An image-change detector comprising: a photodetector array of pixels; a phase grating to receive incident light, wherein curved boundaries of odd symmetry extend radially from a center of the phase grating to a periphery, the curved boundaries of odd symmetry radiate out between elevated arms and recessed arms near the center, the phase grating defines a point-spread response illuminating a pattern over R of the pixels and defining a convex hull enveloping an area of S>2R pixels of the photodetector array, the convex hull being the smallest convex set of pixels that includes all illuminated pixels of the point-spread response captured by the photodetector array of pixels; and logic coupled to the photodetector array to successively poll, the pixels to produce sets of image data, and to compare the sets to identify changes in the image data; the logic to poll a first number of the pixels in the convex hull in a first mode, which is a low-power mode, and the logic to poll a second number of the pixels in the convex hull higher than the first number in a second mode, which is a high performance mode, wherein the logic is configured to poll the pixels in the low-power mode less frequently with respect to the high-performance mode. a photodetector array of pixels; a phase grating to receive incident light, wherein curved boundaries of odd symmetry extend radially from a center of the phase grating to a periphery, the curved boundaries of odd symmetry radiate out between elevated arms and recessed arms near the center, the phase grating defines a point-spread response illuminating a pattern over R of the pixels and defining a convex hull enveloping an area of S>2R pixels of the photodetector array, the convex hull being the smallest convex set of pixels that includes all illuminated pixels of the point-spread response captured by the photodetector array of pixels; and logic coupled to the photodetector array to successively poll, the pixels to produce sets of image data, and to compare the sets to identify changes in the image data; the logic to poll a first number of the pixels in the convex hull in a first mode, which is a low-power mode, and the logic to poll a second number of the pixels in the convex hull higher than the first number in a second mode, which is a high performance mode, wherein the logic is configured to poll the pixels in the low-power mode less frequently with respect to the high-performance mode. curved boundaries of odd symmetry extend radially from a center of the phase grating to a periphery, the curved boundaries of odd symmetry radiate out between elevated arms and recessed arms near the center, the phase grating defines a point-spread response illuminating a pattern over R of the pixels and defining a convex hull enveloping an area of S>2R pixels of the photodetector array, the convex hull being the smallest convex set of pixels that includes all illuminated pixels of the point-spread response captured by the photodetector array of pixels; and logic coupled to the photodetector array to successively poll, the pixels to produce sets of image data, and to compare the sets to identify changes in the image data; the logic to poll a first number of the pixels in the convex hull in a first mode, which is a low-power mode, and the logic to poll a second number of the pixels in the convex hull higher than the first number in a second mode, which is a high performance mode, wherein the logic is configured to poll the pixels in the low-power mode less frequently with respect to the high-performance mode.