A method includes receiving a 3D image of a portion of an organ including a cavity, the image comprising voxels having respective values. A wall of the cavity is identified in the image, by (i) positioning virtual solid objects inside respective sub-volumes of the cavity, (ii) moving the virtual solid objects inside the cavity according to a predefined rule of motion, (iii) while the virtual solid objects move inside the cavity, adapting the traversed voxels to predefined value indicative of the interior of the cavity, (iv) responsively to detecting that a virtual solid object comes into contact with the wall, rolling the virtual solid object over the wall, and adapting a surface of the virtual solid object with the values of the voxels over which the surface rolls, and (v) converting adapted voxel values that are lower than a threshold voxel value into the predefined value indicative of the interior.