Would like to apply a Portra 400 or lomograph color grading filter to a digital photograph. Do these exist in the Darktable software for use in batch lighttable editing? If not, what is a similar solution
Transform RGB values with a 3D LUT file.
A 3D LUT is a tridimensional table that is used to transform a given RGB value into another RGB value. It is normally used for film simulation and color grading.
LUTs are most commonly used in darktable for color grading or film look simulation. For this reason, by default, the module is placed after the filmic rgb module in the pixelpipe and should be applied to a neutral image (without first applying a specific look). While you can find hundreds of free LUTs on the internet, you should note that that not all of them are compatible with the darktable environment and workflow – incompatible LUTs will not produce the advertised look. To limit the risk, a color grading LUT should have been created to work with one of the available “application color spaces” (see below), for both the input and the output of the module.
Darktable has a built in Velvia module.
The external site dtstyle.net has many prebuilt styles including film simulation. It has been around for a while.
Google search suggests there are additional third party libraries containing film styles such as t3mujinpack.
However, film simulation in digital files is more art than science. Film has non linear responses to light, responds to UV, and involves chemical reactions that vary with temperature and technique and human interventions during processing, scanning, and printing.
Which is to say that simulating film is only as good as a photographer thinks it is. Unless the data in the picture file is appropriate, film styles won't give desirable results.