Krita

Krita is a free 2D painting and image editing program. It is more similar to Photoshop than to GIMP and features advanced brushes, layers, and blending modes.

Usage

Users familiar with Photoshop's Channels window may be confused by Krita's, which shows the output channels of the whole image and are not directly editable. The recommended document setup for Krita is to have a layer group containing all layers that comprise the texture, then giving that group a transparency mask layer to serve as the alpha channel:

  1. Select all layers which should be in the group (exclude things like UV templates).
  2. Press Ctrl + G to group them.
  3. Right click the group and select Add > Transparency Mask. You can rename this mask layer to alpha if you want its purpose to be more obvious.
  4. Right click this mask/alpha layer and select Split Alpha > Save Merged to export your .tif file. Don't forget to set compression to None since Tool does not support deflate compression.

Don't use File > Export to save TIFF files because this method does not preserve RGB data where the alpha channel is black (transparent). Use the above process instead.