triangle wave form to sine form through band pass filter
Any repeating wave form - including a triangle wave - can be represented as a sum of sine waves.  Look up "Fourier analysis" for details.  These sine waves consist of a "fundamental", the lowest frequency, plus a series of "harmonics" at higher frequencies.

Applying a bandpass filter to the waveform just selects the frequency you want, and discards all the others.  Pick the right filter and, in theory at least, you'd get a perfect sine wave out.