JPEG Compression
Overview
JPEG compression is a lossy compression format for images. Professor Delp claims he was in a tight race against this format :P
Step 1:
Divide the image into non-overlapping 8 by 8 blocks (8x8 pixels). If the image width or size does not divide evenly into 8, the image may be cropped or pixels may be added to make the image divisible by 8. Each 8 by 8 block will contain 64 values. For a grayscale image, each pixel may be anywhere from 0 to 255, where 0 is pure black and 255 is pure white.
Step 2:
The Discrete Cosine Transform (DCT) is applied to each 8 by 8 block. The DCT is chosen over transforms like the FFT because the frequency coefficients are represented by only real numbers. The FFT would most likely require two 8 by 8 matrices, one to store the real part of the frequency coefficient, another to store the imaginary part. The result of the DCT is also an 8 by 8 block, where the top left corner represents the DC or average pixel value for that specific 8 by 8 block. The bottom left corner represents the fastest vertical change. The top right corner represents the fastest horizontal change. The values in between are a mix of the two. The image is comprised of a weighted sum of the 64 possible AC/DC configurations.
Step 3:
