The best open source alternative to Adobe Substance Designer is Material Maker. If that doesn't suit you, our users have ranked more than 10 alternatives to Adobe Substance Designer and five of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Adobe Substance Designer are TextureLab, Imogen, FilterJS and Symbinode.
Material Maker is a free and open-source, procedural texture authoring tool based on Godot Engine. Its user interface is based on Godot's GraphEditor: textures are described as interconnected texture generators and operators.
TextureLab is a tool designed to make procedural texture generation fast, simple and free. It is the first in a line of tools dedicated to make the process of making beautiful textures easy and artist-friendly.
FilterJS is a node-based procedural texture generator, meaning that you will compose your filter, generator, or algorithm to make a texture – you name it – out of special blocks. Each filter has two unique blocks: an input image and an output image.
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Definetely not as resource efficient as SD, and not a drop in replacement. But pretty much the only serious attempt to make an open source application similar to Designer Much better than trying to make something useful in Blenders Shader Nodes,