Berthold K. P. Horn. Height and gradient from shading. International Journal of Computer Vision, 5(1):37-75, 1990. [doi]
@article{Horn90a, title = {Height and gradient from shading}, author = {Berthold K. P. Horn}, year = {1990}, doi = {10.1007/BF00056771}, url = {http://dx.doi.org/10.1007/BF00056771}, researchr = {https://researchr.org/publication/Horn90a}, cites = {0}, citedby = {0}, journal = {International Journal of Computer Vision}, volume = {5}, number = {1}, pages = {37-75}, }