Image attributes:

img.date - creation date & time (text)

img.expprog - exposure program

img.et - exposure time

img.expbias - exposure bias

img.ext - file extension name

img.fl - focal length

img.fl35 - focal length (35 mm equivalent)

img.flash - flash information

img.fn - F/Stop (F Number, aperture)

img.fovc - field of view crop

img.h - image height

img.iso - sensitivity

img.lightsrc - light source

img.make - camera make

img.metmode - metering mode

img.model - camera model

img.name - file name

img.path - complete path to the image

img.portrait - true if image is in portrait orientation, false if it's in landscape

img.rating - number of stars

img.size - file size in bytes

img.srgb - true if sRGB color space, false otherwise

img.subdist - subject distance

img.tags - dictionary of image tags

img.time - creation time stamp (number)

img.w - image width

String functions: string.lower, string.upper, string.len, string.sub(str, from, to), string.find(str, what)

Mathematical functions: math.abs math.acos math.asin math.atan math.atan2 math.ceil math.cos math.cosh math.deg math.exp math.floor math.fmod math.frexp math.huge math.ldexp math.log math.log10 math.max math.min math.modf math.pi math.pow math.rad math.random math.randomseed math.sin math.sinh math.sqrt math.tan math.tanh math.mod

For a complete list of functions and documentation check

www.lua.org