After you have set the options and the rasterbation begins, you may end up in a queue. Rasterbation uses a lot of memory, hence this conserves server resources and actually reduces the average time users have to wait. When your turn begins, the server examines the original image (preview image is not used here to ensure maximum quality). The image is divided into squares. Each square maps to a dot in the output. The average brightness and color values of the origin square determine the size (and color if desired) of the output dot - bright area produces a small dot. Vector representations of the dots are written into a pdf document. When the entire image is processed, your browser is sent to a different page, which will instruct it to download the pdf file.