You'll find on these page two methods to make angle maps by automatically converting a drawing/image into an angle map. One is using two Photoshop actions, the other is a "Substance", which uses the free Substance Player application from Allegorithmic®, available here.
The first video shows you in detail how to create angle maps in Photoshop manually and how they work. You can however skip directly to Part 2 if you just want to see how to use the Photoshop actions available on this page which allow you to quickly create different angle maps.
Download the two Actions here: GrassAngleMaps.zip
This Substance was made by Jason Maranto, to quickly create angle maps with many options to tweak the maps. The substance is available here. To use it also download the free Substance Player application available here (assuming you are not using 3DS Max or Maya, which support Substances directly). The substance has a wide array of options that will allow you to do any of the following:
Auto creation of Swirl Maps or Radial Maps (your choice)
Input of custom greyscale maps for any of the RGB channels (to create your own angle patterns)
Auto clump generation (this can be randomized by changing the seed value) -- which is the real point of this Substance
Clump Threshold -- this is based off the green input channel (so you can have any custom shaped clumps you desire)
Maximum and Minimum Clump Size (to control the randomization a bit)
Number of Clumps
Minimum Height Angle (90 degrees being straight up and 0 degrees being lying flat on the ground)
Length map (derived from the Green channel)
Clamping (threshold position, softness and inverting) of each RGB channel
Preview of each RGB channel (to see exactly what is happening to each channel when clamping)
Clamping operations for the matching density map (invert was not included because you can do this easily in Maxwell)
Output resolutions up to 2048 x 2048 dynamically
The default settings will give this result on a 1m x 1m plane:
