Picture plane

In painting, photography, graphical perspective and descriptive geometry, a picture plane is an located between the "eye point" (or oculus) and the object being viewed and is usually coextensive to the material surface of the work. It is ordinarily a vertical plane perpendicular to the sightline to the object of interest. In the technique of graphical perspective the picture plane has several features: Given are an eye point O (from oculus), a horizontal plane of reference called the ground plane γ and a picture plane π... The line of intersection of π and γ is called the ground line and denoted GR. ... the orthogonal projection of O upon π is called the principal vanishing point P...The line through P parallel to the ground line is called the horizon HZ The horizon frequently features vanishing points of lines appearing parallel in the foreground. The orientation of the picture plane is always perpendicular of the axis that comes straight out of your eyes. For example, if you are looking to a building that is in front of you and your eyesight is entirely horizontal then the picture plane is perpendicular to the ground and to the axis of your sight. If you are looking up or down, then the picture plane remains perpendicular to your sight and it changes the 90 degrees angle compared to the ground. When this happens a third vanishing point will appear in most cases depending on what you are seeing (or drawing). G. B. Halsted included the picture plane in his book Synthetic Projective Geometry: "To ‘project’ from a fixed point M (the ‘projection vertex’) a figure, the ‘original’, composed of points B, C, D etc. and straights b, c, d etc., is to construct the ‘projecting straights’ and the ‘projecting planes’ Thus is obtained a new figure composed of straights and planes, all on M, and called an ‘eject’ of the original." "To ‘cut’ by a fixed plane μ (the picture-plane) a figure, the ‘subject’ made up of planes β, γ, δ, etc., and straights b, c, d, etc.