There is a notion called the "orthogonal convex hull," or
the "digital convex hull," which may be what you seek.
For example, in this paper,

> Karmakar, Nilanjana, and Arindam Biswas. "Construction of 3D Orthogonal Convex Hull of a Digital Object." In *International Workshop on Combinatorial Image Analysis*, pp. 125-142. Springer, Cham, 2015.
[Springer link](https://link.springer.com/chapter/10.1007/978-3-319-26145-4_10).

the authors define it this way:
"Orthogonal convex hull of a digital object in 3D domain is defined as the minimum volume orthogonal polyhedron enclosing the object such that its intersection with an axis-parallel face plane is either empty or a collection of projection-disjoint convex polygons."
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<img src="https://i.sstatic.net/wkwba.png" width="300" />
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Here's another image, specifically in 2D:
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<img src="https://i.sstatic.net/8uydh.png" width="300" />
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<sup>
Image from [P. Bhowmick's slides (PDF)](https://cse.iitkgp.ac.in/~pb/convex-hull-talk-nit-dgp-2014a.pdf).
</sup>
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>A. Biswas, P. Bhowmick, M. Sarkar, B. B. Bhattacharya, "A
linear-time combinatorial algorithm to find the orthogonal hull of
an object on the digital plane," *Information Sciences*, 216,
pp. 176–195, 2012.
[Elsevier link](https://www.sciencedirect.com/science/article/pii/S0020025512004100).


  [1]: https://i.sstatic.net/8uydh.png