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Fast JPEG (Down-) Scaling for Mobile Applications Disclosure Number: IPCOM000016825D
Original Publication Date: 2003-Aug-25
Included in the Prior Art Database: 2003-Aug-25

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JPEG (Joint Photographic Experts Group) files usually have a higher resolution than the displays of mobile phone devices and therefore have to be scaled down before they can be used on mobiles. This invention shows a way to save processing time by operating directly on the DCT (Discrete Cosine Transform) domain of the compressed image, rather than first restoring the image to the pixel domain before processing it. The advantage of this approach is that the decoder of the mobile phone only has to decode a JPEG file of the size requested by the resolution of the mobile application, thus making the decoding process faster. This also enables mobile phone devices to process large JPEG files (e.g. 1024 x 1024 pixels with full color) for which they otherwise wouldn't have enough memory and/or processing power available. In the compression process, the image is first segmented into blocks of NxN-samples (JPEG standard is N = 8). Then the encoder transforms the blocks according to their frequency components in a uniform sampling process. Today, standard modes of decoding (JPEG Baseline, JEPEG Progressive) are based on the Fast 1-D DCT-II (Fast one dimensional DCT Type Two) algorithm, which is given by: