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I recently started building a visual editor for Processing available through my GitHub repo. I thought I'd share it with you in case anyone has interest in this project (sadly I won't have much time to continue it and its still mainly "Work in Progress", thats also why I put it into this category)
Alright then, check out the gif showcase in the readme if you want to peek at the user interface and see how it works. To sum up (taken from the readme):
Components that will be rendered / animated follow certain cutomizable rules. Rules are attached to each other through a tree structure. As the user builds that tree structure (visually) he specifies each rule as a tree node. The tree edges describe the control flow starting at the root node. When rendering, each rule is executed sequentially just like the order in the tree implies. Rendering happens once per frame for each root node. The root node specifies the context of the rendered component. This includes available colors, variables and a cursor. Existing rules are "Draw Elipse" and "Update Variable". Those rules accept parameters like "width" for "Draw Ellipse" or "function" for "Update Variable". Basically, you can just type in the variables available through the root node and thus modify them each frame through your own custom function.
That allows you to manipulate shape, color, size and other properties of rendered objects during animation. One use case I started building this project for is e.g. generative art.