I have created custom workflow, which is generating additional renditions for assets after they are created/modified. Problem is that (after workflow finish) value of property dam:assetState remains "processing". Workflow is not transient. Assets are fully modifiable and I can use them normally.
I guess that probably I've forget about adding some step in workflow model, or whatever.
To create renditions I'm using com.day.image.Layer and its methods: resize() and crop(). After that, using InputStream from the layer I'm adding new rendition using com.day.cq.dam.api.Asset.addRenditionMethod().