Paint in 3D is an asset I developed that delivers realtime painting to Unity, both in editor, and in game.
This asset performs all painting operations on the GPU, which required me to experiment with and develop custom shaders to achieve maximum performance.
This asset can paint between UV seams without producing texture seams, which required me to develop new painting techniques (even professional painting software usually can't do this).
This asset generates no garbage, which required me to carefully manage memory allocation and implement pooling.
This asset is very well optimized, which required me to batch paint operations to minimize state changes, as well as implement spatial partitioning code.
This asset works across many versions of Unity, and most platforms (e.g. VR, mobile), which required me to carefully write the code to take any differences into account, and to test it on many devices.