freehand perspective #2

In class, we reviewed a procedure for detecting the perspective orientation between a subject space and a camera. Given appropriate visual cues, a scale can be assigned with a vertical measuring line (VML). Thus, a photograph serves as a mannequin for spatial design ideas. Dress your photo with prime focus on window openings. In two separate sketches, show the same mannequin dressed in two ideas that you generated during visual note taking B.

when it rains, it pours

The current batch of perspectivists at UC Extension recently reverse-engineered some photos. Reverse engineering is the process of analyzing a subject system to create representations of the system at a higher level of abstraction. A photograph is a representation of the system of perspective construction. The camera uses a station point, line of sight and picture plane; it has a field of view and focuses along a line of sight. These geometric abstractions are common to both perspective and still photography. Our perspectivists located abstract components in the photographs and roughly sketched out plan versions of the scene the camera most likely faced.
Google SketchUp simulates the same kind of reverse engineering on photos to get them to merge their represented reality into the virtual reality of SketchUp. The programmers then had to find a way to make visible the abstract components WITHIN its 3D modeling software, since the software itself, being no more like sight than photography, is a representation of the system of perspective drawing. Looks a lot like the perspectivists' work!The whole process reminds me of staring at the rainy-day illustration on the Morton Salt container. A girl was holding an umbrella and a container of Morton Salt, which had the same illustration with a smaller girl, smaller container, with yet another illustration of a girl with an umbrella and an illustrated container. Or so I imagined.

linear perspective basics

Two drawings are needed to construct a perspective drawing. A third is often necessary, but can be embedded in the second. The first is a map that gives scalable, 2-dimensional information about the orientation of viewer-to-object. I call this the Setup. It's often a plan that includes the viewer's location (SP). What's missing is vertical measure: eye height above ground of the viewer and the height/elevation of the object. (that missing elevation is very often the useful third document) Kevin Forseth's book has very clear illustrations. Here is an example of a Setup from his book:
station point (SP): the putative position of the viewer

line of sight (LOS): a straight line, often parallel to the ground, from the eye of the observer to the center of a scene

cone of vision (COV, FOV) : a peripheral limitation to the field of view -- a cone whose apex is at the station point and whose angle is between 35ºmin-60ºmax

picture plane (PP): an imaginary plane, perpendicular to the line of sight, from which the scaled measure of objects may be taken. In a one-point perspective, the picture plane can be coplanar with an elevation or section.

horizon line (HL): appears to cross through anything standing at the same height above ground as the eye level of the viewer; once it has been assigned a numerical value, the height of volumes on the ground can be inferred. Its usefulness is broad but still limited, and works most predictably when the line of sight is perfectly horizontal and parallel to the ground.

center of vision/central vanishing point (CVP): The geometric center of a perspective construction. The point where the line of sight intersects the picture plane, and the vanishing point for any lines oriented perpendicular to the picture plane. A perspective composition can be cropped such that the center of the construction is no longer located in the center of the frame.

linear perspective

One of the hardest skills to acquire as I became more self conscious about drawing was that of depicting space. I wanted ever so much to depict rooms with people in them and scenarios "beyond" the front-and-center. At university, Dennis Nechvatal handed me an illustration-free, 1930s how-to book on perspective. How NOT to!! Such dense text encouraged me to work it out on my own. Roger Tibbets heartily supported that effort, which has been the root of my understanding of spatial relations. A fascination with how we perceive and depict space and how visual images link us to deeper fields of information has fueled my studies and my work life.
For those who don't want to work out perspective drawing on their own, an abundance of well-illustrated books and websites covers the practice of linear perspective. Much dwells on the built environment as content. 3D modeling software, the slide rule for 21st century perspectivists, will do much of the work for us, faster than humanly possible. But it makes the process opaque, yields a less-than humane-looking product, and severs the proprioceptive feedback loop that makes drawing so rich. (Check out the dedication at the bottom of this page!)
I've taught perspective drawing (both measured and freehand) since 1981.