code: func (td *TrianglesData) SetLen(len int) {
  	if len > td.Len() {
  		needAppend := len - td.Len()
  		for i := 0; i < needAppend; i++ {
  			*td = append(*td, zeroValueTriangleData)
  		}
  	}
  	if len < td.Len() {
  		*td = (*td)[:len]
  	}
  }
docs: // SetLen resizes TrianglesData to len, while keeping the original content.
  //
  // If len is greater than TrianglesData's current length, the new data is filled with default
  // values ((0, 0), white, (0, 0), 0).