Introduction to JPEG m Akram Ben Ahmed Outline Introduction Encoding Decoding Summary and Future work Research Paper Reading22010/7/12 Introduction JPEG (Joint Photographic Experts Group) is one of the most widely used lossy compression method. JPEG has many standards and can be encoded in many ways. 06/22/2011Research Progress Seminar3 Outline Introduction Encoding Decoding Summary and Future work Research Paper Reading42010/7/12 Encoding: Color space transformation The image should be converted first from RGB to YCrCb. Y represents the brightness of the picture while Cr and Cb represent the red and bleu chrominance respectively. This picture shows a color image and the Y, C b and C r elements of it. 06/22/2011Research Progress Seminar5 Encoding: Color space transformation The conversion is done by multiplying the pixels values of the RGB image by Y, Cr and Cb factors as shown below. 06/22/2011Research Progress Seminar6 Encoding: Downsampling In this step, the resolution of the Chroma components (Cr and Cb) is reduced. This reduction came from the fact that human eyes detect the brightness change more than the color differences. 06/22/2011Research Progress Seminar7 Encoding: Downsampling The ratios at which the downsampling is ordinarily done for JPEG images are 4:4:4, 4:2:2 or 4:2:0 (most commonly). 06/22/2011Research Progress Seminar8 Encoding The next process steps are done to each Y Cr Cb components separately. 06/22/2011Research Progress Seminar9 Encoding: Discrete cosine transform We divide first the image into 8x8 blocks. If one block cant be exactly represented in 8x8, the encoder must fill the remaining area of the incomplete blocks with some form of dummy data. 06/22/2011Research Progress Seminar10 Encoding: Discrete cosine transform Before computing the DCT of the 88 block, its values are shifted from a positive range (0-->255) to one centered around zero by subtracting the mid-point of the range (128 in our case) from the original block values. 06/22/2011Research Progress Seminar11 Encoding: Discrete cosine transform 06/22/2011Research Progress Seminar12 m= Original Block matrixg= Resulted shifted matrix Encoding: Discrete cosine transform We perform now the 2D DCT given by: u is the horizontal spatial frequency, for the integers 0