This guide is meant to be a valid help to better understand about DPI, PPI, and their relevance and how the printer's driver work.

• DPI = Dot Per Inch relates to the amount of ink dots for inch. These are correlated to the ink flows putted to use during the printing process. Note: a dot does not mean one single ink drop, but many of them, which its amount varies dot by dot due to the color desidered.
  
  
• PPI = Pixel Per Inch represents the number of pixels parceled out in one single inch. This will be a function of an image resolution and of a printing format.

DPI is a variable parameter which depends by your printer, by the volume of your ink drop, by the amount of nozzle etc. Consider also the DPI value provided by the manufacturer corresponds to the highest value reachable by your printer.

For these reasons we better give up on this parameter which is, additionaly, a non-editable parameter. As we said above your DPI parameter just depends on the kind of printer you ow


Let's get started to talk about PPI

PPI, as we said, represents the number of pixels parceled out in one single inch and it will be a function of an image resolution and of a printing format required.

Every printer, for home or lab use, works on a certain PPI value, regardless of the kind of picture committed to be printed out, its resolution, etc.

Let's move on with an example:
A Canon i560 printer with the following settings: high quality print, 4x6 inches paper format no borders, will work at 619 PPI, 600 PPI by using a standard paper with borders on and standard printing quality, 300 PPI in a draft printing mode.

Higher the PPI amount, higher the image resolution.

This means if we are meant to print a photo in a 4x6 format 300 PPI a resolution 1800X1220 pixels is needed, and again to print a photo in a 4x6 format 600 PPI a resolution 3600X2400 pixels is needed.

We care to remind you that it's not possible at all to modify the printer's PPI value. However, knowing this value might be useful in order to commit the image, already at the proper size, to your printer optimizing this way the print.

If an image not having the proper dimensions comes sent to the printer in the selected format according to the printer's PPI value, the printer's driver will resample the image for you in order to obtain the dimensions needed.


Note: Any image sent to the printer which does not have the proper dimensions will be ALWAYS resampled by the printer's driver! The only way to avoid this resampling process is to provide to the driver the image hasing already the proper dimensions




Example:

To print a photo in a 4X6 format 619 PPI a photo of 3714X2476 pixels is needed. If we send to the printer a 2816X2120 (6MP) photo, the printer driver will resample the photo automatically to 3714X2476 pixels. In this case the driver will make a crop of the image since the image sent does not follow the height-width ratio.



The resampled image by this way often produces a non-satisfactory result due to plain algorithm used for resampling.

In order to obtain better quality results it is good to resample the images via a specific software, then send the images to the printer already having the proper dimensions.

PhotoResampling deals with it! Photoresampling provides many tools for digital image touch up, in addition PR allows to resample photos thanks to a lots of algorithms, including 3 exclusive ones specifically developed by our staff. These algorithms will allow you to obtain the highest results you ever seen.
Besides the tool "ForPrinting" allows you to resample and crop the images all at once according to the print format, sending this way the image to the driver in a proper dimension in order to avoid unpleasant surprises in your printed image like bad crops or bad photo quality caused by a plain algorithm used.

It stands to reason that the knowledge of the PPI value is very important.
If you are going to bring your photo in a photolab ask them info about their printer's PPI value (usually in a range between 200 and 400 PPI).
Otherwise, if you print you photos at home, downlaod the Printer Data software.
This is an easy to use and free utility able to show you important printer's settings that you have to bear in mind during the printing phase like the PPI.