Today's challenge began when GeraldK asked for help getting a featured picture about the Winter Olympics. He's active with the Olympic Games project and is hoping to have something run on Wikipedia's main page in time for the 2010 Olympic Games. The Bundesarchiv material he suggested was far too small for consideration: ranging from 21 KB to 51 KB. Normally 10 MB would be the minimum for my work.
So Staxringold found a 32 MB poster from the Works Progress Administration about the bobsled run at Lake Placid, New York. Lake Placid hosted the Winter Olympics in 1932 and 1938; its bobsled run was converted to public use after the 1932 Olympics.
So we've got a technically adequate file that fits the topic. It comes with a problem: the dimensions aren't rectangular like they ought to be. This is how it looks after 0.7 degree counterclockwise rotation.
This demonstration crop leaves a little background to help illustrate the problem. If you look carefully enough, the caption itself is slanted in conjunction with the distortion. There's an elegant solution to this in the form of a software option which mainly serves the different purpose of perspective control.
When perspective distortion is a problem it usually appears in architectural photography. Back in the days of film photography people corrected for it with a view camera or a perspective control lens. In the digital era this usually gets corrected with software. The same software controls can help the Lake Placid poster too.
In Photoshop this is called perspective cropping. Perspective cropping allows the editor to drag corners into non-rectangular shapes. Then the software performs calculations to generate a rectangular result. To access the perspective crop, select the regular cropping tool and drag a crop selection. A toolbar option will appear above with a check box for "perspective". Check that box and the perspective adjustment becomes possible. In our situation that means aligning cropping tool with the corners of the poster. Just place one corner of crop selection directly onto each corner of poster, then crop. GIMP software users can get a similar effect from the Perspective Tool. Below is the result after additional edits.
Much more satisfactory and probably fairly close to how the poster looked when it was new. If the distortion had been more severe we might have supplemented the perspective crop by separately rotating the poster caption. But this was a fairly mild instance and the perspective crop did the job.
What causes this distortion, you may wonder? Many of the items from the Library of Congress poster collection were photographed for preservation purposes during the film era. These posters were already old and fragile at the time of the photography and they may not have laid perfectly flat. A slight rise or buckling resulted in the distortion. So if the digital file was made from the slide film rather than from the original, the same problems get translated into digital format.