Here is a list of all documented struct and union fields with links to the struct/union documentation for each field:

Average : bbox
BitAccum : bitstream
BitsPerCode : bitstream
Block : bitstream
BlockSize : bitstream
Code : tableentry
Data : image
File : bitstream
Hash : tableentry
Height : image
InputFile : programparams
JpegQuality : programparams
Max : bbox
Min : bbox
NumBits : bitstream
NumChannels : image
NumPixels : bbox
Opt : programparams
OutputFile : programparams
OutputFile2 : programparams
Phi : programparams
Volume : bbox
Width : image