The blue and green colors are actually the same. Previous lectures covered image formation and feature detection and matching. Edge detection aims to identify sudden changes in images that correspond to boundaries and shapes. Edges are caused by discontinuities in factors like depth, color, illumination, and surface normals. The Canny edge detector is widely used and finds edges by applying Gaussian filters, computing gradients, performing non-maximum suppression, and using hysteresis thresholding.