JPG's are often used for web sites and email as they are generally smaller file sizes as they are lossy meaning that some image quality is lost when the JPG is compressed and saved. The resultant 'lossy' file means that quality can never be recovered. The JPG format is often used in digital camera memory cards. The JPG file is a great format as it often manages to compress files to 1/10 of the size of the original file which is especially good for saving on bandwidth.
JPG is a popular graphic format, which is characterized by a high degree of compression, which leads to a decrease in image quality. It uses the technology of encoding of smooth color renditions, providing the ability to reduce the amount of data frequently during the recording of the image. Because of the small size it is requested by the owners of websites, allowing you to save the traffic effectively. It is also frequently used in memory cards of digital video cameras. The JPG algorithm is optimally suitable for compressing photographs and pictures, in which realistic scenes with little contrast of colors are present. It is not recommended to use this format for compression of drawings and various kinds of graphics, since a strong contrast between a number of closely located pixels provokes the appearance of visible artifacts.
JPG is a graphical file format for editing still images, it offers a symmetrical compression technique which is processor intensive and time consiming in both compression and decompression. JPEG is a joint standard of the International Telecommunications Union (ITU-T T.81) and the International Organization for Standardization (ISO 10918-1). JPEG involves a lossy compression mechanism using discrete cosine transform (DCT). Compression rates of 100:1 can be achieved, although the loss is noticeable at that level. Compression rates of 10:1 or 20:1 yield little degradation in image quality.
The procedure for compressing the digital images in JPG format is carried out in several stages. First, the photo is transformed into a color space YCbCr, then it is divided into squares to determine the upper range of the color spectrum. Finally, the colors and brightness are coded. JPEG uses a "lossy" compression system and discrete cosine transform technology. The format acts simultaneously as the standard of ISO and the International Telecommunication Union. The file compression ratio is in the range from 10:1 to 100:1. In this case, the reduction in image quality can vary from insignificant to substantial.
The JPEG Committee
PNG pronounced ping is a format for storing bitmapped (raster) images on computers. It was created originally to replace the GIF file format when it was announced that the companies who created the GIF format wanted royalties. PNG uses indexed colors and lossless compression (like a .GIF file), but without those copyright limitations; it cannot be animated like a GIF image.
PNG is a scan-line graphic format developed as an alternative to GIF, which had a commercial license. It is based on the best features of the predecessor, including lossless compression and transparent background support. The PNG technology ensures the preservation of all stages of editing and the restoration of the step with the preservation of quality. However, it does not allow to save several pictures in one file at once, which limits its use in the development of animated images.
PNG images are in many ways better than .GIF as they also include an 8-bit transparency channel, which allows the colors in the image to fade from opaque to transparent; GIF images only support fully opaque or fully transparent pixels. PNG supports palettes of 24-bit RGB colors, RGB images or greyscale or RGB images. PNG was intended to be able to transfer images on the Internet, not professional graphics, and so does not support other colour spaces (such as CMYK).
The PNG extension was proposed as a tool that helps to transfer and store images on the World Wide Web. The technology is limited to an 8-bit transparency channel, so you can adjust the colors from opaque to transparent. It supports 24-bit RGB colors, matching images and shades of gray. However, as a non-professional format, PNG can not display other color spaces. Most programs allow to open such files, they can also run in any browser. Images are different in small weight with good quality.
PNG Development Group
PNG Development Group (donated to W3C)