In one embodiment, a method includes a system accessing an image, which may comprise covered and uncovered portions, and an overlay image comprising opaque pixels. The covered portion may be configured to be covered by the opaque pixels of the overlay image. The system may generate a data structure comprising data elements associated with pixels of the image. Each of the data elements associated with a covered pixel in the covered portion of the image may be configured to identify an uncovered pixel in the uncovered portion of the image that is closest to the covered pixel. Each covered pixel in the covered portion of the image may be modified by accessing the data element associated with the covered pixel, determining a distance between the covered pixel and an associated closest uncovered pixel using the accessed data element, and modifying a color of the covered pixel based on the distance.