The value of allowed undershoot in the resize mode ExactOrSmaller is currently a compile time constant. It would be great to make this configurable for the developer using the library:

https://github.com/facebookincubator/spectrum/blob/fa9e917f5b062797d51c3e013aaa7d61926c72a5/cpp/spectrum/core/ResizeUtil.h#L22-L27

At the moment, while an encoding is in progress, the encoding settings can be changed. Although I assume changing these will have no effect in the encoding in progress, it is rather odd and distracting that you can change them.

I suggest to grey these out while an encoding is in progress so they cannot be changed. I think greying is better than hiding, so you can see what settings were picked eve