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Better control over upsampling in PDF export


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When exporting to press-ready PDF from Publisher, the software always seems to re-rasterize images that have a resolution lower than the document resolution (or the resolution specified in the export dialog box).

There currently seems to be no way to just pass the images to the PDF at the original resolution. There is no per-image control over how they are resampled/upsampled.

I have a case here where a client has provided me a QR code as a PNG image, with a resolution that is much lower than my document resolution. Exporting with any other sampling than "Nearest Neighbour" will obviously result in an blurry QR code. Exporting with "Nearest Neighbour" will result in all other images potentially getting upsampled using Nearest Neighbour sampling.

As a user, I actually may not want images like the QR code re-sampled at all. Re-sampling that image even in Nearest Neighbour will just generate pointless data and the RIP at the printer will re-sample once more at native print resolution, so depending on the image and how close it is to the document resolution, there could be some generation loss, artifacts or even moiré incurred.

Ideally, we would be able to have a PDF that has images at their native resolution so they will get resampled only once during the RIP phase at the printer at its internal maximum physical output resolution.

InDesign can output such a PDF and in fact will do so by default – it does not upsample images at all.

Another use case where upsampling could be undesirable would be when creating PDFs with web graphics for client review. In that case, the client may actually want to inspect the image at a pixel level by zooming in in their PDF viewer. Just passing the images through to the PDF at original resolution would allow for this (though the user would have to use a PDF reader that does not do image filtering when zooming in for this of course). Just imagine someone places a 299 dpi image in a 300 dpi document and exports it to PDF. The resampling result will not look great. But if the native image is passed though, it may actually look great (depending on the zoom level in the PDF viewer).

Another use case are screenshots in magazines. If these are resampled like photos, they can become blurry or exhibit ring artifacts when resampling with a filter like Lanczos. Of course one could just export the document with Nearest Neighbour filtering, but that would cause all photos in the magazine that are even slightly under document resolution to become Nearest Neighbour filtered as well.

Don't get me wrong, upsampling to document resolution is actually a good idea in most cases as printers in my experience only ever do Nearest Neighbour filtering and you may gain quality.

It might be useful to allow the user to control these settings at an object level. For instance, for the QR code in my example, an option not to upsample could be activated. Upsampling of images could be deactivated globally in the PDF export settings. Or, with upsampling activated, all regular photos in the document, except the QR codes and screenshots with upsampling off at object level, would be upsampled on export for slightly better quality.

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Could you not Rasterize the QR codes and Screenshots before exporting?

Mac Pro (Late 2013) Mac OS 12.7.4 
Affinity Designer 2.4.0 | Affinity Photo 2.4.0 | Affinity Publisher 2.4.0 | Beta versions as they appear.

I have never mastered color management, period, so I cannot help with that.

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That would have the following disadvantages:

  • Bigger file size in both Publisher and the PDF (compared to pass through)
  • Does not prevent rasterization happening twice (once inside Publisher and once by the printer's RIP)
  • Will cause triple rasterization if the image is then resized in Publisher after it has already been rasterized to document resolution
  • Does not prevent quality loss if the effective image resolution is very close to document resolution (say, a 300dpi image placed and resized so it ends up at 298 dpi)
  • Breaks link to original image asset file
  • Has to be re-done manually when file is replaced (eg. when client sends an updated version)
  • Will not update if, for whatever reason, the document resolution is changed later
  • The "Rasterize" command has no setting for Nearest Neighbour sampling, so rasterizing an image that requires Nearest Neighbour resampling would just cause it to go blurry one step earlier in the process
  • If I hand such a document to someone else to work on, they would have to be aware of the situation or risk messing it up when making changes

I think being able to choose per image whether the data is to be passed through without upsampling (better quality and smaller PDF file size for images like QR Codes, screenshots and pixel art as well as matching InDesign's behaviour) or upsampled to document resolution on export (which, for especially for photos will lead to slightly better quality results) would be a better solution.

In my case, I worked around the issue for now by rasterizing all the photos to document resolution (which probably uses bilinear or bicubic sampling) and then setting the resampling in the PDF output settings to Nearest Neighbour. I was only dealing with a two-page document, but if I was working on say, a tech magazine with a mix of screenshots and photographs, that would be rather error-prone and cumbersome, to the point where frankly, I'd just do the project in InDesign instead and not worry about it.

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