Resetting the dpi of export images

[Stuart444]

I'm trying to prepare some images for uploading to a photo printing company for them to print. However I'm a bit confused over various print settings and hope someone can help.

 

Their requirements are "images must be set to the print size required and at 402ppi". I'd like them to print the images around 18" x 12" (that would be the paper size I'd print onto if it were my printer) although they've all been cropped a little and so won't have exactly a 1.5:1 size ratio and I expect to have to trim the prints when they come back.

 

Firstly, I don't know how to how to set a paper size in AF. I've tested the "document/resize canvas" and this just seems to be a way to crop the image to a size smaller than the existing, so that is not what I need here. How do I set my image to 18"x12"?

 

Secondly, I've discovered the "document/resize document" command, and I have been setting that to 402dpi (which I presume is the same as ppi) and unticking "resample" else I get much bigger images. However if I then export the image, I find that the resultant file is no longer 402dpi but back at whatever dpi setting the image had before I resized it. How can I get it to retain the new dpi value?

 

Thanks

[anon1]

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[Magnus_vb]

Hi Stuart

MBd beat me  :)

 

One thing to remember - tick "Resample" when changing the resolution (most of the time anyway).

[Stuart444]

Thanks, guys.

OK, I've done as you suggested and watched the video. And hey presto, it seems to work. I made my new document at 18"x12", 402dpi. I copied my 72 dpi, 1899 x 1561 px original, pasted it into the new document, and it only filled under 25% of the available space. I then dragged the image out to fill the new canvas width (leaving an unfilled area in the height direction as they are not the same aspect ratio). It looks good. If I click document / resize document, it confirms it is still 18x12, 402 dpi.

It's still a bit puzzling to me though. Obviously, dpi multiplied by inches gives dots (pixels) along that dimension.  So an 18inch print side with 402 dpi needs 18 x 402 = 7236 pixels. My original image had only 1899px. So how come it can now fill the new document? Has it been altered somehow?

[anon1]

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[Stuart444]

I'd wondered about that, but had thought it would only be possible to do this if the "stretching" was in integer units. However the stretching is continuous rather than in discrete steps, so I'd thought I was wrong.

 

OK, that all looks good now, thanks. I figure that as long as the resolution looks good on the screen with due regard to the final print size, as you say, then I'm OK. So I'll send the files off processed like this and see what comes back. I've not done commercial digital printing before, thought I'd give it a try before shelling out for an A3+ printer as many of my photo club friends only use commercial printers.

 

Before closing this, could I ask Magnus to elaborate? I know the sampling box is ticked by default, originally I'd left it so, but I was getting very large files (1GB sometimes) so I figured I was getting that wrong in this instance. Is the stretching procedure being used to fill the new document effectively upsampling as done when we resize with sampling left on? When generally do you leave sampling on and when off?

[anon1]

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[R C-R]

It might be a good idea to view the "stretched" image at actual size on screen (shortcut command+1) to see what an effective dpi of 105 looks like & compare that to using a high quality resampling method like Lanczos 3. My guess is the stretched version will look much worse, displaying the pixelated effect we used to call the "jaggies" back in the 1980's.

 

As for larger file sizes, if you resample an image that is only about 25% of the required size in both height & width, you need to increase its pixel count by four times in each dimension, or by an overall factor of 16 (because every original pixel is replaced by a 4 X 4 matrix of new ones).

[anon1]

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[R C-R]

MBd,

 

In Affinity Photo for me the resize document option has a dpi setting & the width or height values accept all the expressions without problems.

[anon1]

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[Magnus_vb]

This might be old, bad Photoshop habits of me. But, if I need to adjust the image to meet some printing demand  - like the output size or resolution - resampling must be done. That is pixels are removed or added.

 

In AFP you have the option to chose a sampling algoritm (quality) when changing the size of the image, and I don't know which sampling type is used when you just resize an image that is past into another image with higher resolution.

[R C-R]

R C-R it does not make any difference, you either stretch first without resampling and then choose the resampling option during final export or you resample through the resize option. It´s both exactly the same, I´ve just done it and it was pixel perfectly the same.

 

Your source image is about 2,9MP and the target is at least capable of printing 34MP so there is quite a difference but if you're okay with the final output it can be totally fine.

The two methods do seem to result in identical pixels in the image, but when I tried this to exactly double the size of the image & export using the PNG-24 preset with Bilinear resampling, I got different files sizes, with the stretched first version about 15% larger. But going through the same steps except using the Nearest Neighbor resampling, the file sizes were identical. They were also exactly the same file size as the original unstretched .afphoto file, whereas the Bilinear exports were more than 2X larger.

 

I have no idea what to make of that.  :(

[anon1]

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[R C-R]

In AFP you have the option to chose a sampling algoritm (quality) when changing the size of the image, and I don't know which sampling type is used when you just resize an image that is past into another image with higher resolution.

As I understand it, as long as you do the stretching/shrinking to the native .afphoto file, the pixel count does not change -- unlike resampling it is a non-destructive process. Only on export is resampling applied, so you always have control over which algorithm is used.

[Magnus_vb]

@Magnus: The one that you specify in the export dialog is used. It´s only applied on export.

 

Not sure I follow  :unsure:  Export setting?

 

I did a quick and dirty test.

1. A 500x500 image at 200 dpi

2. Same file resized (Resize document) to 1000x1000 and 300dpi. Resampling and Lanczos 3 non-sep.

3. Created a new document with the size 1000x100 at 300dpi. Paste in the layer from 500x500.

It's a small difference, but to my old eyes, version #3 is a little softy.

[Magnus_vb]

As I understand it, as long as you do the stretching/shrinking to the native .afphoto file, the pixel count does not change -- unlike resampling it is a non-destructive process. Only on export is resampling applied, so you always have control over which algorithm is used.

Okay. Thanks

 

Edit. When I compared the images above, it was native afphoto files

[anon1]

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[Magnus_vb]

This is complicated. And my head hurts now  :D  :wacko:

[Stuart444]

Your head hurts! Mine's in a spin.

Anyway, thanks for your help and explanations. I'm new to digital photography, and having scurmished with PSE and other softwares in Windows, I've bought a mac and AF specifically because a very experienced friend recommended and gets wonderful results from both. Steep learning curve, but it's all getting clearer now.

Stuart

[R C-R]

It is complicated enough to cause everyone headaches because the only thing "real" about it is what comes out of a printer. Everything else is virtual, just a bunch of ones & zeros processed by complex hard & software into an approximation of a printed page. Dots per inch on a print has an unambiguous physical meaning; pixels per inch does not. It can refer to the physical pitch of the pixels on a computer screen but we routinely use the same phrase to refer to any of the several ways the image information encoded in a digital file is mapped to the screen, & sometimes even to a print.

 

That is an abstraction, one without a clear physical meaning. It is no wonder that it makes our heads hurt just trying to find the right words to describe it, much less to apply it to practical purposes.

[anon1]

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[R C-R]

It does not matter at all if we display something on a device that uses pixels or if we print it and print pixels on a paper.

But it does. We are not printing pixels on paper; we are printing dots of ink or toner. There is no fixed mapping between the geometry (or color) of those dots & the pixels of the display. The maps are linked mathematically to the same digital source data but the software generates different maps for each device.

 

If it did not, you would get very different results when printing to two different printers with different nozzle pitches, or when using inks & papers with different spread & blend characteristics, & so on.

[anon1]

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[R C-R]

the dots are on a sub pixel level so we´re not talking about them in dpi nor in ppi description 

 

Which is yet another reason this is so complicated. It isn't complete but https://en.wikipedia.org/wiki/Dots_per_inchhas a reasonably concise description of the differences between dpi & ppi as measurement units for different devices.