Pixels per inch is just a tag
Image pixel density for web and app design is often seen as a confusing topic. I’m not sure why this is — when stripped back to the bare essentials, the reality is far less complex than you might think.
Bitmap images are two dimensional grids of pixels, with a width and a height. They often also contain meta data, which may include pixel density. Pixel density is often described as pixels pixels per inch (PPI) or dots per inch (DPI).
The two names, PPI and DPI, mean essentially the same thing — the number of pixels contained within a vertical or horizontal inch. 300PPI means that there’s 300 pixels contained within each one inch row. A one inch by one inch square in a 300PPI image contains 90,000 pixels, because it is a 300 × 300 pixel square. Most displays have a fixed resolution grid, so their pixel density can’t change. A Retina iPad display is 264PPI, because there’s 264 elements per inch in the display that can each only ever represent one pixel.
The pixel density stored in image files is different. It’s just meta data that’s saved alongside the image data. It can be changed at any time without affecting the image data at all. It should be considered a relic from the printing industry, because in general, web and app development display all images so that one pixel in the image equals one on screen, ignoring the embedded pixel density information.
Pixels per inch (PPI) vs dots per inch (DPI) vs lines per inch (LPI)
Purists will probably use PPI to describe on screen pixel density and DPI to describe print density, but I think this is a battle that’s drawing to a close. Apple, Microsoft and Google all seem to prefer using DPI in their literature. Curiously, Apple’s iPhone 4S and iPad specs describe their displays using PPI.
For screen design, PPI and DPI are interchangeable.
For print design, DPI only exists to indicate the final size the image might be printed. It’s sometimes used to generate thumbnail images to help performance in layout apps. It can also be used to indicate optimal print size — InDesign, Xpress and others treat 100% image size as meaning “the physical size you’ve used the image in your layout matches the pixel density tag contained in the image”. That’s a great feature. If the images are set up correctly, you know how large you can make images without risking pixelation. It’s still just a tag associated with the image. A handy tag, but a tag nonetheless.
Lines per inch (LPI) is a printing-specific term that describes how close together the halftone lines are. Not all printing is 300DPI — coarser line screens, like those used for newspaper and large format printing, mean lower pixel density images can be used.
The history of 72PPI
The original Mac, the Macintosh 128K, featured a 512 × 342 pixel, 72PPI black and white display. Because of this, OS X has considered 72PPI to be baseline where one image pixel equals one display pixel, even though most Mac displays are far higher density than 72PPI.
OS X’s Preview can view one image pixel as one display pixel, or it can scale using the PPI tag within the file.
It’s just metadata
If you’d like to set up your Retina iPad Photoshop documents at 264PPI, be my guest. You could also set them up at 72PPI. It really doesn’t matter, as long as the final result is images with the correct pixel dimensions.
However, if you plan to use Photoshop and different pixel densities for each document, dragging layers and copying layer styles between documents scales layer styles — dragging a layer from a 326PPI Retina iPhone document to a 264PPI Retina iPad document will mean all layer styles get scaled by 20% (then rounded to the nearest integer). And that’s probably not what you want. Also, OS X’s Preview will display 72PPI images at the exact size, no matter how you have it set up.
It is for these reasons I assign 72PPI to all my design documents, and I recommend you do the same. To change the pixel density of your Photoshop document without resizing the image data, open the Image Size dialogue, uncheck Resize Image and type in the desired pixel density.