One reason for the effectiveness of camouflage, of various kinds, is our innate tendency to perceive similar components as "belonging together" (called unit-forming) and to see dissimilar components as "belonging apart" (unit-breaking). There was widespread interest in this and related aspects of vision, among artists and scientists, prior to and during World War I. Here are two wartime examples of the playful use of grouping.
Above is a photograph that appears to be an image of US President Woodrow Wilson. It was made by carefully arranging more than twenty thousand soldiers at Camp Chilocothe in Ohio. By wearing certain clothing and standing in designated locations, the soldiers were able "to produce all the lines and shading in a likeness of the face…The lighter portions of the picture were made by soldiers who wore no coats or hats, while in the darker sections, the men were in full uniform. Nearly 50 men were required to represent one lens of the president's eyeglasses. There were 21,000 men in the picture."
Below is President Wilson's image again. It was produced in 1917 by Harvey Parsons, a cartoonist for a Kansas newspaper, and a typesetter named O.W. Kelly. The portrait is made entirely of metal letters, produced on a linotype machine. The letters can also be read as the text of a statement that Wilson had written to Pope Benedictus. As explained at the time, "light-faced type composes the high lights of the picture, and black or bold-faced, the half-tones and darker portions. The proper spacing of the letters is not destroyed, and the reply to the Pope is legible in spite of the underlying likeness." This second method of portraiture can today, of course, be easily made on a computer with an app that is made for the purpose.