Thanks!

The requirements are to fill the area whose amplitudes are bigger than a certain value, for instance, in your sine curve example, only fill the area whose amplitudes are bigger than 0.7, not to fill the entire polygon...

I didn't specify render hint, so the default hint is used which I think should be a faster one...

The data set has dimension of 256 x 370, the plot widget has a size of 800 x 800...

There is no multi-thread involved because the software I am comparing to doesn't use multi-thread...