code: public static void interpolate(RectF out, RectF start, RectF end, float factor) {
          out.left = interpolate(start.left, end.left, factor);
          out.top = interpolate(start.top, end.top, factor);
          out.right = interpolate(start.right, end.right, factor);
          out.bottom = interpolate(start.bottom, end.bottom, factor);
      }
docs: Interpolates from start rect to the end rect by given factor (from 0 to 1),
  storing result into out rect.
  
  @param out Interpolated rectangle (output)
  @param start Start rectangle
  @param end End rectangle
  @param factor Factor