Drawing shadows is an important part of a 3-D drawing. You need to understand the geometry behind this process to make your drawings most life-like. It is not as hard as it looks.
Drop shadow is a shape that is created by an object on a surface (or multiple surfaces) by blocking the light.
To see a drop shadow you need a light source.
There are two types of light sources: natural (sun, moon) and artificial (lamp, candle, flash-light, etc).
Our natural light sources ( Sun and Moon) are quite far away. So the light rays that come from them and lit an object are considered to be parallel (because of the distance):
Artificial (spot) light
Spot light is much closer to us. We can position it more accurately in our environment - we can tell exactly how far it is from the object and how high it is from the ground (or ant surface).
To build a drop-shadow
1. Light source (yellow dot)
3. Light ground mark - position of the light source on the surface or ground (green dot)
4. Line, connecting the position of the light source and the bottom of the
object (black line)
5. Line connecting the light source (1), the top of the object (red dot), and
Line 4 - defining the end point of the drop shadow (6)
6. The end point of the drop shadow on the surface
This rule is applied for both types of the light source and for all forms and objects:
The only difference is that lines 4 are parallel as the light source is extremely far away, so we don't need to draw a yellow dot (1). Lines 5 are also parallel for exactly the same reason.
Each corner point follows the above rule to form an exact shape of the drop shadow.
Steps to build a drop shadow:
Shadow of a point
Imagine that you need to draw a drop shadow of a very small round object, almost a dot. Let's draw one of a fly. Why not?
So here we have a room with one spot light and a fly. The ground surface is the brown floor - this is where we are going to place the shadow.
The first thing you do is mark the position of the light source on the floor (light ground mark). You do it by drawing a straight vertical line down to the floor and marking the point on the floor that is exactly below the light:
Now you need to mark the position of the fly on the floor (object ground mark). Please note the position of the light and the fly in respect to each other - in my case, the light is closer to me than the fly (you don't need to draw the brown horizontal lines - they are just for a reference).
Now connect the light ground mark and the object ground mark on the floor with a line - it will define the direction of where the shadow will appear on the floor.
Great! One more construction line, and you're done! Draw a line from the light through the fly - all the way to the ground. The intersection will mark the position of the drop shadow!
Shadow of a line
Drawing a shadow of a line is, actually, a bit easier - you don't need to define the position of the object on the floor!
So, start, again, with defining the position of your light on the ground surface (floor) - place the light ground mark. Please note, that in this case my object (pencil) is closer to me that the light.
Connect the light ground mark with the bottom of the line (pencil, in this case).
Now draw a line (light ray) from the light through the tip of the line (pencil). That will define the length of your drop shadow:
In case you need to draw a shadow that falls on multiple surfaces (floor and walls) - use the same steps, just tweak them a little.
Let's say your line (another pencil) is close to the wall. The drop shadow will fall on both the floor and the wall. This is how you do it:
Just bend the line when it hits the wall!
Draw a line from the light through the top of the line (pencil) as shown:
Shadow of a cube
Drawing a drop shadow of a cube (box) is a repeated use of the same technique as we did with drawing s shadow of a line. You will have to repeat the steps with all corners that form a shadow.
As you already know, the best way to draw 3-D forms is to draw them with "see-through sides" - as if the form is made of glass.
Connect the light ground mark with all bottom corners:
Draw lines from the light through the top corners of the box. The intersections are the points that form a shadow shape. All you have to do now is to connect the dots!
Shadow of a cylinder
The same technique is used to draw a shadow of a cylinder, with a few minor adjustments.
Keep all construction lines (axis of symmetry, ovals):
Draw lines from the light ground mark through the widest parts of the bottom oval:
Also, draw a line through the bottom of the axis of symmetry.
Draw vertical lines from the bottom ovals to the top ovals as shown.
Draw lines from the light through the top oval's dimension points like this:
Time to trace the shadow. The two sides are straight lines that follow the ground lines. The middle (axis) projection is the furthest point of the shadow. Connect the three dots with a curve.
Drawing a shadow of a cylinder on multiple surfaces is like drawing a shadow of a line on multiple surfaces:
Shadow of a cone
Start with placing a cone on the floor.
Have all construction lines in place (axis of symmetry, bottom oval)
Draw a line from the light ground mark and the bottom of the axis of symmetry
Draw a line from the light through the top of the cone to the ground line.
Draw lines from he point of the top's shadow through the widest parts of the oval
The shadow is done!