Why JPEG compression processes image by 8x8 blocks instead of applying Discrete Cosine Transform to the whole image?
8 X 8 was chosen after numerous experiments with other sizes.
The conclusions of experiments are: 1. Any matrices of sizes greater than 8 X 8 are harder to do mathematical operations (like transforms etc..) or not supported by hardware or take longer time. 2. Any matrices of sizes less than 8 X 8 dont have enough information to continue along with the pipeline. It results in bad quality of the compressed image.
Read, my blog, http://nboddula.blogspot.com/2013/05/image-compression-how-jpeg-works.html
Because, that would take "forever" to decode. I don't remember fully now, but I think you need at least as many coefficients as there are pixels in the block. If you code the whole image as a single block I think you need to, for every pixel, iterate through all the DCT coefficients.
I'm not very good at big O calculations but I guess the complexity would be O("forever"). ;-)
For modern video codecs I think they've started using 16x16 blocks instead.
来源：https://stackoverflow.com/questions/10780425/why-jpeg-compression-processes-image-by-8x8-blocks