Image size does not stick

Is something not working like it should? Let us know.
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2021-01-04 11:45:24

If I resize either the canvas or the image to 216 mm x154 mm (resolution 300dpi), the result is an image of 215,98 mm x 154,01 mm. I even tried to open a new image with these sizes and the new image without doing anything the results are the same.

I know the difference is very small, but when I upload these files to the printer's website, they get scaled. I do not want the files to be processed at all by software I don't know and cannot see the result of before I receive the final print.

Image Image

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2021-01-04 19:09:36

The problem here is purely an issue of units and mathematics – in a raster editor like Pixelmator Pro (and others), the print size is really only a theoretical thing, and images, layers, and tools still work in pixels. This is because an image cannot be, say, 95.5 pixels wide. You're using millimeters for the size and PPI (pixels per inch) for the resolution. This gives you a fractional pixel size because 300 PPI is ~118 PPCM (pixels/centimeter).

Basically, what happens when you create a new image is that the print size of the image (in your case, 216 mm or 21.6 cm) is multiplied by the resolution in pixels per cm (which was converted from 300 pixels per inch to ~118 pixels per cm) and that is used to create an image sized in pixels rounded to the nearest whole pixel. Under the hood, the rounding happens at the last stage, so really everything is accurate, but the problem remains, that multiplying, say, 21.6 by 118 doesn't give you a round number of pixels, it gives you 2548.8.

Everything just works with a number like 300 because that gives you round values for pretty much any (imperial) size. But 300 isn't some magic number in terms of quality, it's just handy for things like this, so I would say you can use 120 pixels/centimeter for your images (which is around 304 PPI).

In fact, we should probably change the metric presets to use PPCM instead of PPI to avoid issue like this
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2021-01-05 08:05:20

Hi, thanks for your answer. This makes perfect sense. Maybe at the export stage it should be possible to ‘round off’ the values to prepare better for print. Also setting up a bleed size in the exported pdf (without adding lines or cutting marks) would help achieving this.
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2021-01-05 09:49:03

I think the value that we'd need to round off would be the resolution, which would mean changing the pixel size of the image, which can visually change it (even if only a little), so that would be a big no-no. This is something that should really be set up at the beginning, but we might be able to make any discrepancies more obvious in the new image dialog.

As for a bleed size, we'll keep that in mind!