The best alternatives to BigArt are Think with Google, Tinkersynth, and Public Art. If these 3 options don't work for you, we've listed a few more alternatives below.
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Tinkersynth is an experimental art-creation tool. By tweaking whimsical, inscrutable controls, you create one-of-a-kind generative art. Experimentation is encouraged! When you come up with a design you like, you can purchase it as a fine-art print.