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JPEG is good for images where the color changes fluidly throughout the image, like in a photograph, whereas PNG files are good for images with relatively few colors, such as a drawing of a flag, a chart, or a map; note that sometimes SVG is better. Just automatically converting to PNG will almost never produce a good image.
JPEG for photographic images. GIF for animated images. PNG for everything else. While some formats offer multiple compression systems, in general the format and the compression system are tied together. Other image formats should be avoided in most cases: BMP - Images are uncompressed, resulting in larger file sizes. Should usually be converted ...
When converting an image from the PNG format to GIF, the image quality may suffer due to posterization if the PNG image has more than 256 colors. GIF intrinsically supports animated images. PNG supports animation only via unofficial extensions (see the section on animation, above). PNG images are less widely supported by older browsers.
Like any resampling operation, changing image size and bit depth are lossy in all cases of downsampling, such as 30-bit to 24-bit or 24-bit to 8-bit palette-based images. While increasing bit depth is usually lossless, increasing image size can introduce aliasing or other undesired artifacts.
Filesize reduction factor 100 or so when converting to png is typical if the image is a line drawing; if the image is a photo, it is best converted to jpeg, which yields a greater filesize reduction. The PPM format is generally an intermediate format used for image work before converting to a more efficient format, for example the PNG format ...
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.
The conversion makes it possible to change a pixel to have a value of, for instance, FF80 C340 56A0, but the conversion itself does not do that, only further manipulation of the image can. Converting an image or audio file in a lossy format (like JPEG or Vorbis) to a lossless (like PNG or FLAC) or uncompressed (like BMP or WAV) format only ...
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