Designing virtual game worlds is often a long and labor-intensive process. Moreover, when a game world needs to be slightly altered in appearance, the entire process needs to be repeated, or will at least require some repetitious tasks. Ideally, when the same game world is needed under different circumstances (e.g. in another season, before and after a war, in prosperous or poor economic conditions), the designer should be aided in this process using procedural generation techniques.We propose an approach for the specification of procedural filters that describe how (parts of) virtual worlds should be customized to fit a particular situation based on their semantics and the conditions of the situation. This description will guide the customization process by triggering and parametrizing, among others, procedural instructions that can change the appearance of the virtual world. We will discuss how the generic nature of this approach, which favors reusability, and its integration with semantics, which increases the intuitiveness of the design process, can eliminate many of the repetitious tasks involved in performing these actions manually.We describe an implementation of this approach that shows how some simple procedural filters can i) age an urban environment and simulate the effects of poor living conditions on the look of that environment, and ii) apply a party atmosphere to an ordinary office scene.