This paper studies the usage of the GPU as a collection of groups of related processing units, where each group communicates in some way to complete a computation efficiently and effectively. In particular, we use the GPU to perform jump flooding to pass information among groups of processing units in the design of two simple real-time soft shadow algorithms. These two algorithms are purely image-based in generating plausible soft shadows. Their computational costs depend mainly on the resolution of the shadow map or the screen. They run on an order of magnitude faster than existing comparable methods. Categories and Subject Descriptors (according to ACM CCS): I.3.1 [Computer Graphics]: Graphics processors; I.3.3 [Computer Graphics]: Bitmap and framebuffer operations; I.3.7 [Computer Graphics]: Color, shading, shadowing, and texture