Sentence:  The area adjacent to one edge of a piece of affixed material - or simply of a painted in form - will be shaded to pry that edge away from the <WSD>surface</WSD> , while something will be drawn , painted or even pasted over another part of the same shape to drive it back into depth . 
Meaning of the word marked with <WSD> tags:  the outer boundary of an artifact or a material layer constituting or resembling such a boundary