Tips and Techniques/Natural Phenomena/Snow
From VFXPedia
Creating a realistic snowfall in Fusion is more tricky than it may seem at a glance. While the whole immense compositing power is at your fingertips to work with the sprites, the snow dynamics is a real challenge.
Compared to raindrops, snowflakes are much slower and more contrasty to the background, they live longer in the frame and the spectator has a much better chance to notice flaws in their behaviour.
- The main forces which affect snow motion are wind, gravity and turbulence. For the first two you can use pDirectionalForce, but with turbulence it's more complicated.
- The problem of the standard pTurbulence tool is that you can't adjust the temporal frequency and the resulting trajectories are jerky. This is especially noticeable if you are creating a slow snow.
- The only way to solve this is to apply pCustomForce which uses three FastNoise textures as a XYZ-velocity map (see the pCustomTurbulence setup).
- Another important feature of snow is that every snowflake is unique and the different shapes react differently to the applied forces. So, snowflakes don't move smoothly in a common flow, each of them has its own, secondary dynamics.
- Such secondary fluctuations can be simulated with an additional pCustomForce. The noise function in the expressions should be based on particle id.
- As snowflakes are pretty small objects, you can simulate the depth of field effect using the pQuickDOF macro. In this case the sprite image should resemble a bokeh shape rather than a snowflake.
- If the DOF is deep and the focal plane is far away, you can create an additional emission only for the defocused particles close to the camera.
- Similarly, if you deal with a macro shot (shallow DOF), you can use emission with sharp snowflakes as sprites near the focal plane and separate additional emissions with the pQuickDOF and bokeh-shaped particles for the defocused areas.
