ðɪs ɪz kˌɑːmpjuːtˈeɪʃənəli ɛkspˈɛnsɪv, bɪkˈʌz ðɪ ɛntˈaɪɚ ɹˈɛndɚɹɪŋ pɹˈɑːsɛs mˈʌst biː ɹᵻpˈiːɾᵻd fɔːɹ ˈiːtʃ sˈæmpəl loʊkˈeɪʃən.
This is computationally expensive, because the entire rendering process must be repeated for each sample location.