Can learning of mental imagery skills for visualizing shapes be accelerated with feature masking? Chemistry, physics fine arts, military tactics, and laparoscopic surgery often depend on mentally visualizing shapes in their absence. Does working with "spatial feature-masks" (skeletal shapes, missing key identifying portions) encourage people to use visualization strategies? This experimental study tested that hypothesis using an online computer game involving rotating and stamping a 3D cube on a 2D pattern. According to a chi-squared test, people who trained with 3D feature-masks reported using significantly more visual imagery strategies on a related visualization posttest. Spatial feature-masks provide a new building block for instructional designers to address educational outcomes involving visual imagery of shapes. (Contains 2 figures.)