With the growth in the computation capacity of sensor nodes, they are increasingly equipped to handle more complex functions. Moreover, the need to realize the complete loop of sense-control-actuate as the wired sensing facility, demands for more in-network processing. To this end, some primitives related to event data processing can be made available for event-driven applications. In this paper, we propose a middleware to provide efficient basic and higher-level event processing services through collaborative processing. As concrete service task instances, we propose an integrated algorithm that performs both event center localization and area estimation. To the best of our knowledge, this is the first such attempt. Using edge detection as a core middleware module, it is demonstrated that the middleware is able to support accurate estimation services for these event properties.