There are two kinds of shadow: form and cast. Form shadows are the shaded areas on a surface where a light source cannot reach. Cast shadows are what we typically think of as shadows: the shadows projected by an object onto a surface (such as the ground) when the object comes between the light source and the surface. Shadow plays an important role in your artwork, giving it solidity and form and well as creating drama and suspense.
Shadow can be used to create a sense of realism, giving objects weight and presence or, as here, used to produce extreme drama.
Types of shadow
If we were somewhere where ambient and reflected light didn't occur, such as space, the values of form shadows and fast shadows would be identical: absolutely black. However, here on rjrth things aren't that simple - the atmosphere acts as a giant diffuser, nattering light everywhere, and our world is also full of reflective surfaces that scatter this light even further. This means that shadows are, in most cases, tilled with some secondary light, and generally more of this secondary light will illuminate form shadows as opposed to cast shadows. The diagrams demonstrate why this happens.
When light bounces off a large area, such as the ground, the reflected light is effectively coming from a large and therefore bright source. This means that the form shadow on the sphere is getting quite a lot of fill light coming from the ground, as well as light coming from the atmosphere.
Because the sphere itself is much smaller than the ground, it reflects less light. The cast shadow therefore gets only a small amount of fill light from the sphere, and is mostly lit by the atmosphere. Notice also that the terminator is dark and saturated.
This means that in ordinary outdoor situations cast shadows will usually be darker than form shadows. This is just a rule of thumb, and there will be occasions where some source of light might be providing more fill to the cast shadows than the form shadows, therefore making them brighter. It is important to observe for yourself rather than blindly follow a ‘rule’.
Form shadows give objects depth, especially with side lighting, and really help to convey the form of an object. The absence of form shadow is why frontal lighting can look so flat. Form shadow also reveals texture. The 3-D renders of a tree trunk, below, illustrate these points clearly: The second image has more depth, and the trunk looks more solid and dimensional. The texture is also greatly emphasized by the stronger more directional light, so the form shadow plays a very important role in conveying both the form and the texture of the object.
Here is a rare case of the form shadow being darker than the cast shadow. This is because there is more reflected light filling in the cast shadow.
This tree is lit by a large diffuse area light, such as occurs on an overcast day, making the form shadow soft and gradual.
Here the tree is lit by a harder light (simulating the evening sun), giving a harder edge to the form shadow. There is also much greater contrast'between the lit and shadowed sides.
AMBIENT OCCLUSION
In 3-D imaging another layer of realism can be added to rendered images by creating an ambient occlusion pass.
This is created by darkening surfaces that are adjacent to others, and creating a shadow around the area where they meet, simulating the effect of a large diffuse light source. Although there is no actual lighting in the scene below, the technique creates a sense of depth and solidity, illustrating perfectly how shadows contribute to the illusion of depth and form. Darkening areas where surfaces meet in this fashion will really help any image, whether it is a 3-D render or a painting.
In diffuse lighting this is the main way we have of reading forms: as objects get closer to each other, less light is available from the environment and so the surfaces darken. The form of the
grapes (right) is perfectly defined by this shadowing. It is also worth noting how the more obstacles there are in the path of the environmental light, the darker the shadows become. The grapes that are behind or underneath others are darker than those that are on the outside of the bunch
BASE SHADOW
It is also very useful to give objects a base shadow, since most things do not sit perfectly flush to the ground, but will have rounded edges at their base that will leave a gap where a shadow will form between them and the surface on which they are resting. This effect is most pronounced in diffuse lighting, where it will often provide some of the strongest shadow in the image.
This CAD rendering has no light sources, but the inclusion of shadow creates the illusion of depth.
These rounded grapes give the perfect ] illustration of how surfaces darken whfl they get close together.
ABOVE Occlusion can happen over large surfaces as well as small ones: notice how the ceiling gets darker as the walls converge. This creates a gradient over the walls and ceiling which helps to convey the depth of the space.
ABOVE Note the base shadow below this cup, which is the darkest area in the whole image. If you look at the image below left you will find a similar base shadow below the plant pot.
LEFT The effect simulated by ambient occlusion can be seen here. Note how the areas on the sofa and the plant that are in close proximity to others have shadows near the points of contact. This helps to give a sense of solidity and form despite the very diffuse lighting.
■ Light source
The most important factor in determining the appearance of shadows is the size and distance of the light source: a small or distant light will cast hard-edged shadows, whereas a large or adjacent light will cast soft-edged shadows. The reason for this is that the rays of light cast by the small or distant light will be more or less parallel, but in the case of the large and adjacent light there will be considerable overlap of the light rays.
This will affect form shadows, cast shadows and the texture contrast, as illustrated in the tree trunk render on page 55. Larger light sources provide soft, diffuse light, which creates soft, diffused shadows. The quality of the light, whether hard or diffuse, has a big impact on the overall appearance of any scene, its effect on shadows being the most pronounced result. The overall aesthetic quality of any image will be greatly influenced by the quality of the light, and by the resulting shadows. Many studio photographers make use of large softboxes to create soft lighting, since it is very flattering and avoids strong contrast and dark shadows.
When recreating natural light, the
sun is a very distant light source, which casts hard shadows, and the higher up the sun is in the sky the harder the shadows will be. There is some softening of the edges on the sun's shadows, as it does cover about half a degree in the sky: the further the shadow is from the base of the object the softer its edge becomes, and as the shadows get longer when the sun is lower in the sky this softening becomes more pronounced.
With a small or distant light source, the rays of light have little or no overlap, giving the shadow a crisply defined hard edge. The darkest part of the shadow, known as the penumbra, covers most of the area and the lighter, softer part of the shadow (known as the umbra) is found only at the far fringe. The umbra increases in area as it gets further away from the shadow-casting object
Small or distant light source
Large or close light source
However, with a large adjacent light source there is far more overlap of the light rays, which results in a shadow with a much softer edge. In this case the umbra becomes much more prominent, as the edge is being filled by overlapping rays from the larger light, and it is the penumbra that is less prominent. The larger the light and the closer it is to the object, the softer the shadow becomes as more rays are able to overlap into the shadow area.
As the distance of projection increases the shadows can become very soft indeed, even in harsh midday light. I This softening is not a uniform blur, but is much closer in appearance to the optical blur produced by a lens, so the soft forms become rounded, resulting in spots of light and shadow.
During a partial eclipse of the sun the rounded spots actually become crescents, as the pinhole projection of the sun changes shape.
This image makes use of relatively hard light from a setting sun. Note the hard edges to the shadows, the sharp highlights and the clear demarcation where form turns from the lit to the shadowed side.
This image taken in overcast daylight has a very different character - there is no sharp highlight, shadows can be found only in the deepest recesses and the form turns very gradually from lit to shadowed.
ABOVE Note how the shadow is very crisp at the base of the pot, becoming softer as the shadow recedes from the plant.
BELOW In this image, captured during a partial eclipse of the sun, you can see how the gaps between the leaves become crescents instead of spots. They are, in fact, projections of the sun's image.
ABOVE In this shadow (top) the gaps between the leaves have become spots of light, with the small apertures acting as pinhole cameras and projecting an image of the sun onto the ground. Compare this with the shot taken from directly above with the lens thrown out of focus (above), and note the very similar optical effect. This looks nothing like a uniform blur, but has a distinctive look of its own.
ABOVE Here the spots have softened even more, despite the midday sun, because the leaves are high above the ground, creating very soft spots. Note the contrast with the much sharper adjacent shadow.
OVERCAST DAYLIGHT
Overcast sunlight is completely different in character from direct sunlight. All shadows will be very soft since the entire sky will be acting as one enormous light source. Shadows can be so soft as to be almost non-existent, except for dark areas beneath cars or trees.
The soft quality of overcast daylight is very underrated, although most studio photography goes to great lengths to reproduce this effect, because the low contrast and soft shadowing is flattering. It is quite simple to simulate an expensive photographic studio by taking photographs on white backgrounds outdoors on overcast days.
It is worth bearing in mind that any light source, whether reflected, diffuse or direct, will cast a shadow. In the case of a fill light this shadow may well be overpowered by the main light, or be so diffuse as to be invisible. However, whenever multiple light sources, including light coming from reflective surfaces, are being considered (single light sources are very rare in everyday
life), there will be multiple shadows of some sort — even if they are very hard to spot. In many artificially lit environments these will be quite obvious, since multiple lights are the norm in any large area lit by artificial light.
There are significant differences in the characteristics of shadows cast by adjacent or distant light sources, and these are worth examining. Our only source of natural light is the sun, and it is very far away, which means that the shadows it casts are parallel (or so close to it as to be identical), but other sources of light are much closer to us and the shadows they cast radiate out from the light.
ABOVE This image is typical of overcast diffuse shadows - there are some very soft shadows behind the leaves and underneath the window but the contrast is low. Lack of glare, bright highlights and dark shadows can also increase the saturation of colours.
A typical interior lighting scheme where an array of spotlights is used to lights kitchen, resulting in multiple shadows being cast around every object, as weflfj multiple highlights on reflective surfaces.
In this extreme example of diffusion, the overcast light is further diffused by the mist and the reflective snow, resulting in the almost complete absence of shadows, except in the densest vegetation.
Shadows cast from a very distant light source such as the sun will be parallel to each other, and remain at the same relative thickness from the shadow-casting object.
In complete contrast to the previous example, shadows cast from a nearby light source, such as an artificial light, will radiate out from that source. Note how the width of the shadows themselves also increases as they radiate out from the light.
With multiple nearby light sources the shadows radiate out in multiple directions. Note the different amount of fill light received by each individual shadow, depending on its position in relation to all of the different lights.
■ Colour & shadows
Shadows are themselves illuminated by what is known as fill light, usually a secondary or reflected light source.
In the vast majority of cases fill light will be coming from a soft and diffuse source, such as light reflected from a wall or the ground, or from skylight. This means that even in harsh sunlight, the shadow side of a form will be lit by diffuse light. Shadows are rarely black, but will generally contain some light and colour, which tends to be soft and muted.
In stronger light the contrast will tend to obscure the shadows, and in the case of photographs these can then become almost entirely black: our eyes are able to discern far greater levels of contrast than current photographic
methods can reproduce. Even to the naked eye, though, the relative colours within a shadow will tend to be muted and homogenized, even if overall the shadow is deeply coloured, for example the blue shadows created by skylight.
TRANSPARENCY & TRANSLUCENCY
Transparency and translucency can also have a major impact on shadow colour, in two different ways. First, the shadow-casting object may be transparent or translucent, and will cast a coloured shadow as a result. The most obvious example of this is glass, such as stained glass, casting what is essentially a projection of its image onto the walls or floor. Second, if the surface in shadow is translucent, light from within the surface can then emerge and affect J colour of the shadow. This is the pj nomenon known as sub-surface scatt J ing in CG terms, and is commonly® in human skin, marble, liquids su^H milk, foliage, or any other substance # through which light can travel. Tt® materials should have enough satujjJ tion in their shadows to represent this extra light emerging from within the surface itself. The effect of this on h® man skin is covered in more detail UtJ (see Chapter 11).
Within the shadow, colour relationships tend to be muted and colour is homogeneous because the shadow is overall a bright blue, overpowering the local colours of the surfaces. The contrast with the strong light nearby exaggerates this effect.
The sun is so weak that the blue fill from the sky isn't that much darker. In this case it is more helpful to think of two distinct light sources (the orange sun and the blue sky), rather than shadow and light. Both are very strongly coloured but there is less tonal and colour variation overall in the blue shadowed side due to the lower contrast from the skylight.
The effect of translucency on the shadows is very clear indeed, since some of the grapes are in direct light and others are in diffuse fill light.
The coloured glass projects its colours as shadow on the window sill and the far wall.
■ How we perceive colour
Benjamin Thompson, also known as Count Rumford, was the first person to notice that colour perception can be influenced by psychological factors. He noted that in strongly coloured lighting shadows took on the complementary hue of the predominant colour, and only by observing the shadow in isolation from the primary light colour — by looking through a tube — did he realize that the effect was an optical illusion. Not all shadows will therefore take on the complementary hue of the light source, but in very strongly coloured light this does take place.
There are also many physical factors affecting the colour of shadows. For example the colour of the sky will have a much greater effect on shadows than will optical effects that may take place
in our minds. This subject is examined in greater detail in Chapter io.
It is important to realize that there is often a lot of colour found in shadows, and just as our minds are capable of adding imaginary colours to shadows in strongly coloured light, they are also able to ignore them in other circumstances, and nothing will make an image look more dead than grey shadows. You should carefully consider the coloration in shadow areas, whether you want to reproduce nature or make a more psychological use of colour, because colourful shadows will lend a lot of life and realism to an image. Colour can be found in shadows for many reasons, be it strongly coloured light from the sky, an optical effect, or light scattered through skin or some other translucent material.
This lamp has a purple shade, which ff-.J a very strong tint to its light: note the complementary green cast to the shadows which is probably a very similar phenom-1 enon to that observed by Count Rumford. j
