Dust and Scratches
Dust and scratches on the film can be a big problem for scanning. Because of their reduced size (compared to prints) the scanners are capable of resolutions much higher than a regular flatbed scanner; typically at least 2000 samples per inch (spi), up to 4000 spi or more. At these resolutions dust and scratches take on gigantic proportions. Even small specks of dust, invisible to the naked eye, can obscure a cluster of several pixels. For this reason, techniques have been developed to remove their appearance from a scan, see film restoration.
The simplest is the median filter, often called despeckle in many graphic manipulation programs, e.g. in Adobe Photoshop and the GIMP. It works by examining a pixel in relation to the pixels surrounding it; if it is too different from the surrounding pixels then it is replaced with one set to their median value. This and other methods can be quite effective, but have the disadvantage that the filter cannot know what actually is dust or noise. It will also degrade fine detail in the scan.
Read more about this topic: Film Scanner
Famous quotes containing the words dust and/or scratches:
“‘All that was, seemed as if it had been not;
And all the gazer’s mind was strewn beneath
Her feet like embers; and she, thought by thought,
‘Trampled its sparks into the dust of death;”
—Percy Bysshe Shelley (1792–1822)
“Farmers in overalls and wide-brimmed straw hats lounge about the store on hot summer days, when the most common sound is the thump-thump-thump of a hound’s leg on the floor as he scratches contentedly. Oldtime hunters say that fleas are a hound’s salvation: his constant twisting and clawing in pursuit of the tormentors keeps his joints supple.”
—Administration in the State of Arka, U.S. public relief program (1935-1943)