Various images of the M̈uller-Lyer figures have been obtained by digital filtering. The filtered images contained a definite spatial-frequency spectrum, predominantly low, medium, or high frequencies. The filtering was carried out by convolution of the images with wavelets that are the difference of two Gaussoids with half-width differing by a factor of 2. The equalization threshold of the M̈uller-Lyer figures was measured by presenting images subjected to digital processing and without processing, thereby measuring the threshold for bringing about the illusion. The M̈uller-Lyer illusion was caused by all the stimuli, but it was reliably larger in response to the presentation of the image with a predominantly low-frequency component. The modelling of the M̈uller-Lyer illusion must take into account the spatial-frequency spectrum of the test image and the characteristics of the pass-band filtering in the spatial-frequency channels of not only the primary but also the higher divisions of the visual system, which construct the envelope based on the primary filtering of the image.