These constants define the different kind of filter brands available.
Values:
| IG_FREQIP_FILTER_BRAND_IDEAL | 0 | Ideal Filter. |
| IG_FREQIP_FILTER_BRAND_BUTTERWORTH | 1 | Butterworth Filter. |
| IG_FREQIP_FILTER_BRAND_GAUSSIAN | 2 | Gaussian Filter. |
| IG_FREQIP_FILTER_BRAND_CHEBYSHEV_I | 3 | Type 1 Chebyshev Filter. |
| IG_FREQIP_FILTER_BRAND_CHEBYSHEV_II | 4 | Type 2 Chebyshev Filter. |