https://doi.org/10.1007/3-540-54414-3_49 Wiebren de Jonge Peter Scheuermann Ardie Schijf Encoding and Manipulating Pictorial Data with S<sup>+</sup>-trees. 401-419 SSD/1991/P401.pdf 1991 conf/ssd/91 SSD db/conf/ssd/ssd91.html#JongeSS91