Image downsampling

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eleigh
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Image downsampling

Post by eleigh »

I have run a lot of tests with PDF-XChange, comparing it with print drivers from Microsoft, Adobe (old PDF driver) and Foxit. It seems to me that the image subsampling algorithms used by Microsoft and Foxit are better at handling line art (graphs, charts and illustrations). PDF-XChange softens edges/lines, whether bilinear or bicubic downsampling is selected. I don't think JPEG compression isn't the issue here. PDF-XChange produces excellent results from photos with embedded text when JPEG2000 is set as the compression method. The following sample images are from PDFs of almost identical file sizes (5277KB v 5250KB v 5215KB).

Sample image from PDF-XChange, output via Standard Printer from MS Word with bicubic downscaling to 150ppi and JPEG2000/ZIP @ 50% quality:
PDF-Xchange- jpeg2000Q50.png
Sample image from Microsoft Export-as-PDF from MS Word at "Standard" optimization:
Microsoft-jpegHigh.png
Sample image from Foxit medium quality:
Foxit-jpegMedium.png
Viewed at 100%, the first is noticeably softer than the other two. Have I overlooked a permutation of settings that would produce sharper graphics (in a document that also contains photos)? Could the issue be fixed relatively easily in a future update?

This is the only issue causing to me hesitate buying PDF-XChange PRO.
eleigh
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Re: Image downsampling

Post by eleigh »

Here's a side-by-side comparison of Microsoft's "Standard" export to PDF (left) versus PDF-XChange's version (right), bicubic-resized to 150ppi, JPEG2000/ZIP-compressed @ 50% quality. The output pixel dimensions of the image is the same in both cases.

MS2016 v PDF-X.png
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TrackerSupp-Daniel
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Re: Image downsampling

Post by TrackerSupp-Daniel »

Hi, eleigh

Thank you for the large selection of sample images to compare, while I admire your attempt at accuracy here, simply making files of comparable size is not a good point to compare, when we generate PDF documents, we can include much more information in the document compared to say "MS print to PDF" would. Indeed image compression could be part of what causes differences in size, but in this case, the cause of the "extra fuzz" in our document is almost certainly your decision to use 50% quality. Does it look ideal if you use 100% quality, disregarding the file size?

To investigate this, we will need more than just images of what you see, of course. To begin, could I ask that you please send us a copy of the original Word document you are using as a test here, and each of these output PDF documents, so we can compare the results you are seeing?

Kind regards,
Dan McIntyre - Support Technician
Tracker Software Products (Canada) LTD

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eleigh
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Re: Image downsampling

Post by eleigh »

The issue is definitely with downsampling, not JPEG-compression, as the attached files show: even with 'Flate' compression, graphics are still softened. One oddity is that, when I placed the graphics by themselves in a new Word document, the output is better quality than when they were embedded in a much larger document (extracts also included). So, there seems to be a context-dependent inconsistency in the downsampling algorithm in addition to it softening graphics more than the algorithm used by Microsoft et al.

Note too that the softening has a knock-on effect on compression: the output file size increases significantly for a given quality setting because the blurring creates more non-white pixels to encode.

Although a quality setting of 50% sounds low, PDF-XChange actually produces similar output file sizes for JPEG2000 @ 50% and JPEG @ 90%.

PDF-XChange.zip
(1.75 MiB) Downloaded 145 times
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TrackerSupp-Daniel
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Re: Image downsampling

Post by TrackerSupp-Daniel »

Hi, eleigh

Thank you for the sample documents, I see what you mean here, and have brought it to our Dev team. They asked me to create a formal ticket to track the issue for you as well, so please see the ticket number below:
RT#5521: Image downsampling inflates size and severely impacts quality
This is an internal ticket, but you can ask any member of our support team for an update on this in the future.

Kind regards,
Dan McIntyre - Support Technician
Tracker Software Products (Canada) LTD

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Ivan - Tracker Software
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Re: Image downsampling

Post by Ivan - Tracker Software »

The first looks show a possible reason for such effect.
The original image (from the very first post) has a dimension of 976 x 681 pixels.
The image received by PDF-XChange is 979 x 683 pixes - a little bit scaled.
And please note, this scaling is not done by PDF-XChange -- the image is received from the MS Word during the conversion (printing) process.

I'm trying to understand why this scaling is happening and how to avoid that, but I'm not sure it is under our control.
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When attaching files to any message - please ensure they are archived and posted as a .ZIP, .RAR or .7z format - or they will not be posted - thanks.
eleigh
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Re: Image downsampling

Post by eleigh »

this scaling is not done by PDF-XChange -- the image is received from the MS Word during the conversion (printing) process
Both the Microsoft export and Foxit print driver output the image pixel-for-pixel identical to the image in the Word document, which is in PNG format.
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TrackerSupp-Daniel
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Re: Image downsampling

Post by TrackerSupp-Daniel »

Hi, eleigh

As a temporary workaround, you should find that directly opening the Word docuemnt in our Editor does not have this issue. But our dev team is looking into this, and are considering other options for this that may be less reliant on Microsoft word.

Kind regards,
Dan McIntyre - Support Technician
Tracker Software Products (Canada) LTD

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Our Web site domain and email address has changed as of 26/10/2023.
https://www.pdf-xchange.com
Support@pdf-xchange.com
eleigh
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Re: Image downsampling

Post by eleigh »

TrackerSupp-Daniel wrote: Sat Mar 13, 2021 12:51 am As a temporary workaround, you should find that directly opening the Word docuemnt in our Editor does not have this issue.
Thanks for this tip. The downside though is that it bypasses the opportunity to generate bookmarks and links, which the Word Add-in does very neatly.
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Tracker Supp-Stefan
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Re: Image downsampling

Post by Tracker Supp-Stefan »

Hello eleigh,

Then maybe your workaround could be to generate two copies of the file - one with the bookmarks and links, and the other with the high quality images - and you can then just replace the pages in the file generated by the Add-in, or copy your bookmarks tree and paste them in the file you drag-dropped.

Kind regards,
Stefan
eleigh
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Re: Image downsampling

Post by eleigh »

Tracker Supp-Stefan wrote: Mon Mar 22, 2021 12:54 pm Then maybe your workaround could be to generate two copies of the file - one with the bookmarks and links, and the other with the high quality images - and you can then just replace the pages in the file generated by the Add-in, or copy your bookmarks tree and paste them in the file you drag-dropped.
This could be a viable workaround, thank you. The ability to copy and paste bookmarks between documents is neat! However, the conversion options available when opening a Word document are minimal. Is it possible to fine-tune the resizing and compression of graphics this way?
eleigh
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Re: Image downsampling

Post by eleigh »

Just wondering if there is any progress on this? I still cannot produce high quality PDFs of Word documents in PDF-Xchange.

The problems remain with image degredation (and now also a path rendering bug - https://forum.pdf-xchange.com/viewtopic.php?f=62&t=37157). Workarounds that do not use the Word add-in do not allow me to set the image down-sizing and compression settings.
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Ivan - Tracker Software
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Re: Image downsampling

Post by Ivan - Tracker Software »

I'm afraid we cannot do much here, as the issue is not on our side, I'm afraid.
source.png
On the left side is the image we got from Word - it is already upscaled, not the original one which is used when Word exports to the PDF itself (right side on the screenshot).

I'm using Word 2019 on Win 10, maybe it prints in a bit different way than your Word 2016, but I'm not sure.
And I'm afraid we have no control over that.
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When attaching files to any message - please ensure they are archived and posted as a .ZIP, .RAR or .7z format - or they will not be posted - thanks.
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