DPI significance in document setup

[David Green]

In Affinity Publisher, what exact affect does the 'DPI' setting do in document setup? The help text is too vague, and says nothing about how this setting is used by Publisher and how it interacts with the resolution in generated files:
"DPI—Sets the resolution of your document. For example, for professional print quality, set your resolution to 300 dpi or above."

If I initially set the 'DPI' setting to a low value, will it prevent me generating high-res PDFs? If not, what purpose does it serve?

Perhaps DPI 

I've been using Serif PagePlus for years, and I'm used to only controlling DPI in the files which I generate, like in PDFs, not in the source file.

[Dan C]

Hi David Green

The DPI of the document technically doesn't matter until the document is exported, as it is this time the DPI is embedded into the document and cannot be changed easily. 

The DPI setting will control any pixel data in the document, whilst you're still working on the file in Affinity publisher you can change this DPI on the fly, and any pixel data already in the document will be resampled to match this new value.

On 6/22/2019 at 12:28 PM, David Green said:

If I initially set the 'DPI' setting to a low value, will it prevent me generating high-res PDFs?

It will not prevent you from this, as the DPI can be changed at any time, should you wish.

Certain users require a set DPI for their projects, depending on what they're doing with the file. I expect most files in Publisher to be created at 300DPI and above, but we like to offer this option so we can suit any workflow.

On 6/22/2019 at 12:28 PM, David Green said:

If not, what purpose does it serve?

Certain documents will use a lower DPI for images to reduce the file size when exported (such as serving the file over the web), the lower the DPI, the less information raster images can carry and therefore they have a smaller file size. I hope this helps!

[Carlsson]

I came here to understand this as well. I’ve just started to create my first document in Affinity Publisher (been an Indesign user), and this is the first thing I’d like to know more about.

I’m creating a folder that will be printed on a 1200 dpi digital press later. I will have technical scans containing black lines against white background, I have scanned them in 600 and 1200 dpi to maintain the “crispness”.

 

- Should the DPI setting be 1200?

- What happens if I change this later?

- Could I adjust the DPI if I do a PDF export and that will adjust accordingly in the PDF file?

- What if I get a document from another user, that didn’t set the DPI correct from the start?

- Will all imported pictures be down/upscaled to the current documents DPI?

- What if I have imported a file while the document was set with 300 DPI, and later adjusted the DPI to 1200, will the photo relink to adjust accordingly?

 

Some important things to know, that I think you should update in your documentation. It’s important for us that do prints still.

 

However, I am enjoying the trip into Affinity land so far…  

 

[lacerto]

11 hours ago, Carlsson said:

I’m creating a folder that will be printed on a 1200 dpi digital press later. I will have technical scans containing black lines against white background, I have scanned them in 600 and 1200 dpi to maintain the “crispness”.

You should know that Affinity apps do not support monochrome images so 1-bit bitmaps will be handled as RGB images (and can be made to behave like grayscales). They also do not support separate setting for downsampling color and grayscale images so if you need to output raster graphics at 1200 dpi, it is.a good idea to import most of the raster graphic at the resolution that is actually needed, without needing to rely on export time downsampling. 

If you intend to rasterize anything in place (in layout) it also happens by using document DPI (and involves upscaling whether wanted or not; upscaling happens also silently in certain situations). Objects imported via Clipboard (including from other Affinity apps) are affected by DPI, based on rather complex rules (e.g. depending on whether objects are grouped), and will be rescaled (often producing conflicting results since part of the features pasted might have scale-related independent sub options). DPI dependence in general, in context of vector graphics, is guaranteed to cause some surprises (in perspective of being accustomed to use Adobe apps). EDIT: This oddity seems to have been recently fixed on both platforms as when I just tested this I can no longer reproduce this behavior when exchanging objects with Affinity apps and documents, other than when "pasting special" (possible on Windows) and choosing "PDF" as the paste format. Exchange between Affinity and [many] non-Affinity apps still happens in PDF format and there having the document DPI at 72 will retain the dimensions when exchanging data [and will otherwise result in scaled sizes].

As for PDF export, whether raster content is affected by export DPI depends on if a file is placed for passthrough or interpreted. The former won't be touched no matter what export setting is used, but the latter will, and then both to upscale and downscale.

As for relinking, I have some quite confusing experiences, both color- and DPI-wise, so I would avoid relinking automatically content that has changed in this way and either would relink manually and confirm the change, or re-import and place fully manually. 

[thomaso]

5 hours ago, Carlsson said:

- What if I have imported a file while the document was set with 300 DPI, and later adjusted the DPI to 1200, will the photo relink to adjust accordingly?

In this example I placed an image + copied it twice, at a document resolution of 600 dpi. Then I switched the document to 60 DPI, the visual dimensions of the placed objects did not change. (but if I now click-place the same image once more it would get placed in 10x size because it gets placed relative to the document resolution). So far a change of the document DPI does not alter the existing layout content.

But the current document resolution matters when an object gets rasterised. Here the left image was rasterised at 60 dpi document resolution, the right at 6 dpi. Because they all had been placed at 600 dpi in their 100% layout size their resolution gets changed during rasterisation and they appear different.

[Carlsson]

Thanks for the input. I should have mentioned that the scanned documents are scanned in color, not bitmap. 

There really is a difference scanning these in 300, 600 or 1200 dpi in the printed output. Photos usually don't have this problem, but documents with a lot of details etc really shows a difference.

So if I have understood this correctly; If I create the documents with a 1200 dpi, and import the scans "as normal", there shouldn't be a problem.

[lacerto]

31 minutes ago, Carlsson said:

So if I have understood this correctly; If I create the documents with a 1200 dpi, and import the scans "as normal", there shouldn't be a problem.

That would at least guarantee that whatever is explicitly rasterized in the layout, would have that resolution, and some export options would have downsampling trigger by default set to document resolution. But many export options seem to have fixed default DPI settings (300 with print/press oriented export presets and less for some other).