The frei0r plugin description says, “Unsharp masking (port from Mplayer).”
The mplayer man page says it does gaussian blur or a sharpen using an unsharp mask routine.
The underlying plugin code range is [-1.5, 3.5] where 0 is the switching point between blur and sharpen. Thus, 50% in Shotcut converts to 1.0 in the plugin code, and the switch point where there is no change is 30% in Shotcut.
The Size parameter controls the amount of blur just like nearly all blur filters have as a parameter. Sharpening still creates a blur, but it uses this as a mask to sort of subtract from the original. This also explains why it can also be used to blur and why many tools call this function “unsharp” or “unsharp mask.” However, I do not like to call it that in Shotcut because it is confusing and deceptive for non-advanced users who are generally looking to increase sharpness (while also not limiting it to that).
I know the unshap masking process from adobe photoshop and used it a lot. The size normally means the area around the pixel that is beeing effected and the amount means the “force” that is applied in sharpening. In general you should use these filters with care and subtle as it is easy to spoil an image with too much sharpening. There is always a negative effect to shapening if you do too much. In Shotcut i normally tend to use the sharpen filter below 50% (in amount and size). If you leave it at 50% you already get a lot of shapening effect and you can already see artefacts (like halos and overcrisp areas). Generally i would suggest using values between 40-50% for subtle sharpening. I didn’t experiment much with the size amount, maybe its worth trying that out. It all depends on your footage content but i would never go to values up to 70% unless you are really after such effects.
@Leader: your info about the 30% meaning 0 change is good to know! You normally would expect the slider in the middle position of 50% doing nothing but in this case 50% is already crisp sharpening - in most cases already too much for my taste.
