three point perspectiveAt this point it's customary to explore the capabilities of 2PP in a variety of specific drawing problems. I want to keep the momentum and look at three point perspective, which allows you to construct a form in any orientation (from any viewpoint). Three point perspective is often illustrated with aerial views of Manhattan, looking down on a skyline bristling with skyscrapers. But artists will find 3PP equally useful in still life or figure paintings — where the view downward onto a table of objects or a piece of furniture can be just as steep — and in landscape views up toward soaring cliffs or a stand of tall trees. The 3PP perspective problems and construction methods are complex, and it may seem we lose more in clarity than we gain in drawing power. Many artists have come to the same conclusion, and avoid 3PP for simpler approaches, including freehand modification of drawings blocked out in 2PP, or the expedient of tracing photos. I won't disagree with those solutions; they can be convenient and effective. They fall short, however, if you must add new forms around the primary form — for example, if you have traced the photograph of an existing building, and want to insert new or different buildings around it — or if you want to show the building from a different point of view, or require more precision than freehand perspective can provide. For these common situations, 3PP is invaluable. three point perspectiveAs we add vanishing points, we remove aspects of perspective that we can take for granted. In 1PP or central perspective, the relationship of the vanishing points and horizon line to the direction of view are taken for granted. In 3PP both the vanishing point locations and the relationship between in the direction of view and the ground plane (horizon line) must be specified. Defining Features of Three Point Perspective. The diagram shows the simplest 3PP situation: a cube centered in view but first rotated 45° to one side and then downward until all front faces appear of equal size. In all three point perspective views there are no faces or edges parallel with the picture plane. In particular, because the direction of view is still assumed to be perpendicular to the image plane, the direction of view is no longer parallel to the ground plane when the primary forms are constructed as buildings are, with walls perpendicular to the ground. The canonical view places the three front edges of the cube in a 54.7° angle to the direction of view, so that all three vanishing points are outside the circle of view. The planes of the three front faces are at a 35.3° angle to the direction of view, with vanishing lines defined by the triangle of three vanishing points. three point perspective: the basic geometry The three vanishing points (vp_{1}, vp_{2} and vp_{3}) control the recession of all lines parallel to the edges of the cube. This means the outline of each face is determined by two vanishing points, rather than one as in 2PP. Connecting the vanishing points are three vanishing lines, which control recession of all planes parallel with each front and matching back face of the cube and all planes parallel to them. Each vanishing line also contains the vanishing points for all lines parallel to their respective planes, including the diagonal vanishing points (dvp_{1}, dvp_{2} and dvp_{3}) for the planes. A vanishing line perpendicular to the viewer's vertical orientation (parallel to the ground plane) is typically the horizon line in architectural or landscape uses of perspective. It is the vanishing line for all planes parallel to the ground plane, and contains all vanishing points for lines parallel to the ground plane (perspective rules 13 and 14). Each vanishing line is connected to the vanishing point opposite to it by an auxiliary horizon line (shown in orange in the figure). These are the vanishing lines for measure points for each of the three dimensions of the cube. In 2PP, the horizon line was a vanishing line for both the vanishing points and measure points, but in 3PP these functions can be separated. The auxiliary horizon lines always intersect at the direction of view (the principal point) — that is, they link the vanishing points of the object to the vanishing point of the viewer's central recession (perspective gradient). Therefore the principal point is always inside the vp triangle formed by the three vanishing lines: if it is not, then the primary form does not define right angled vanishing points (it is a pyramid or a lopsided cube). The measure points become significantly more complex in the 3PP orientation: two vanishing points define the edges of each face, and each edge requires its own measure point. So we have in all six measure points (mp_{1} to mp_{6}) — two for each vanishing point in relation to the two faces it governs. Finally, with the visual ray method we had a simple way to rotate the vanishing points in 2PP, but this also becomes significantly more complex in 3PP. In 2PP we just had to rotate two faces joined in one right angle, which we could easily diagram in two dimensions as two lines joined in one angle. In 3PP we must rotate three faces joined in three right angles, and that complicates the visual ray approach to a perspective solution. Direction of View & Horizon Line. A 3PP construction allows the direction of view to be oblique to the ground plane, so that we are looking down or up on objects rather than looking at them directly from one side. Consequently in 3PP it is necessary to distinguish between (1) the object geometry (the vanishing points defined by the edges of the primary form), (2) the central recession defined by the direction of view, and (3) recession on the ground plane, for example in the visual texture of forests, grassy plains, deserts or bodies of water. For example, we can redraw the cube illustrated above in two point perspective so that it has exactly the same angular size in the field of view (using a measure bar), and is positioned below the direction of view so that we look down on its upper face at a 35° angle. This locates the top front corner on the 71° circle of view and the bottom front corner just in front of the ground line (diagram, below). the 3PP canonical view in two point perspective Because both the angular size of the cube and the angle of its faces to the direction of view are identical, we are viewing it from exactly the same location in physical space. All we have done is shift our gaze from the object itself to the horizon line behind it. This keeps the same visual angle between the front corner of the cube and the horizon line. But changing the direction of view in 3PP means that: (1) the horizon line no longer must intersect the principal point, and in fact may no longer be within 90° circle of view; and (2) the geometrical relationship between any two vanishing points (the size and shape of the triangle the vanishing points define on the image plane) depends on the location of the third vanishing point and the location of the direction of view (the orientation of the image plane to the perspective problem). the perspective sketch construction methodThe solution is basically to draw the form first, so you can locate the vanishing points and measure points which will produce that perspective view. You then use these to reconstruct the primary form in accurate perspective, and to add objects around the primary form within the same perspective space. 


Why not draw the primary form by freehand perspective alone? Because, as we've already seen in 2PP, inaccurate placement of vanishing points results in a distorted perspective view; even small distortions can be obvious in a finished drawing. There is a better way. You start with a freehand perspective sketch or scaled down perspective drawing at the center of a fairly large piece of paper (a 3' section from a roll of wrapping paper or white butcher paper is ideal). three point perspective: perspective sketch of the primary form Your drawing or photograph of the primary form should be small enough to fit all perspective points on the sheet of paper, yet large enough to work with accurately — usually a drawing about 10cm or 4 inches on its longest side is practical. Take your time with the freehand drawing, and try to capture the relative proportions of the dominant edge angles and faces as accurately as you can. Don't worry about extraneous features (such as doors, windows or domes): you want to capture the basic perspective shape as it recedes in three directions. Be sure to define the edges and corner points clearly. You can also start with a drawing or photograph of a building or monument that presents clear vanishing lines in its edges or surfaces, in the perspective orientation you want to duplicate. This photograph is only used to specify the approximate perspective view of the primary form in the drawing, so it does not have to look anything like the primary form you actually want to draw. Once the drawing is finished to your satisfaction, or you have taped your photograph to the sheet of paper, you draw prospecting lines from the edges of the front planes to find the three vanishing points. Using a ruler or yardstick, extend the outer edges of the form until these prospecting lines intersect at three separate points. In a cube you have three edges tending to each vanishing point; use these in combination to reconcile discrepancies and find the point that gives all three the best definition. The Vanishing Line Triangle. Next, connect these three vanishing points with three vanishing lines. You have defined the vanishing line triangle that will define (and usually contain) the primary form. three point perspective: the vanishing line triangle This is the point to look at the overall placement of the vanishing points in relation to the primary form and the space around it that will appear in the finished drawing. You can block in the format outline, or sketch other large forms around the primary form, to make sure you will get the effect you want. Constructing Auxiliary Horizon Lines. Next, draw the three auxiliary horizon lines through each vanishing point and perpendicular to the opposite vanishing line. There are two ways to do this. The quicker is to use a large carpenter's square, laying one side against each vanishing line and sliding it back and forth along the line until the other arm is exactly on the vanishing point. Then draw the line. three point perspective: constructing the auxiliary horizon lines A more accurate method in large drawings is to construct the perpendiculars using a long piece of fishing line, hemp (not stretchable cotton) string or strip of cardboard as a compass measure. With your thumb, a tack or a piece of tape, fix one end of the measure at the vanishing point, and with the other end scribe a wide pencil arc across the opposite vanishing line. (Put the tip of the pencil through a loop in the string or a small hole in the cardboard strip.) The arc must intersect the vanishing line at two widely spaced points. Then either scribe two intersecting arcs centered on each of these new points, or measure with a ruler 1/2 the distance between them. In the figure, the two arcs have been scribed around vp_{1} and vp_{2}, and through vp_{3}, to define the new points X and Y. Intersecting arcs drawn from X, Y and vp_{3} create the new points P1 and P2; lines to these points from the corresponding vanishing points create two auxiliary horizon lines. The direction of view (dv) is always at the intersection of all three auxiliary horizon lines, so the third line can simply be drawn from vp_{3} through dv to the opposite vanishing line. You end up with a vanishing line triangle similar to the one shown above. Didn't I say elsewhere that the freehand placement of vanishing points leads to distortions? No: it's the clumsy scaling of drawing size in relation to the distance between the vanishing points that introduces distortions. If your three auxiliary horizon lines are at right angles (perpendicular) to their vanishing lines, if they meet in a single point (dv), and if this point is inside the vanishing line triangle, then the triangle defines a valid (physically possible) perspective space for a rectilinear solid. Constructing the Circle of View. Now we insert the 90° circle of view. This requires you to (1) find the midpoint of any of the three vanishing lines (connecting two vanishing points), (2) draw a semicircle of Thales over the vanishing line, (3) extend to the semicircle the auxiliary horizon line that intersects the vanishing line, (4) construct a line parallel to the vanishing line, and finally (5) draw a second arc back to this parallel line. The intersection of this arc with the parallel line defines the radius of the 90° circle of view around dv. three point perspective: constructing the circle of view In the traditional solution, the artist uses either a ruler or the method of intersecting arcs to find the midpoint M on the vanishing lines between two vanishing points. In the diagram, I've chosen the vanishing line between vp_{2} and vp_{3}. When arcs of equal radius are inscribed across the vanishing line from the two vanishing points, they intersect at two points, x and y. (1) A line through these points defines the midpoint M of the vanishing line. (2) From point M the artist constructs a semicircle of Thales between the two vanishing points, then (3) extends to the semicircle the auxiliary horizon line that intersects the inscribed vanishing line at P. This defines a new point C. (For visual clarity, the semicircle is shown outside the perspective triangle, but to save space it can just as well be drawn to intersect the interior auxiliary horizon line.) (4) Next, the artist constructs a line through dv that is parallel to the vanishing line. (5) Finally, the artist inscribes an arc from P with radius equal to PC, the extended segment of the auxiliary horizon line. This intersects the line parallel to the vanishing line at either H1 or H2, depending on where it is more convenient to construct the arc. (6) The line segments dvH1 or dvH2 are equivalently the radius of the 90° circle of view. The artist draws this circle from H1 (H2) with dv as its center. It is often useful to include the 60° circle of view, which is a second circle with a radius equal to 0.58 (58%) of the radius of the 90° circle of view. This completes the perspective space. Locating Measure Points. The last step is locating the measure points. Six are required if they are marked along the vanishing lines, but only three if you locate them on the auxiliary horizon lines. Auxiliary Horizon Line Measure Points. To find the measure points on the auxiliary horizon lines, use a protractor or architect's triangle (or the traditional method for constructing a perpendicular) to construct finish perpendiculars on each auxiliary horizon line, from dv to the circle of view: the intersection with the circle of view defines three new points, C1, C2 and C3. Draw arcs from each of these C points back to the auxiliary horizon line perpendicular to it, using the vanishing point on that auxiliary horizon line as the center of the arc. three point perspective: finding the measure points This completes the perpective space at a reduced scale. I find that this entire procedure, starting with a blank sheet of paper and ending with the finished perspective space, requires about 20 minutes to complete. Once you understand how to do it, the work goes quickly and smoothly. You must carefully make seven measurements on this drawing (using a metric ruler) to rescale it to full size: (1) the longest distance between any two vanishing points (in the example, vp_{3} to vp_{2}), (2) the distance from one of these vanishing points to the intersection with the auxiliary horizon line (vp_{3} to h), (3) the length of this auxiliary horizon line (h to vp_{1}), (4) the length to the direction of view (h to dv), and finally (57) the distance from dv to each of the three measure points. Divide the radius of the circle of view you want in the full sized drawing (say, 160cm) by the radius of the circle of view in your perspective sketch: multiply all the measurements by this number. This gives you the full scale perspective space. Your perspective work surface needs to be at least as long as the longest vanishing line and as wide as the 90° circle of view. In the example drawing, assuming a 3m circle of view, this would be roughly 5m by 3m. On a surface large enough to accommodate these distances (a very large table, or a clean hardwood or linoleum floor, or a clean, flat patio, garage floor or driveway), measure out the longest vanishing line (in the figure, vp_{2} to vp_{1}), and the auxiliary horizon line to vp_{3}. Connect the three vanishing points to define the vanishing line triangle. Measure the distance from the vanishing line to dv, and draw the remaining two auxiliary horizon lines from the vanishing points through dv. Finally, mark the three mp's on each auxiliary horizon line, measured from dv. Use the drawing scale shown in the distance to size table to compute the drawing scale — the percentage of the actual object size (for a given viewing distance) that the drawing of the primary form should have. On a piece of paper, make a rough sketch of the primary form at this size, and lay the sketch on the format (size of support) you intend to use, to make sure the proportions work. Vanishing Line Measure Points. The 3PP method of using three measure points is convenient, but it fails when the anchor point for measurements is close to the direction of view (dv). In this case, you may want to use the vanishing line points instead. The construction of the circle of view required a semicircle of Thales drawn around one of the vanishing lines, centered on M and intersecting the vanishing points at either end of the vanishing line, then extended the auxiliary horizon line to intersect the semicircle in a point h'. This is all you need to define the measure points on that vanishing line. (Note that you can save steps and work space by intersecting the auxiliary horizon line inside the perspective triangle, to define interior h', and construct the measure points from there.) three point perspective: alternate method to define measure points The point h' will always define a 90° angle with the two vanishing points on the vanishing line. That is, it is equivalent to the viewpoint in a 2PP rotation of vanishing points. So you can draw two arcs from this point back to the vanishing line, using each vanishing point as the center of an arc, to define the measure points for the vanishing line — just as you would in two point perspective. Confusion about the choice of vanishing line measure points is usually dispelled by the following two criteria: • The controlling vanishing point is the vanishing point for the convergence of the edges that are being sized by the measure bar. Thus, edges converging to the right side vanishing point (vp_{2}) are controlled by that vanishing point. • The measure point to use was defined by an arc from the controlling vanishing point. Thus, mp_{4} was defined by an arc centered on vp_{2}, so mp_{4} is the measure point to use when sizing edges that recede to that vanishing point. The height dimension is controlled by the vertical vanishing point (vp_{3}), which was the center of the arc used to define mp_{3}. Measure bars to the vanishing line measure points always must be parallel to the vanishing line containing the measure point being used, not to any auxiliary horizon line as before. Note that two measure points are always available for each dimension. In the example, mp_{6} can be used to size the vertical edges receding to vp_{3}, if for some reason mp_{3} is inconvenient to use — but in that case, the measure bar must be parallel to the vanishing line containing mp_{6}. The measure bars in the illustration are the same length as those used previously, and as you can see, they define the same reduction in perspective depth. You do not need to rescale or recompute the measure bars you already have; just align them parallel with the appropriate vanishing line. Because the semicircle on M is part of the circle of view procedure, and any vanishing line can be used to define the circle of view, you should consider the location of your anchor points in the perspective space, and place the semicircle of Thales around the vanishing line where measure points will be most convenient. For example: I had originally put the anchor point at the front bottom corner of the cube; in that location mp_{3} worked fine, but the other two points created badly slanting measure lines that would introduce inaccuracies. The best alternative points would be found on the top vanishing line (between vp_{1} and vp_{2}), so I should have started building the circle of view by putting the first semicircle on that side. constructing a 3PP cube


A discussion of the 3PP geometry will clarify how this method works. Because all parallel lines converge to the same (single) vanishing point (perspective rule 6), and the 3PP vanishing points define visual rays at right angles to each other, the 3PP vanishing points are equivalently defined by the three right angled edges of a cube that can be turned or rotated around a front corner fixed on the direction of view (diagram, right). These edges converge to the three right angle vanishing points at the vanishing lines for the three planes defined by the three front faces of the cube (perspective rule 14). Therefore the vanishing lines between the pairs of vanishing points will be parallel to the line intersections of the three front faces of this cube with the image plane (green, corollary to perspective rule 11). As a result, we have reduced the geometry of the 3PP vanishing points to the geometry of a three sided pyramid thrust through the image plane in any arbitrary angle and rotation. As explained earlier, the circle of view framework provides a method to specify exactly the location of any vanishing point as a line rotated to the required angle around the viewpoint folded into the image plane. What we require is a way to perform this folding for elements of the 3PP "pyramid". This is done by moving the fixed corner of the cube forward until it coincides with the viewpoint. In that position its three edges define three visual rays to the vanishing points (magenta lines, diagram above right). More important: the altitude of the pyramid is now equal to the viewing distance and therefore to the radius of the 90° circle of view (diagram, below). folding a pyramid right triangle into the image plane Two kinds of folding operations are possible in this 3PP geometry. First are the auxiliary line folds that define the interior angle between a pyramid edge, or the pyramid face perpendicular to it, and the direction of view. These are found by folding into the image plane a vertical section of the pyramid defined by an auxiliary horizon line, for example the interior triangle PVC defined by the auxiliary horizon line PC in the diagram above. This triangle contains the two triangles VdvC and VdvP, each containing a right angle at dv. The fold brings line Vdv into the image plane as x'dv. Because the edge Cdv is continuous with edge Pdv, the right angle at dv is preserved. And the image edges Cx' = CV and Px' = PV. Therefore, by triangular equalities, the image angle 1' equals the interior angle 1, the angle between the direction of view and the face ABV. This fold also identifies (at Cx'dv) the angle between the vanishing point C and the direction of view, so this folding down of an interior section of the perspective pyramid is geometrically identical to the folding of the viewpoint into the circle of view that is used to rotate vanishing points to the direction of view. The second kind of folding operations are the vanishing line folds that define an exterior angle of one face of the perspective pyramid (angle 2) as a "plan view" of the angle in the image plane (angle 2'). This is the angle, on the face of the 3PP pyramid, between the edge of triangle ABV and its altitude PV. The fold is achieved by constructing a line (ab) that intersects the direction of view parallel to the vanishing line (AB). This line intersects the circle of view at x'. Because Vdv equals x'dv, the line Px' equals line PV, the altitude of ABV. Therefore an arc constructed on P with radius Px' intersects the auxiliary horizon line at x, and Px = PV. Therefore the right triangle ABx is the perpendicular view of the foreshortened triangle ABV, and x is the auxiliary viewpoint for the horizon line AB. three right triangles folded out of the 3PP pyramid The diagram (above) shows the three possible vanishing line folds and auxiliary viewpoints (x, y and z) constructed from a 3PP vanishing line triangle. Study this diagram carefully until you understand how each fold has been done. The geometry of triangles is efficient: defining any one side with its two adjacent angles, or any two sides with their common angle, defines the rest of the triangle. Therefore only two folding operations are necessary to define the image of a 3PP vanishing line triangle: one auxiliary horizon line fold and one vanishing line fold. This is sufficient to define the location of all three vanishing points and vanishing lines in relation to the direction of view and circle of view. Finally, the 3PP construction releases the direction of view from its parallel position to the ground plane, and this creates several novel features in the perspective geometry which affect in particular the scaling of the 3PP drawing. For now I only want to describe this geometry and define a few new terms (diagram, below). elevation view of 3PP geometry In this example we assume the perspective view is downward in relation to the ground plane: it can just as well be upward (as the top of a skyscraper viewed from the ground) or tilted (as a city viewed from a turning airplane), a problem I leave for the reader. In the downward view case: • The image plane is oblique to the ground plane, as is the direction of view. As a result the direction of view does not terminate in a vanishing point, but in a fixation point, some physical point on the ground. This fixation distance is typically different from the object distance from the station point to the primary form. • The station point S is still directly under the viewpoint, but now the station point appears on the image plane, where it is the image s equivalent to the vertical vanishing point (vp_{1}). • The horizon line is now located above the direction of view in the circle of view, which means the principal point, the vanishing point for the viewer's central recession (at p), is no longer the same as the orthogonal vanishing point (at h), the vanishing point of ground plane recession. • The primary form appears in rotation foreshortening — the vertical and horizontal dimensions are in a different scale. Foreshortening is corrected by using the measure points; measure bars parallel to the image plane may be rotated in the image plane to any other angle. However, it is sometimes useful to estimate the amount of vertical foreshortening, for example when planning the image layout. This is found by a cosine correction for foreshortening: where θ is the horizon angle. Because the angle of view to the ground plane is only equal to the horizon angle at the fixation point, a measure bar established at any other point must be calculated with the correct angle of view to that point on the ground plane. • An object's angular size or image size is determined by the sight line distance from the viewpoint, which is simply the hypotenuse of the right triangle formed by the object distance and viewing height. These points need to be understood in order to apply the correct distance & size calculations when scaling the drawing in a 3PP construction. constructing a 3PP drawing

the 3PP vanishing points defined by three edges of a cube 

With a plan and elevation of the primary form (diagram, right), the artist is ready to construct the perspective drawing. three point perspective: constructing the primary form diagram enlarged to 60° circle of view for clarity
It is usually convenient to establish the plan or "street map" dimensions of the drawing first, as the plan outlines do not intersect one another and clearly establishe the front to back ordering of large forms. Here the drawing is being constructed from the elevation only, as the base is square. Only the 60° circle of view is shown for clarity. Using the measure point guidelines (above), the controlling vanishing point for the right side of the base of the tower is vp_{3}; and this vanishing point defined the arc for mp_{3} (refer to the diagram above). The controlling vanishing point for the height of the tower is the vertical vanishing point, vp_{1}; and this vanishing point defined the arc for mp_{2}. The fixation line measure bar is used to establish the base width of 125 meters, and this dimension is projected in depth by the lines to the opposite measure points mp_{3} and mp_{4}. The measure bar is parallel to the vanishing line containing the measure points, Then vanishing lines from the anchor point to vp_{2} and vp_{3} establish the sides of the plan. The tower is symmetrical on its four sides, but the sides are not vertical: they define an exponential function designed to maximize the tower's strength against strong winds. To facilitate the perspective construction we have to find the central axis, which is simply the intersection of the diagonals of the plan. three point perspective: finished drawing diagram enlarged to 60° circle of view for clarity The major stages of the tower are marked off on a vertical measure bar, this bar is rotated to be parallel with the vanishing line of the appropriate measure point, and the tower stages are projected onto a vertical axis. Two strategies are available. The existing elevation drawing can be used to create the measure bar: this drawing is in the image scale defined by the anchor point on the fixation line, 579 meters from the viewpoint. Elevation points are projected onto a vertical line constructed from the anchor point. These elevation points are the front corners of new squares, of equal size as the base of the tower, containing the tower platforms. These are constructed "scaffold style", by vertical lines from the four corners of the plan. At each level the scaffold squares are recessed to the side vanishing points from the front corner, and the diagonal found as before. Then the plan of the tower platform is constructed within this square, its four corners along the diagonals. The alternative method is to project the elevation points onto the central axis, and project the tower platforms out from these central points. This method is also shown in the diagram: the measure bar must be anchored at the base of the central axis. Note however that this point is over 80 meters farther away from the viewpoint than the front corner (as shown by the 50 meter distance transversals in the ground plan), therefore the measure bar must be sized, using formula 18, to the new image distance. First the added distance is derived from the whole diagonal, which is then aligned to the direction of view by the cosine correction: base diagonal = [125^{2}+125^{2}]^{1/2} = 177 m and the new ground plane distance (579+80 = 659) is used to compute the new image scale: (18) image scale (at central axis) = 1.5/[659^{2}+270^{2}]^{1/2} (Note that the central axis distance could be estimated from its position just beyond the 650 m transversal distance line.) Once the major external points of the tower profile are established, the outside curves of the tower can be drawn with a French curve or freehand, and details of the tower filled in as appropriate. three point perspective: finished drawing diagram enlarged to 60° circle of view for clarity And here is the finished drawing. The point of using the exact rotation method is that the Arc de Triomphe could be precisely positioned behind the Tour Eiffel, and both positioned in relation to the direction of view and horizon line, to produce a specific effect. The plan of the distant streets is taken from a Michelin map of Paris, projected onto the ground plane using the foreshortened and recessed orthogonal squares and plotting the major streets, square by square, as far back as useful. N E X T : Advanced Perspective Techniques 
elevation of the primary form  