As the drum rotates, each line reaches the toner, which contains electrostatically sensitive black powder. This powder is attracted to the dots that didn't lose their charge. Then the drum is pressed against the paper, transferring the black powder to the paper. The paper passes through heated rollers to fuse the powder to the paper. The drum is recharged and coated for the next page.
Vendors sell print engines, which are complete assemblies.
Laser printer manufacturers use these print engines and add their own software, a CPU, and memory to hold bit maps and fonts. Halftoning is used to print gray values, since a printer pixel can only be black or white. This technique involves breaking up the image into cells, usually 6x6 pixels. For each zone, a number of black pixels equal to the zone number is printed in the cell. This creates the illusion of gray.