JPG compression and noise

Question

I am studying jpeg compression and it seems to work by reducing high frequency components in images. Since noise is usually high frequency, does this imply that jpeg compression somewhat works on reducing noise in images?

Answer 1

Strictly speaking, no.

JPEG does remove high frequencies (see below), but not selectively enough to be a denoising algorithm. In other words, it will remove high frequencies if they are noise, but also if they are useful detail information.

To understand this, it helps to know the basics of how JPEG works. First, the image is divided in 8x8 blocks. Then the discrete cosine transform (DCT) is applied. As a result, each element of the 8x8 block contains the "weight" of a different frequency. Then the elements are quantized in a fixed way depending on the quality level selected a priori. This quantization means gaining coding performance at the cost of losing precision. The amount of precision lost is fixed a priori, and (as I said above) it does not differenciate between noise and useful detail.

You can test this yourself by saving the same image with different qualities (which technically control the amount of quantization applied to each block) and see that not only noise is removed. There is a nice video showing this effect for different quality levels here: https://upload.wikimedia.org/wikipedia/commons/f/f3/Continuously_varied_JPEG_compression_for_an_abdominal_CT_scan_-_1471-2342-12-24-S1.ogv.

Answer 2

JPEG compression can reduce noise by smoothing out the high-frequency components of the image, but it also introduces visual noise in the form of compression artifacts. Here is a zoomed-in (3x) view of part of my avatar (a high-quality JPEG) and part of your avatar (a PNG drawing), on the left as downloaded and on the right as compressed with ImageMagick using -quality 60. To my eye they both look "noisier" when JPEG-compressed.