Raster image

A raster image is made of pixels, a fixed grid of tiny colored dots. Photographs, scans, and screenshots are all raster: zoom in far enough and you see the individual squares. This is the opposite approach to vector graphics, which are defined by math and scale infinitely.

The defining trait of raster is resolution. An image has a set number of pixels, so blowing it up past its native size makes it soft and blocky because there is no extra detail to invent. This is why a low-resolution scan looks fine small but falls apart when enlarged, and why DPI matters so much for print.

In PDFs, raster is how photos and scanned pages are stored, usually as JPEG, PNG, or TIFF data embedded in the file. Raster images are also the main driver of file size, so re-encoding or downsampling them is where compression earns most of its savings.