Techniques of detecting edges of objects in images include applying an adaptive threshold to a sigmoidal membership function of a fuzzy gradient of the image brightness. For example, when a computer configured to detect edges of an object of an image receives a grayscale image, the computer computes gradients of the image brightness at each pixel to form a gradient matrix. The computer then creates a fuzzy gradient matrix by applying a sigmoidal membership function to each element of the gradient matrix to form a smoothed gradient matrix. The computer then performs an adaptive threshold operation on the smoothed gradient matrix to produce the edges of the object of the image.