![]() ![]() ![]() Whilst effective, the use of a DCT has led to some significant problems in the quality of the final images. The DCT is "fitted" to the underlying image data - the accuracy of the fit determines the degree of compression, with a closer fit requiring more storage. The algorithm underpinning the file format uses a discrete cosine transformation (DCT) over 8x8 windows of pixels (or kernels). However, the benefit is smaller files, with compression ratios of 10:1 common, along with faster write speeds. The primary setting is "quality" which aggressively increases the amount of compression applied at the expense of efficacy. Regardless of the input data, the RGB layers (or the single grayscale) are reduced to 8-bit data before smoothing to the point that it looks the same - or almost the same - as the original image. More specifically, it had to look appealing to the human eye. That was the key to unlocking the easy storage and transfer of digital images. Philosophically, the JPEG committee didn't mind if they butchered the original image, as long as it looked good. For simplicity, it was known as ISO/IEC JTC 1/SC 29/WG 1! Given the importance of the problem the two main standards organizations (ISO and IEC) came together and released the JPEG format for digital still images in 1992. JPEG is an acronym for the Joint Photographic Experts Group, which was formed as a sub-group of the ISO/IEC Joint Technical Committee 1, Subcommittee 29, Working Group 1. An alternative was needed that worked with photos over low bandwidths. ![]() Of course, these compression systems didn't work particularly well for continuous-tone images, such as photos. Even within this context, there was a competing trade-off between resolution, file size, and loading speed. I distinctly remember waiting tens of seconds for individual grayscale photos to load on my PC running the MS-DOS operating system. Methods to compress images were rapidly developed, such as Run Length Encoding (RLE), which identified contiguous pixels that were the same value and just stored this information. This was vastly inefficient for data storage and transfer but was the cornerstone for simple algorithms. That could be a hard disk drive or floppy disk, over dial-up networks, or perhaps the more favored sneaker-net! Up until that point, images had been stored as raw bitmaps (BMP) which contained each individual RGB or grayscale pixel values. Low bandwidth applied to both the storage medium, as well as the transfer channel. The need for JPEG format was a very real one - transfer images across extremely low bandwidth networks for display. ![]()
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