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Images with 256 colors are not compressed with JPEG in exported PDF

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May 10, 2020

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I'm confused about how InDesign compresses images in pdfs.
I have to submit 15 photos to a photography contest, so I put these 15 images (each image in its own page) in an InDesign document that has no more than these 15 pages. All the images have 2000 pixels in the larger side and have in average 689 KB.
In the Export to pdf dialog, I choose "Smallest file size" preset and JPEG compression (not JPEG (Automatic)) for both "Color images" and "Grayscale images".
However the resulting pdf size is aprox. 26 MB! I expect the pdf to be a little more than 689 KB * 15 = 10335 KB or 10 MB.
An inspection of the pdf with pdfimages, a tool from a pdf library called Poppler, reveals that only 3 out the 15 images are compressed with jpeg.

$ pdfimages -list mypdf.pdf 
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image    2000  1337  index   1   8  image  no        78  0    72    72 2195K  84%
   2     1 image    2000  1337  index   1   8  image  no         4  0    72    72 2155K  83%
   3     2 image    2000  1333  icc     3   8  jpeg   no         8  0    72    72  120K 1.5%
   4     3 image    1500  2000  icc     3   8  jpeg   no        12  0    72    72 98.3K 1.1%
   5     4 image    2000  1333  index   1   8  image  no        16  0    72    72 1919K  74%
   6     5 image    1334  2000  index   1   8  image  no        20  0    73    73 2058K  79%
   7     6 image    1500  2000  index   1   8  image  no        24  0    72    72 1781K  61%
   8     7 image    2000  1332  index   1   8  image  no        28  0    72    72 1943K  75%
   9     8 image    2000  1333  index   1   8  image  no        33  0    72    72 2168K  83%
  10     9 image    2000  1337  index   1   8  image  no        37  0    72    72 2354K  90%
  11    10 image    1500  1998  index   1   8  image  no        41  0    72    72 2080K  71%
  12    11 image    1500  2000  index   1   8  image  no        45  0    72    72 1869K  64%
  13    12 image    2000  1339  index   1   8  image  no        49  0    72    72 2465K  94%
  14    13 image    2000  1332  icc     3   8  jpeg   no        53  0    72    72  120K 1.5%
  15    14 image    1337  2000  index   1   8  image  no        57  0    72    72 2381K  91%

 

Inspecting each image in detail, I found that the 3 images that were compressed with JPEG have more than 256 unique colours, while the others have exactly 256 unique colours. Also notice that the images that have 256 unique colours have an "Indexed color space" in the pdf.

Using LibreOffice writer, a word processor commonly used in Linux distributions, I could export to a 8,2 MB pdf.

So my questions.
How can I instruct InDesign to compress with JPEG all the images and not only ones that have more than 256 colours?
Is this an expected behaviour of the pdf compression process in InDesign?

Thank you all in advance.

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Images with 256 colors are not compressed with JPEG in exported PDF

New Here ,
May 10, 2020

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I'm confused about how InDesign compresses images in pdfs.
I have to submit 15 photos to a photography contest, so I put these 15 images (each image in its own page) in an InDesign document that has no more than these 15 pages. All the images have 2000 pixels in the larger side and have in average 689 KB.
In the Export to pdf dialog, I choose "Smallest file size" preset and JPEG compression (not JPEG (Automatic)) for both "Color images" and "Grayscale images".
However the resulting pdf size is aprox. 26 MB! I expect the pdf to be a little more than 689 KB * 15 = 10335 KB or 10 MB.
An inspection of the pdf with pdfimages, a tool from a pdf library called Poppler, reveals that only 3 out the 15 images are compressed with jpeg.

$ pdfimages -list mypdf.pdf 
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image    2000  1337  index   1   8  image  no        78  0    72    72 2195K  84%
   2     1 image    2000  1337  index   1   8  image  no         4  0    72    72 2155K  83%
   3     2 image    2000  1333  icc     3   8  jpeg   no         8  0    72    72  120K 1.5%
   4     3 image    1500  2000  icc     3   8  jpeg   no        12  0    72    72 98.3K 1.1%
   5     4 image    2000  1333  index   1   8  image  no        16  0    72    72 1919K  74%
   6     5 image    1334  2000  index   1   8  image  no        20  0    73    73 2058K  79%
   7     6 image    1500  2000  index   1   8  image  no        24  0    72    72 1781K  61%
   8     7 image    2000  1332  index   1   8  image  no        28  0    72    72 1943K  75%
   9     8 image    2000  1333  index   1   8  image  no        33  0    72    72 2168K  83%
  10     9 image    2000  1337  index   1   8  image  no        37  0    72    72 2354K  90%
  11    10 image    1500  1998  index   1   8  image  no        41  0    72    72 2080K  71%
  12    11 image    1500  2000  index   1   8  image  no        45  0    72    72 1869K  64%
  13    12 image    2000  1339  index   1   8  image  no        49  0    72    72 2465K  94%
  14    13 image    2000  1332  icc     3   8  jpeg   no        53  0    72    72  120K 1.5%
  15    14 image    1337  2000  index   1   8  image  no        57  0    72    72 2381K  91%

 

Inspecting each image in detail, I found that the 3 images that were compressed with JPEG have more than 256 unique colours, while the others have exactly 256 unique colours. Also notice that the images that have 256 unique colours have an "Indexed color space" in the pdf.

Using LibreOffice writer, a word processor commonly used in Linux distributions, I could export to a 8,2 MB pdf.

So my questions.
How can I instruct InDesign to compress with JPEG all the images and not only ones that have more than 256 colours?
Is this an expected behaviour of the pdf compression process in InDesign?

Thank you all in advance.

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May 10, 2020 0
Most Valuable Participant ,
May 10, 2020

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This sounds entirely expected (if also surprising). JPEG is designed specifically and exclusively for photographs (the P in the name) and is effective on real world images with lots of very similar colours that gradually change. It cannot effectively compress images with a few very different colours; they would often be catastrophically bad or huge. So a good PDF producer won’t even try. Typically ZIP is used but compression may be negligible or negative in some cases. Remarkably, LibrePDF apparently succeeds in this case by ignoring indexed colour anyway, which Acrobat won’t do. 

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May 10, 2020 1
New Here ,
May 10, 2020

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Thank you very much for your response.

As a user I would like to force the pdf producer to use JPEG anyway, by setting an advanced option that would warn me of the possible effects of using JPEG in images with few distinct colours.

In my case, the JPEG compression that LibreOffice writer applied to the images with 256 colours was pretty fine and showed no differences (perceptually) with the images with more than 256 colours.

Considering this behaviour, I don't know of any workflow involving InDesign that would allow me to produce small sized pdfs with images that have 256 colours.

Kind regards,

diego

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Adobe Community Professional ,
May 10, 2020

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Just to double check, if you open one of the files that are listed as index into Photoshop does the title bar show (RGB/8) after the title and not (Index)?

 

Screen Shot 10.pngScreen Shot 9.png

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New Here ,
May 10, 2020

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Hi rob_day.

It shows RGB/8# in images with more than 256 unique colours (first screenshot) and images with just 256 unique colours (second screenshot)

256_color_image_rgb.jpg256+_color_image_rgb.jpg

Kind regards,

diego

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Adobe Community Professional ,
May 10, 2020

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Is there a reason you are using index color? If you inspect an index image in AcrobatPro the compression method is zlib/deflate not JPEG. I think you will get better compression leaving the images as 8-bit per channel:

 

Screen Shot 8.png

 

 

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New Here ,
May 10, 2020

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Hi rob_day.

I'm not using Indexed Color. All the images used in the InDesign document have sRGB with 8 bit per channel. Apparently the pdf producer (InDesign in this case) chooses to encode images that have 256 unique colors with an Indexed Colorspace when it exports to pdf. Images with more than 256 unique colors are encoded with sRGB and compressed with JPEG.

Kind regards,

diego

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