部分内容来自：https://en.wikipedia.org/wiki/Convolution
上图的英文描述（来自wiki）：
- Express each function in terms of a dummy variable
- Reflect one of the functions:
→
- Add a time-offset, t, which allows
to slide along the
-axis.
- Start t at −∞ and slide it all the way to +∞. Wherever the two functions intersect, find the integral of their product. In other words, compute a sliding, weighted-sum of function
where the weighting function is
The resulting waveform (not shown here) is the convolution of functions f and g.
If f (t) is a unit impulse, the result of this process is simply g(t). Formally: