Pixels are the individual building blocks of every digital photograph and most other digital images. In addition to pixels in a digital image, pixels can refer to pixels in a digital display, a digital camera sensor, and other devices. But, in this tutorial, we’ll mainly be talking about the pixels in digital images.
The word ‘pixel’ was invented by combining the words ‘picture element’. You can think of the pixels in digital images as colored squares. The images you see on screens usually have hundreds of thousands (and often millions) of pixels — when enough of these colored squares are placed next to one another and displayed at a small enough size, you see continuous images instead of individual pixels. Unless you zoom an image far enough to see the pixels, in which case they appear as colored squares.
Pixels themselves don’t really have a size. While physical units like inches or centimeters have an exact, real-world size, a pixel is more of a logical unit than a physical one. However, the pixels of digital images are most often displayed at a size so small so as to not be visible, so they usually exist as very small elements.
Although pixels don’t have a size of their own, their relationship to physical units becomes important when you want to print an existing image. Or if you want to create a new image to be printed at a particular size. For example, if you’d like to print an image at 6 inches wide and 4 inches tall, that says nothing about how many pixels has or should have. At this point, it’s important to make sure that, just like when displaying an image on a screen, the individual pixels are small enough to not be visible. The standard for high-quality prints is to print 300 pixels per inch of a page. How many pixels should fit into an inch is called the resolution of the image. And with a resolution of 300 pixels per inch (PPI), 6 inches are now equal to 1,800 pixels and 4 inches are 1,200 pixels.
Pixels can come to exist in a few different ways. For example, digital cameras have sensors made up of light-detecting pixels. When you press the shutter, the sensor captures the subject you’re photographing and the information detected by each of the pixels of this sensor are translated into the pixels of a digital image. Instead of capturing a photo, if you create a new image in Pixelmator Pro, the first step is setting its size. Therefore, you immediately say how many pixels you’d like your image to have. So all digital images start out as collections of pixels and as long as they exist in digital form, they continue being collections of pixels.