This disclosure describes techniques for determining a shape of a signal. In particular, a kernel is applied to a portion of a signal to compute at least a first, first order derivative of the portion of the signal and a second, first order derivative of the portion of the signal in a single pass of the kernel. The shape of the portion of the signal is determined based on the first and second first order derivatives. In one example, the shape of the portion of the signal is determined based on the ratio of the first, first order derivative and the second, first order derivative. These techniques may be particularly effective for detecting edges within image signals. However, the techniques may be used to detect the shape of significant changes within any signal that represents a variable that is changing over time, space or other dimension.