Viewing Position, Step 2

Finding the correct viewing position

Step 1: Find the Primary Vanishing Point

(a) First, find lines that are parallel to the picture plane.

The only parallel lines in 3D whose images remain parallel to each other in perspective are those lines which are parallel to the picture plane.

In this image, the front and back edges of the rows in the tile floor appear to be parallel to each other, and so in real life they would be parallel to the picture plane.

(b) Use those to find orthogonals:

Find lines which your experience of life in 3D tells you in real-life would be perpendicular to the lines you found in (a) and parallel to each other, but which aren't parallel to each other in the perspective image.

Extend the lines found in (b) until you find the one point where they all intersect. That point is the primary vanishing point

Conclusion 1: Viewers should place their viewing eye opposite this primary vanishing point.

Step 2: Find the Viewing Distance

	(a) Are there any squares that are parallel to the "floor" or the "side wall"? If not, time to move on to some other plan. In this picture, the tiles are clearly square and parallel to the floor.
	(b) We're going to be looking for the vanishing point of a diagonal of the square. Draw the diagonal of the square. If the square is parallel to the "floor", the diagonal's vanishing point is going to be on the horizon line -- the horizontal line through the primary vanishing point. In preparation for finding this vanishing point, draw in the horizon line. (If the square is parallel to the "side wall", the diagonal's vanishing point is going to be on the "verizon" line -- the vertical line through the primary vanishing point. In preparation for finding this vanishing point, draw in the verizon line.)
	Extend the diagonal until it intersects either the horizon line or the "verizon" line, depending on whether your square is parallel to the floor or the side wall. This gives you the vanishing point of the diagonal. We will call that the secondary vanishing point.
	(c) The distance between the primary and secondary vanishing points will be the viewing distance.

Conclusion: Place your eye directly opposite the primary vanishing point; it should be the same distance back from it as the distance between the two vanishing points away.

Next - Why does this work?

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