Positional information in developing tissues often takes the form of stripes of gene expression that mark the boundaries of a particular cell type or morphogenetic process. How stripes form is still in many cases poorly understood. Here we use optogenetics and live-cell biosensors to investigate one such pattern: the posterior stripe ofbrachyenteron (byn)expression in the earlyDrosophilaembryo. Thisbynstripe depends on interpretation of an upstream signal — a gradient of ERK kinase activity — and the expression of two target genestailless (tll)andhuckebein (hkb)that exert antagonistic control overbyn. We find that high or low doses of ERK signaling produce either transient or sustainedbynexpression, respectively. These ERK stimuli also regulatetllandhkbexpression with distinct dynamics:tlltranscription is rapidly induced under both low and high stimuli, whereashkbtranscription converts graded ERK inputs into an output switch with a variable time delay. Antagonistic regulatory paths acting on different timescales are hallmarks of an incoherent feedforward loop architecture, which is sufficient to explain transient or sustainedbyndynamics and adds temporal complexity to the steady-state model ofbynstripe formation. We further show that an all-or-none stimulus can be blurred through intracellular diffusion to non-locally produce a stripe ofbyngene expression. Overall, our study provides a blueprint for using optogenetic inputs to dissect developmental signal interpretation in space and time.