ImageMagick sampling factor

Question

I have been using ImageMagick command line tool for jpeg compression of the images. However, I'm trying to understand the role of sampling factor. It says the sampling factor is used to specify the block size i.e. 8x8 or 16x16. However, I can't find the default values used if I omit this parameter.

Also, I tried fetching properties of an image by using the following command:

identify -verbose image.jpg

It shows sampling factor values as 1x1, 1x1, 1x1. What does it mean? Either it should be 8x8 or 16x16 or 32x32....

Could anyone please give me an idea where to find these details?

Answer 1

From ImageMagick's command line options:

-sampling-factor

This option specifies the sampling factors to be used by the JPEG encoder for chroma downsampling. If this option is omitted, the JPEG library will use its own default values.

Here is a good thread.. on Knowing which chroma-subsampling was used in a JPEG

Chroma Subsampling in JPG Compression

I hope this helps :)