A novel algorithm to derive the isothetic convex envelope (ICE) of an object using a digital geometric technique is proposed in this paper. ICE is defined as the isothetic convex polygon that contains the digital object, with an aim of capturing its underlying shape information. Slackening the tightness of an ICE corresponding to a digital object is achievable by increasing the grid size, and for a slackened ICE with lesser output complexity (i.e., with lesser number of vertices), the runtime of the algorithm falls significantly. The proposed algorithm is marked by its dependence on object boundary instead of object size, and usage of primitive integer operations in the digital domain, which, in entirety, ensures its speedy execution and acceptability in a real-world application. Experimental results including CPU time demonstrate the elegance of ICE and the efficiency of the proposed algorithm.