There are three core ideas to recreate the portal VFX from Dr. Strange:
- Motion blur
- Rotating the particle emitter
- Asymmetry in the particle emission
The look of flying sparks can be achieved by using motion blur in combination with an emissive material for the particles. The particle emitter should be a circular mesh. It's also important how the particles are emitted, since in the movie they fly away from the circle in a tangential direction while seemingly unaffected by gravity. This can be achieved by spinning the emitter, setting the normal velocity to zero, setting the tangential velocity of the particles to a suitable value and deactivating gravity. A more subtle effect in the movie is that even when the portal is fully open the emission intensity and particle distribution wobbles slightly. This can be achieved by removing segments from the circular particle emitter. The amount of wobble depends on the speed of the emitter's rotation, lower speed results in more wobble.
You can download the project by clicking on the link below:
A quick breakdown of how a scene with the portal was filmed for the movie and the result with the added VFX can be seen in this video.