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Some PNG files will no longer auto-upsize to widths larger than the original, without causing the image to become skewed and blurry. Otherwise, PNG images still auto-thumbnail to smaller sizes and retain most of the clarity when smaller. All through 2008 and early 2009, JPEG images seem to thumbnail OK as well.
Unsharp masking applied to lower part of image. Unsharp masking (USM) is an image sharpening technique, first implemented in darkroom photography, but now commonly used in digital image processing software.
As mentioned at the beginning of this chapter, an image doesn't have to be a photo. It could be a map, an illustration, a graphic, or even an animated gif. The SVG format is preferred for drawings and line-art illustration. The PNG format is preferred for non-vector graphic iconic images, including screenshots.
The difference between a small and large Gaussian blur. In image processing, a Gaussian blur (also known as Gaussian smoothing) is the result of blurring an image by a Gaussian function (named after mathematician and scientist Carl Friedrich Gauss).
In general, if you have a good image that is in the wrong format, convert it to the correct format before uploading. However, if you find a map, flag, etc. in JPEG format, only convert it to PNG if this reduces the file size. For further advice on converting JPEG to PNG, see Wikipedia:How to reduce colors for saving a JPEG as PNG.
Watermarks present an issue when archival videos are used for a documentary that strives to create a coherent story. In some cases, watermarks are blurred or digitally removed if possible to clean up the picture. In the absence of visually perceptible watermarks content control can be ensured with visually imperceptible digital watermarks. [2]
For color images with three RGB values per pixel, the definition of PSNR is the same except that the MSE is the sum over all squared value differences (now for each color, i.e. three times as many differences as in a monochrome image) divided by image size and by three.