How do resolution and pixels work?

[adelaisr39143285]

Hi,

I was wondering if you could help me understand this concept:

I'm trying to design illustrations for this print-on-demand t-shirt website. They encourage users to post images which are 7632 x 6480 in size. Yet, they say that DPI and PPI have no effects on the final upload. How is that possible? I was under the impression that for an image to be of high quality for print, the PPI should be fairly high and in the 300 region. What exactly am I missing to understand here?

The other thing is that:

When I create a new document which is 7632 x 6480 and add an illustration to it, when I save it, depending on the PPI used the actual resolution changes in the saved file. As an example: I started a new document 7632 x 6480 and created a circle on it. I then saved it as a PNG file with a PPI of 150 (tried to save it at 300, but Illustrator doesn't even allow me to save at 300 - I get an error). When I checked the file's properties panel, under "details" it was listed that the file has a resolution of 14170 pixels by 12550 pixels. I tried adding a few more shapes to the document and sure enough the final resolution went on higher still. So...does this mean that I can never know how big of a resolution the final PNG file that I save will have even though I start knowing very well how big I want my art-board to be?

Thank you in advance.

[jdanek]

Resolution depends on the output device and how the shirt is printed.  Illustrator artwork is vector based and resolution independent.  When effects are applied, then document raster resolution becomes an issue and can be set as low, medium, or high.  Your original file is then exported once it is finished and ready for upload to the online vendor. Upon export you can set the final image resolution, whatever is appropriate for their output device. It must be established before you proceed.

[jdanek]

When you say "post an image", is that for their page display?  If it is, they should be telling you the size based on 72ppi. A thumbnail if you will.  The final artwork should be based on their output device.  There may be a possibility they use a third party for production printing and may not know their size and resolution limitations, because it could vary.

[adelaisr39143285]

Sorry I should have mentioned: they don't actually require a vector file to be uploaded to their website. Just PNG or JPEG. That's why I can't understand how it is that the PPI has no effect on the final image considering how the graphic uploaded to the website would be raster based and not vector based. (Just to explain things a bit further: they ask for a 7632 x 6480 file to be uploaded which can then be used on may different products. The 7632 x 6480 is for one of their largest products and then of course for t-shirts it can be scaled down on the website using some of their online tools.)

[Mylenium]

Resolution is just meta information, hence it's ultimately irrelevant to many workflows. They likely simply check the absolute pixel dimensions and then re-assign the meta data in their own internal database system rather than relying on potentially wrong embedded image file tags. Nothing magical to it beyond that.

Mylenium

[Doug A Roberts · Accepted solution]

adelaisr39143285  wrote

they ask for a 7632 x 6480 file to be uploaded which can then be used on may different products. The 7632 x 6480 is for one of their largest products and then of course for t-shirts it can be scaled down on the website using some of their online tools.)

The resolution will be determined by the size of the product they are printing on. They have determined that 7632 x 6480 px is what they need for the resolution they require on that product.

At 300 ppi, that is 25.4 x 21.6 inches

At 144 ppi, that is 53 x 45 inches

Then for anything smaller than their largest product, they have far more resolution than they need.

For your second point, export at 72 ppi and ensure that Use Artboards is checked in the file browser window. Illustrator treats 'pixels' as an actual measure, the same as points (1/72 inch). If you then export that at 300 ppi, you're multiplying the pixel dimensions by 4.16.