Its a simple case of some basic maths....just easy multiplication.
'Resolution' can be used in two ways:
1. To re-sample the image.
2. To assign a 'resolution' for printing.
The first may or may not be needed, depending on the original document size....IN PIXELS.
The second doesn't affect the image whatsoever and is just 'metadata' information for the printer to read.
You'll need to look at your ORIGINAL document dimensions IN PIXELS to determine if 300ppi is enough...
Size of PRINT = width and height in PIXELS / ppi
So for example:
Doc size = 3000px x 1500px (W x H)
3000 / 300 = 10" (Wide)
1500 / 300 = 5" (Height)
Printing this document at 300ppi will produce a print 10" wide by 5" high. (This would probably be too small for a T-Shirt.)
If your PRINT size is around 10" x 5" then your document size is enough....but if you need a bigger or smaller PRINT size then you'll have to re-sample it.
Re-sampling ADDS or REMOVES pixels to or from the document.
ADDING pixels is to be avoided at all costs but sometimes it cannot be helped.
REMOVING pixels is not as bad, although either method will reduce what is perceived as 'quality' of the image.
If the document does not contain enough pixels then you'll have to re-sample the document so it does.....you can use the 'resolution' setting in the 'Image Size' dialog to calculate the amount of pixels you need for you.....to do this you check the 'Resample' checkbox.
Entering the PPI AND PRINT size will change the document dimensions for you and display them at the top of that dialog....note that they will be 'greyed out' indicating that you cannot edit them directly and that they are calculated from the other two values....PRINT size and PPI.
If you just want assign a 'resolution' to a document for printing then un-check the 'Resample' checkbox.
You can now enter ANY PPI value without changing the document size.
The PRINT size at whatever PPI you enter will be displayed in the dialog.
Again, the PRINT size is dependant on the amount of PIXELS in the image and the PPI.....change one of them and the PRINT size will change accordingly.
DPI is a printer setting and not found in PS (Although Adobe do confusingly use the term DPI.)
Regards.
MrToM.
