There is a show on Frans Hals at the Metropolitan Museum in NYC (open until Oct. 10). He is known as an early practitioner of the "painterly" approach, where the "finish" of a painting is loose and lively. As has been known for some time, he did not just toss off a portrait in one sitting, but instead worked up an image in layers in the traditional manner. Once the portrait was nearly done he would "finish" it with a flourish that would bring a spontaneity and life to the portrait.
I've never been a great fan of Hals. His compositions always seemed limited, and sometimes even awkward. The painting Yonker Ramp and his Sweetheart is one of the few that I like from a compositional point of view.
The addition of gestural strokes to a carefully layered painting is one of the answers to the problem of computer rendering.- the problem of dull, literal detail. Whatever you have built into a computer model will show up in the rendering, no matter haw inconsequential it is. Many surfaces will render with the bland sameness that is mathematically correct, but is visually boring. And any natural elements, like trees or people will lack the liveliness that is expected from living creatures.
The usual solution to this problem is to "loosen" the final rendering in some way. You can process it through a graphic "filter" to make it emulate a pencil drawing or a gouache painting. You can scribble the final print with pencil or pastel. Ink rendering over a print is also possible. The use of computer tablets has made this process more "forgiving" and variable then ever before. The following are a series of detail examples with some explanatory notes.
The possibilities are endless, and the problem has yet to be "solved". The client will always want detail, and the illustrator will always have to balance that against the artistic needs.