docs: public static float[] subbandAbsVal(GrayF32 subband, float[] coef ) {
  		if( coef == null ) {
  			coef = new float[subband.width*subband.height];
  		}
  
  		int i = 0;
  		for( int y = 0; y < subband.height; y++ ) {
  			int index = subband.startIndex + subband.stride*y;
  			int end = index + subband.width;
  
  			for( ;index < end; index++  ) {
  				coef[i++] = Math.abs(subband.data[index]);
  			}
  		}
  		return coef;
  	}