Understanding Resolution, Image Size, DPI/PPI

The image size of a graphic file is specified in pixels. It refers to the dimensions (width x height) of a digital image, for instance, 4608x3072px. Typically, the width is mentioned first.
A digital image on a computer never uses DPI, cm, or inches for sizing, only pixels.

Resolution

The resolution of digital images is understood as the product of height and width of the image in pixels. For an image with a size of 4608x3072px, the resolution is therefore 14,155,776 pixels, or roughly 14.2 megapixels. Camera manufacturers often use the megapixel count to indicate the resolution of their digital cameras.

• DPI stands for Dots Per Inch and indicates the number of printing dots per inch.

• PPI stands for Pixels Per Inch and indicates the number of pixels per inch.

DPI and PPI essentially mean the same thing, though PPI is the more accurate term, with DPI specifically used for some printers and print processes.

In image editing, specifying only DPI/PPI never makes sense. The DPI/PPI value must always be given together with the desired image size in cm/inches. From these two values, the minimum size of the digital image in pixels can then be calculated.
Example: A poster with 90x60 cm should be printed with at least 150 PPI.
From this, one can calculate that the digital image size must be at least 5315x3543 pixels (~18.8 megapixels):

Width :
Height :

Resolution Information in Windows Explorer Image Properties

The resolution indicated in the properties dialog of an image file in Windows Explorer or in graphic editing programs can be disregarded. These values have no significance and only cause confusion.
The size of a digital image is only given in pixels. A resolution in DPI/PPI is only relevant when the print size is defined. Thus, a resolution of 300 DPI for a 4608x3072px photo is only relevant when the photo is printed at a size of 39x26 cm.

What Resolution Do I Need?

For posters, 150 PPI is more than sufficient. Although many printers can print at higher resolutions, in practice, it makes little difference. At viewing distances typical for posters, the human eye cannot distinguish between 150 PPI and higher resolutions. The print preparation scales the image data accordingly, so a resolution higher than 150 PPI only results in unnecessarily large file sizes without improving visible detail.

Resolution for rapid.art Products:

For Rapid-Mosaic and Rapid-Map, images are rendered for a specific print size. Elements such as mosaic tiles or line thickness and map labels in city maps are optimized for a specific print. For the selected print size, the digital image at Rapid-Mosaic has a resolution of 150 PPI and at Rapid-Map 225 PPI.

When creating photo collages with Rapid-Collage, the created collages can be printed in any size. Thus, the digital image is rendered in maximum resolution, which corresponds to roughly 150 PPI for a print size of 150x150 cm, which is over 75 megapixels.

Image Size and Resolution Settings for Rapid-Mosaic

With Rapid-Mosaic, you only need to specify the size of the poster or canvas/acrylic/Dibond. The algorithm calculates the right resolution itself and sets it automatically for a good print result. You don't need to make complicated calculations or settings. Just give it a try: