by A shadow is the object drawn again on the ground by the light, and it obeys perspective exactly as the object does; it simply has its own vanishing points. This lesson is how to draw shadows in perspective with two points instead of guesswork: the shadow vanishing point on the horizon and the light point above or below it. Three cases cover almost every picture (sun in front, sun behind, light from the side), a fourth handles lamps and candles, and the last section deals with shadows that climb walls and steps.
Cast shadow is the shadow an object throws onto other surfaces, as opposed to the form shadow on its own unlit side. In perspective a cast shadow is constructed from two points: the shadow vanishing point (on the horizon; where shadows of vertical edges aim) and the light point (the sun or lamp, or its opposite point for light behind the viewer). Every cast shadow is the intersection of light lines through the object's top with shadow lines along the ground.
The two points
Sunlight travels in parallel lines, and parallel lines in perspective converge. The horizontal direction the light travels in converges to a point on the horizon: the shadow vanishing point (SVP). The light itself, being a direction rather than a place, also has a vanishing point: the sun point, which sits on the vertical line through the SVP, above the horizon if the sun is in front of you and below it (the anti-sun point) if the sun is behind you.
With those two points marked, any vertical edge's shadow is drawn in three lines. From the foot of the edge, draw a line to the SVP: the shadow lies along it. From the sun point, draw a line through the top of the edge until it crosses the first line: that crossing is the end of the shadow. The shadow is the segment between the foot and the crossing. For a box, do it for the vertical edges that matter (the ones on the shadow side), join the ends with lines that go to the box's own vanishing points, and the shadow is a box-shaped patch on the ground, correctly converging.
Sun in front of the viewer
The sun is somewhere ahead, above the horizon, so shadows come toward you and lengthen as the sun sinks. Mark the sun point where the sun is (in or above the picture) and drop a vertical to the horizon for the SVP. Shadows of verticals run from their feet toward you, spreading out as they come; the lower the sun point, the longer they get. This is the backlit, late-afternoon picture, and it is the most dramatic use of shadow because the shadows point at the viewer.
Sun behind the viewer
The sun is over your shoulder, so shadows go away from you, toward the horizon. The sun's own vanishing point is now below the horizon (the anti-sun point), on the vertical through the SVP. The construction is the same: shadow lines from the feet of verticals to the SVP (now going away from you), and light lines from the anti-sun point up through the tops. The shadows converge toward the horizon as they recede, which reads as "sunlit scene from the sunny side", the typical mid-morning street.
Light from the side
When the sun is directly to the left or right, its rays are parallel to the picture plane and have no vanishing point at all: the SVP is infinitely far along the horizon. Shadows of vertical edges run horizontally, parallel to the horizon, and their length depends only on the sun's height: a 45° sun throws a shadow as long as the object is tall, a 30° sun one 1.7 times as long. Draw each shadow as a horizontal from the foot, ending where a line at the sun's angle through the top meets it. The shadows of the object's receding edges still go to the object's own vanishing points. This is the easiest case, and most classroom perspective shadows use it.
A lamp, a candle, a streetlight
A close light source is not parallel light, so the rules change shape but not kind. Mark the light where it is (the bulb) and its foot, the point on the ground directly beneath it. The foot replaces the SVP: shadows of vertical edges radiate away from the foot. The light replaces the sun point: lines from the light through the tops of edges give the shadow ends. Because the rays diverge, shadows get longer and fan out the further objects are from the lamp, and an object directly under the lamp has almost no cast shadow at all. Two lamps give two shadows, one per light, overlapping darker where both fall.
Shadows on walls, steps and other objects
A shadow follows the surface it falls on. When a shadow running along the ground reaches a vertical wall it turns and climbs the wall vertically, as far as the light line through the object's top allows. Construct it in two parts: the ground segment from the foot toward the SVP until it meets the base of the wall; then a vertical up the wall from that point, cut off where the light line from the sun point through the object's top crosses it. On a wall that runs to a vanishing point, the shadow's top edge runs to that vanishing point too.
Steps are a wall and a floor repeated: the shadow climbs each riser vertically and crosses each tread toward the SVP. A shadow falling on another box does the same, bending at every edge. A shadow on a slope runs to the slope's own vanishing point rather than the SVP; find the shadow of the edge on the level ground first and then lift it onto the slope at the slope's angle.
Shadows of figures and round things
A standing figure is a vertical edge with a shape on it: construct the shadow of the height line first (foot to SVP, top via the sun point), then dress it with the silhouette, keeping the shadow's shoulders and head on the far end. The shadow of a sphere is an ellipse on the ground, found from the shadow of its vertical diameter; a cylinder's shadow is the shadow of its two end ellipses joined by the shadows of its sides. In every case the object's own vanishing points still apply to the shadow's outline, so a box's shadow has edges converging where the box's edges converge.
Checks
Every shadow of a vertical edge aims at the same point on the horizon. The sun point (or anti-sun point) is on the vertical through that point. Shadows in front-light are longer at the top of the picture than at the bottom, and shorter in back-light. Objects touching the ground have shadows starting at their feet; objects floating have shadows detached from them by the height they float. If a shadow breaks one of these it is the shadow that is wrong, not the object.
Practice
On the 2-point grid at 50 mm, draw a box and a standing figure, then cast their shadows three times: sun in front (sun point above the horizon), sun behind (anti-sun point below), and side light (horizontal shadows). Then put a wall behind them and bend the shadows up it. The figures in perspective lesson gives you correctly sized people to cast shadows from.
Related
How to draw perspective · figures in perspective · how to draw a room in perspective · 2-point perspective grid
Questions
How do you draw shadows in perspective?
Two points do all the work. One on the horizon is the shadow vanishing point: every shadow of a vertical edge runs to or from it. One above or below the horizon on the same vertical is the light (the sun) or its opposite: a line from it through the top of the object hits the shadow line where the shadow ends. Join the ends and the shadow is drawn.
Where is the shadow vanishing point?
On the horizon, directly below the sun (light in front of you, shadows come toward you) or directly above the anti-sun point (light behind you, shadows go away from you). It is the vanishing point of the horizontal direction the light travels in, which is why shadows of parallel vertical posts all aim at it.
What is the difference between sun shadows and lamp shadows?
Sunlight is parallel, so all shadows converge on one vanishing point on the horizon and the sun itself is a point at infinity. A lamp is close, so its rays diverge: shadows radiate from the point on the ground directly under the lamp (its foot), and shadow ends are found by lines from the lamp itself. Lamp shadows are longer the further the object is from the lamp.
How do I draw a shadow when the light comes from the side?
Light parallel to the picture plane has no vanishing point: shadows of vertical edges run horizontally across the ground, parallel to the horizon, and their lengths come from the light's angle above the horizon by simple slope. It is the easiest case and the one most sunny-afternoon pictures use.
How does a shadow behave when it hits a wall?
It runs along the ground to the wall, then climbs the wall vertically (for a vertical wall) as far as the light line from the object's top allows. Draw the ground part first, then a vertical at the wall, then find where the light line through the object's top cuts that vertical.
Why does my shadow look wrong even though the object is right?
Usually the shadow vanishing point is not on the horizon, or the light point is not vertically above or below it, or shadows of parallel edges are aiming at different points. Check the shadow of every vertical edge: all of them must aim at the same spot on the horizon.