AIA 1600 Å images showing the X2.2 flare ribbons at different times on September 6, 2017. The red contours represent the flare brightening regions identified at each time step, while the orange contours indicate the cumulative brightening region of the flare across all times. The contour level is (I_1600f − I_1600q) > 20σ_1600q. Here I_1600f and I_1600q refer to the AIA 1600 Å intensities in the flare region and quiet region, respectively. σ_1600q is the standard deviation of AIA 1600 Å intensities in a 100″ × 100″ quiet region.