The existing Add and Multiply blending modes have limited usability,
because the appropriate operation for meaningfully combining values
depends on the channel. This adds a new mode that chooses the operation
automatically based on property settings:
- Axis+Angle channels are summed, effectively averaging the
axis, but adding up the angle. Default is forced to 0.
- Quaternion channels use quaternion multiplication:
result = prev * value ^ influence
- Scale-like multiplicative channels use multiplication:
result = prev * (value / default) ^ influence
- Other channels use addition:
result = prev + (value - default) * influence
Inclusion of default in the computation ensures that combining
keyframed default values of properties keeps the default state,
even if the default isn't 0 or 1.
Strips with this mode can be keyframed normally in Tweak mode,
except that for quaternion rotation keyframing always inserts
all 4 channels, and the channel value sliders on the left side
of Graph/Action editors won't insert keys without Auto Key.
Quaternion keys are also automatically normalized.
Differential Revision: https://developer.blender.org/D4190
de1d3e5f5f