Detail this asset from Unity Asset Store: https://assetstore.unity.com/packages/templates/packs/paint-craft-drawing-coloring-book-engine-63046
But now you can download Paint Craft (Drawing & Coloring book engine) FREE.
Paint Craft (Drawing & Coloring book engine) v2.2
Artist friendly multiplatform drawing app constructor. Perfectly works for coloring books and simple drawing apps. Support pattern brush, mask region filling, stamp/stickers and more.
Works the same fast on iOS, Android, WP8, PC
Get Pro version here. It has SVG support and drawing on 3d scene.
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