JPEG file quantization table definition

Question

When I use Photoshop Save As function, and pick jpeg file format I get following window:

As you can see, I select Baseline ("Standard") format, and maximum picture quality. When I open this picture in Hex editor, I see several FF DB markers (which is start of Quantization table/s). No problem yet, but lets look next picture:

As you can see form picture above, at address row BDA starts FFDB marker. First two bytes are 00 84 which means that this marker holds 132 bytes of data. Doing some math, we can conclude that two Quantization tables are defined by this marker. Values of first table are: 0C 08 08 08 09 etc...

In the same file, there is another FFDB marker, starting from 2885 address row, as you can see in picture:

Again, value of first two bytes are 00 84 which means 132 bytes of data. But this time, first Quantization table values are: 01 01 01 etc...

How to know which of FF DB markers should I use, and why there is several FFDB markers in file?

Answer 1

Without seeing the entire file, it's hard to say with certainty, but it looks like your first quantization table is for an embedded thumbnail which is compressed with a lower quality. The second quantization table is for the main image and has values of 01,01,01,... because you chose the highest quality and therefore the coefficient values are quantized the least possible amount.