files above I'm putting in a max quality JPEG of 11,743,167 bytes and after a trip through InDesign it's coming out at 3,068,891 even at Max quality

8 megs has gone missing and I doubt it's lossless.

Can someone else try loading the file into InDesign and outputting as a Press Ready PDF with compression set to JPEG at Quality Maximum and letting me know your final PDF size

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Community Expert ,
Oct 04, 2013 Oct 04, 2013

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Using PDF/X-4 the file is 2.58 mb. Using Press setting it's 2.1.

I still say you're worrying about nothing.

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