Imaging's low level interface is set of functions operating on TImageData structure and TDynImageDataArray. These functions are implemented in Imaging.pas unit and you can find how to use them in How To ... section. You can also look at Demos section. Low level interface is accessible from programming languages other than Object Pascal. Information on this can be found in Using dll/so Access section.

Here is the table with all low level interface functions currently available.

You can find other functions in Reference but they are not available for other languages thus they are not part of the low level interface. Some of them are even dangerous to use if you do not know what you are doing.