Photomontage (and Atget's display windows) were great enthusiasms of the surrealists in mid 1920s Paris. The Révolution Surréaliste included quite a number, including two by Rene Magritte. These made use of photomontage mainly to juxtapose incongruous objects (such as putting cows at stream edge before the Paris Opera--this by Magritte!). See Dawn Ades, Photomontage, p. 136. In some of his "experiments" with representation in the later 1920s, however, he discovered gradient blending of objects which normally appear with edges. This allowed him to avoid the edge or seam with composite objects. In these experiments he merged wood planking into sky (i.e. one background into another)("The Passion for Ideas," (1927) and "The Cultivation of Ideas" (1927)) and into a woman's naked body ("The Discovery" (1928)). This merging , he declared excitedly in a letter to Paul Nuogé, forces the eye to think in a completely different way. It did not become a favorite technique of his, however, and the reason can perhaps be grasped from "The Discovery," which is not I think one of his most successful paintings.
There is much that is clever here: the planking following different contours with shape and with its grain, but the wood is not a background and so seems gratuitous, not to mention producing an effect rather like an animal skin (tiger coat), so that the woman never mysteriously merges into something else and the texture applied to her doesn't look entirely like wood. The problem I think is in the outline: Magritte likes lines and outlines because they define objects and relatively clear planes in which the objects are located. That is, they help to define the visual contradictions and paradoxes that he set up for the viewer--his version of Breton's "the one in the other" game. (Magritte comes quite close to Escher at times. See his late "Blank Check".) If the outline of one object is broken by another object, we see that as occlusion, and locate the second object nearer to the viewer. Magritte's strong outline here thwarts all kinds of mythological associating with dryads and other people and spirits placed in wood or trees. To have background blend into foreground so that the foreground figure seems to emerge from the background subverts Magritte's rigor which feeds on violations of rules for the perceptual construction of scenes. So rare is this blending that I can only think of one (series) like that, which is of a fully rigged sailing vessel with wave texture riding on the waves ("The Seducer," 1950-53). Magritte is very fond of merging one object into another within a common outline, as for example his "Red Model", thus making a paradoxical or contradictory object, but he does not push beyond that to merge foreground/background.