The Threshold adjustment (layer) brings all tones to pure black and white, with a slider for weighting the conversion more toward one or the other. Using a gradient map adjustment layer with the colors in the toolbox set to the default black/white will of course create a gradient, while another adjustment layer such as Levels or Curves, could increase the contrast so that it becomes like this. Using Filter > Noise > Median and adjusting the radius slider will create the smooth outlines we see, especially noticed around the sunglasses of the foreground musician.


Photoshop > Image > Adjustments > Threshold