: Before 2001, face detection was computationally expensive and slow.

In technical literature, "Viola [3]" refers to Paul Viola's seminal work on efficient face detection. Below is an outline for a paper exploring this context, which you can use as a draft for your own project.

Paper Title: The Legacy of the Viola-Jones Framework in Real-Time Object Detection

: A "cascade" of increasingly complex classifiers that quickly discards non-face regions of an image, focusing processing power only on promising areas. III. Modern Significance

: A method for rapid feature evaluation that allows "Haar-like" features to be calculated in constant time regardless of size.