ModLab is free software that generates and combine Normal maps from diffuse and height maps. The latest 1.4 releases adds the ability to generate normal maps based on sprite's alpha (thus its volume) and once again modify said volume through curve editor.
From the ModLab description:
Modlab is not an image editor. ModLab is a free tool that can generate normal maps in a realtime deferred rendering environment. It was initially created as an asset previewing and mastering tool for the game Wayward Terran Frontier which means it stands out among other content creation utility programs by featuring a full hardware rendering pipeline for previewing results in realtime.
Modlab can generate high quality normal maps without 3d geometry.
ModLab is available for free on Steam:
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