This image is arranged in a rectangular grid of pixels
whose dimensions are 250 by 375 giving a total of 93,750 pixels. The
color of each pixel is determined by specifying how much of the colors
red, green and blue should be mixed together. Each color component is
represented as an integer between 0 and 255 and so requires one byte
of computer storage. Therefore, each pixel requires three bytes of
storage implying that the entire image should require 93,750 3 = 281,250 bytes. However, the JPEG image
shown here is only 32,414 bytes. In other words, the image has been
compressed by a factor of roughly nine.
JPEG first breaks the image into eight by eight blocks
block is processed independently of the others, so we'll concentrate
on a single block. In particular, we'll focus on the block
Here is the same block blown up so that the individual pixels are more
apparent. Notice that there is not tremendous
variation over the 8 by 8 block (though other blocks may have more).
To find out what happens next, go back and read in your lab report.
Here is our eight by eight block again:
along with the three different blocks corresponding to
Lighter shades of gray correspond to larger values of Y, Cb, and Cr. Notice how the Y values form a grayscale version of the image.
Now go back and read in your lab report.