A pixel (picture-element) is the smallest component of a digital image. It represents a “sample” of an image, usually represented as a square or rectangle. It contains the color information for the portion of the image it represents but it can also contain other information, such as opacity. The more samples there are in an image, the more detail that image will have.
Pixels by themselves have no defined size. As previously stated, they are merely samples of an image. In regards to images, PPI describes how many pixels will be in a given area of printed media. For example, if an image is 100 pixels wide and 100 pixels high, and it is set to 100 Pixels-Per-Inch, then the printed image will be 1 inch by 1 inch on paper.
It should be noted that PPI does not affect how large the image will appear on-screen, such as on a webpage. The only way to change the size of an image for display is to change the number of pixels in the image. Although some newer web browsers have the ability to automatically shrink very large images so they fit on smaller screens.
Dots-Per-Inch is a measure of print resolution. It defines how many linear dots can be printed within one inch. The higher the DPI of a print, the closer it will represent the colors in the digital image. This is important because a pixel can be any one of millions of colors, but printers generally only have four colors available: cyan, yellow, magenta, and black (CMYK). Printers can produce the illusion of more colors through halftone screens or dithering. Essentially these are methods that use different combinations of CMYK ink that appear to blend together to the eye to represent a single color. Therefore, printed images need to have many dots of the four colors in order to closely reproduce the color information in a single pixel.
For example, if an image is 400 pixels wide by 400 pixels high and its PPI is 100, the print will be a 4-inch square, regardless of how it is printed. But if this image is printed on a 300 DPI printer, there will actually be three times as many dots on the page as there are pixels in the original digital image. This means that 9 dots on the printed page represent the color information for a single pixel (3 dots wide, by 3 dots high). If the same image were to be printed on a 600 DPI printer, there would be 36 dots to represent the color information of a single pixel (6 dots wide, by 6 dots high).
Generally, a digital image for print should have approximately 300PPI. It is helpful to determine the desired PPI and final print size of an image. This defines how many pixels in the image to be created. For example, if the desired final print is to be 5 inches by 7 inches at 300PPI, then the image should be 1,500 pixels wide (5 inches x 300 Pixels-Per-Inch) by 2,100 pixels high.
Lenticular images are a special case. The lens of a lenticular print is magnifying a portion of the image underneath it. This requires more detail in the digital image than normal. More detail means more pixels. To keep the image the same physical print size, the PPI must be increased, and to keep the colors as faithful to the digital image as possible, a printer with a higher DPI will be required.
Another issue that lenticular creators face is figuring out how many pixels will fit under a lenticule. This is important because if the PPI of the image doesn’t take into account the pitch of the lens, frequency errors will occur in the lenticular print, such as banding.
Part of the “trick” of lenticular images is to reconcile the pitch of the lens with the DPI of the printer and the PPI of the image.