The Fourmilab Shadow Server allows you to add simulated shadows to graphical images; the title image at the top of the page was created using the Shadow Server. Users of Unix machines with Perl and the PBMplus image processing tools installed can generate shadowed images on their own computers by downloading our pnmshadow utility; the documentation for that program includes a step-by-step description of how the shadows are generated.
Regrettably, many Web users do not have access to a computer configured properly to run pnmshadow. The Shadow Server allows anybody with a Web browser which allows file upload to submit an image in a variety of file formats and receive, moments later, the image with a shadow added, ready to save into a file on their machine or download as a ZIP-compressed archive.
The Shadow Server operates on images you upload to the server from your computer by entering the file name (or selecting the file from a Browse dialogue), then submitting a request to transmit the file to the server. HTML file upload does not pose an indiscriminate risk to the security of files on your computer—the only file uploaded is the one you explicitly choose to transmit; the Web page cannot transmit a file unless you first enter its name.
Notwithstanding this, you should be aware that sending any file over the Internet poses security risks. The file you submit to the Shadow Server and the image it returns are transmitted “in the clear” and can potentially be intercepted by any intermediate site or by eavesdroppers on the communications links interconnecting them. Further, you have no guarantee that the site to which you send the image won't make a copy and indiscriminately redistribute it. We don't do that, but you have nothing more than our word and our reputation on the Web to assure you that images you send to the Shadow Server won't be indiscriminately redistributed without your permission. So, as with anything on the Web, temper your enthusiasm with a dose of prudence and don't upload any image you'd regret were it intercepted by a third party.
(In order to verify the operation of the Shadow Server, we randomly sample input and output images and examine them in order to discover bugs in the shadow generation process and to improve the request user interface. These sampled input/output pairs are deleted immediately after being examined and are not disclosed to any third party. But please note that you have only our word to confirm this policy—unless you've independently verified our integrity, you should assume the worst and send no sensitive images across the Internet to our site or any other.)