300 DPI document exports as 400 DPI

[Davis]

I am working on Affinity Publisher document, that consists of text on top of the images (300 DPI). I need to export it as 300 DPI, but it looks like, that it exports as 400 DPI. Am I doing something wrong or is this a bug? Maybe it doesn't even matter that it is larger than 300 DPI, even though the print company is asking for 300 DPI file?

I am on macOS 11.1, Affinity Publisher 1.8.6 (But I also tried out the latest Beta with the same results)

Thanks in advance!

Here are screenshots with document & spread setup and export settings:

And here is the Document Settings for the exported file - it shows that it is 400 DPI.

 

[firstdefence]

10192.4pt = 3596mm approx

7952.2pt = 2805mm approx

That's rather large, is that intentional?

[Davis]

Yes, there are 112 pages in my exported file – I opened the exported .pdf file in Affinity Designer, and it places all the pages as artboards in grid formation, so that is why it has pretty large dimensions.

It looks like it just estimates the DPI incorrectly – when I open the file, the default setting in the import screen for DPI is set to Estimate, if I just click Open, it opens the document in 400 DPI, the images are still 300DPI, though, it just shows that they are 133% scaled to fit the 400DPI file. If I, set the DPI setting in the import screen to 300 DPI, then it unsurprisingly opens it as 300 DPI document (images are also 300DPI at 100% scale), so I guess everything should be fine for the print company? However, I find it weird, that it estimates the DPI incorrectly... Isn't the DPI "baked" in the file somehow?

[Dan C]

Sorry to hear you're having trouble @Davis

20 hours ago, Davis said:

Isn't the DPI "baked" in the file somehow?

It certainly should be, but this value is not always used when Affinity is importing PDFs. I'd like to have a look at the document that's been exported from Affinity. Could you please upload a copy of this file to the below link for me, then let me know once this has been done?

https://www.dropbox.com/request/eJ0y4viMKTPwQnyHbW7U

Many thanks in advance!

[MickRose]

I might be wrong but I wonder if this is an example of a sort of structural problem with Affinity software. There are issues in Designer and Publisher to do with page sizing and resolution. For example, if I have a blank A4 page in Publisher with a Document Setup/Layout/DPI of 1 and export it to PDF, then the resulting PDF is about 228mm x 305mm. A DPI of 72 gives a page width of 210.3mm. I know that a DPI value of 1 is stupid but it just illustrates the point that there seems to be a link between raster resolution and page size which shouldn't really be there. It might be that because all 3 apps are structurally closely linked, then they have to use the same fundamental measurement unit - and this might be causing problems If I am talking complete nonsense I'd appreciate being told so.

[Dan C]

14 hours ago, MickRose said:

there seems to be a link between raster resolution and page size which shouldn't really be there.

What would lead you to believe this shouldn't happen?

As far as I understand it, DPI will affect the export size of any raster format - as both Pixels and mm can be used to display the document size, but when changing the DPI you are affecting the pixel count and therefore the size of the exported document.

[MickRose]

Changing the DPI changes the pixel count and hence the size in kB or MB but it shouldn't change the dimensions of the page of which it is a part.

[Dan C]

The pixel count and the dimensions of the page are intrinsically linked, meaning changing one will change the other. 

I recommend checking out the following article, which explains how DPI is used to determine the final 'print' (or in your case export) size 

https://affinityspotlight.com/article/understanding-dpi/

[MickRose]

I have read the article. It is mainly about screen resolution and resolution/dpi for desktop printers and large format printers. If I use Publisher to create an A4 page I expect an A4 PDF regardless of the resolution of any image within the page. A low resolution image will simply result in a poor quality print. There should be no link at all between PDF size output and the resolution of any linked or embedded images.

[Dan C]

Apologies but I'm a bit confused Mick, originally you had mentioned:

15 hours ago, MickRose said:

For example, if I have a blank A4 page in Publisher with a Document Setup/Layout/DPI of 1 and export it to PDF, then the resulting PDF is about 228mm x 305mm

Changing the DPI within Document Setup/Layout/DPI changes the DPI for the canvas/page. This in turn will change the size of the canvas.

3 minutes ago, MickRose said:

There should be no link at all between PDF size output and the resolution of any linked or embedded images.

There isn't - and I'm not entirely sure what has lead you to say this, as I don't believe we have been discussing embedded/linked documents in this thread?

Importing a liked/embedded image into your document will not affect the documents DPI, resolution or physical size.

Changing the DPI of the placed/embedded layer will not affect the documents DPI, resolution or physical size.

However the dialog you're using to change the DPI (Document Setup/Layout/DPI), is the setting for the canvas itself, not for any linked/embedded layers.

Changing the DPI here will affect DPI, resolution and physical size.

[MickRose]

I agree my initial comment was somewhat off topic and I apologise for that. But could you explain what a canvas is in the context of a page layout program? I'm not aware that InDesign or Quark has anything like that.

[Dan C]

No problem at all!

3 hours ago, MickRose said:

But could you explain what a canvas is in the context of a page layout program?

The 'Canvas' is the working area of the document in any Affinity app, this is also referred to as your page/spread. This canvas is not locked down to any size and can be edited, enlarged or shrunk at any time during editing, regardless of the canvas preset size that was chosen when creating the document.

The canvas size is controlled by 2 things, physical size and DPI.

Any placed/linked/embedded layers in Affinity will have no control over the canvas size, regardless of this layers size or DPI.

I hope this clears things up

[Davis]

I just found the solution for my problem – there was one image that had 356DPI resolution, so I guess that was the reason why Affinity estimated DPI incorrectly. I changed the export settings, so that it downsamples all the images above 350DPI to 300DPI and that fixed the problem – when I opened the exported .pdf file, Affinity correctly showed that DPI is 300 

Thanks everyone for the answers and offers to check out the file! Have a great & safe weekend!

[Alfred]

1 hour ago, Dan C said:

The canvas size is controlled by 2 things, physical size and DPI.

If you have a canvas size of 8″ × 10″, at 300 DPI it will be 2400 px × 3000 px. Changing the DPI setting to 400 should change the pixel dimensions to 3200 × 4000, leaving the physical output size unchanged.

[MickRose]

46 minutes ago, Alfred said:

If you have a canvas size of 8″ × 10″, at 300 DPI it will be 2400 px × 3000 px. Changing the DPI setting to 400 should change the pixel dimensions to 3200 × 4000, leaving the physical output size unchanged.

I'm sure that works fine for all practical uses. But if you use an extremely low DPI setting (1 for example) the physical output size does change. Using a DPI of 72 changes the PDF to 210.3mm wide. I know this is nit-picking but I'm just curious why this is happening. It seems there is some rounding up/down which is giving me a different size PDF to the Publisher document.

[Alfred]

6 minutes ago, MickRose said:

Using a DPI of 72 changes the PDF to 210.3mm wide. I know this is nit-picking but I'm just curious why this is happening.

I’m not too worried about 210 mm being changed to 210.3 mm, Mick, but your earlier example of A4 becoming 228 mm × 305 mm at a DPI of 1 seems too wildly off the mark to be ignored.

[MickRose]

I wonder if this is happening. It looks like for calculation purposes the page is being divided up into a number of whole pixels, the size of which is the inverse of the DPI value. When the page is exported to PDF all the fractional pixels at the page edges are treated as whole pixels and they all contribute to the exported PDF page size.  So a DPI of 1 for an A4 page gives a  pixel grid of 9 x 12 which gives an exported PDF of 228.6mm (9 x 25.4mm) x 304.8 (12 x 25.4mm). So what is being exported isn't the document page size at all, its the page as defined by the whole pixels. Hence the wrong page size values at very low DPI levels (large pixels).  All of this doesn't matter at all for normal DPI values of 200 or 300 because the high DPI makes the pixels very small. I guess it's all a consequence of Serif deciding to use the same structure for Page Layout, Designing and Image manipulation - it's a compromise. But users need to be aware of this issue if using very small DPI values for whatever reason.