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By  William Li, Gallery 6.
Transparency
© 1998 William
Li, except where stated otherwise.
Overview
In viewing the images of the previous chapters, didn't
you think sometimes that they were about glass objects? That's not
very surprising, since glass is also highly reflective.
In this chapter I will first demonstrate a method
in Part 1 to quickly achieve transparent looks on objects. No explanations
will be given here.
If you really want to understand transparency
you can read Part 2. We will have a look at light behaviour and at some
rendered images.
Understanding transparency will help you in drawing
transparent objects, but it will not make it any easier. The advantage
of this added knowledge however, is that you can draw much more oddly formed
transparent objects ;-)
Orbs
| Consider
this background (a test pattern):
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To draw a transparent object
it is obvious that you see through you object.
In this example I will use an reddish oval orb:
a red ruby.
As you can see the background influences the red:
it's not all red you see, but a mixture of colours*.
Add your shadow to the image now, if you need one.
Adding them later will be very difficult (unless you use layers :-)).
* For more information about light
and colours, see the F.A.R.P. index about Color.
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Our light source is at the
top left.
Make a darkening gradient towards
the light source.
Most transparent objects are somewhat reflective
as well. I would advise not to draw any reflections, because they
distract the viewer too much.
If you do want reflections, add them now.
Be sure to keep them transparent.
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Put a highlight in
the direction of the light source (see Chapters 1 or 2).
Highlights are the only necessary reflection I
would put in.
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| Put a second highlight
opposite of the first highlight. You may make this one more blurred.
Voilà, your transparent orb is ready.
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Hey, how about the lens effects I see on round
objects? you ask?
Well, this is more difficult to draw. The "effect"
is called refraction (discussed in Part 2). Adding this effect to
your object will make it much more realistic. A comparison:
Many bitmap editing software have a function or
a plug-in that can do this effect for you, e.g. in Photoshop this
effect is called Spherize. If you have the possibility to use this
effect, do make use of it!
Please note: this is for round objects
only! It should be quite obvious that you can't use this effect
for blocky objects.
If you're drawing with traditional media or you're
drawing a transparent object that's not like a ball, you can still suggest
or simulate this refraction. For this I'd recommend to read Part 2. There
you will find lots of reference material to simulate this refraction. Or
you can collect objects (transparent bottles, orbs, rods etc.) for reference!
Blocks
In round objects like the orb above, the underlying
image is warped or bent. In blocky transparent objects the underlying image
is shifted.
If you're looking straight on a flat blocky object,
there should be little to no shifting.
Because this effect is pretty simple to achieve
I refer to part 2 below. In part 2 you'll see some rendered samples of
blocks. If you want to draw better "shifts" take a good look at how the
lines shift.
— • End
of Part 1 • —
| Part
2: Understanding transparency
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Traveling light
Transparency is the property of a material to let
light pass through it.
When light rays arrive at an object, they do the
following:
-
one part of the light rays bounces back from the material,
-
one part gets absorbed,
-
the rest passes through the material.
While passing through the material these light rays
can get "filtered". This means that only a part of the light spectrum actually
passes through resulting in a particular color. You can find a elaborate
description in the F.A.R.P. main index, called Color.
Light interpreted
What do we actually see when we look at a transparent
object?
2.0 — three types of light rays
reach our eyes
The image above shows the types of light rays that
makes us see the object:
-
Light bounces off the surface of the object: we see
(the reflection on) the surface of the object. This is already
discussed in the previous chapters.
-
Light from behind the object passes through the object
and reaches our eye: we see through the object. So we conclude
that surface A is transparent.
-
Light reflected on the inside of the object makes
us see surface B: we see inside the object. Now we conclude
that the object has two surfaces (A and B). There are lots more reflections
inside the object giving us the impression of the structure of the object.
Refraction
In figure 2.0 you see that I've drawn ray numbers
2 and 3 as a crooked light rays at entering or exiting the block. This
is called refraction. Refraction means "bending" of the light
ray. The ray's direction changes when it travels through different materials.
In physics the level of refraction is indicated with a refraction
index. Each material has its own ray bending characteristic. In
space (vacuum) rays don't get bent; the refraction index of vacuum is exactly
1. The higher this number the more a ray gets bent.
In the previous chapter I introduced the surface
normal, which indicates the direction of a particular spot of the
material's surface. The refraction
index determines how incoming rays
will be bent towards this surface normal.
2.1 — Refraction index and the
surface normal. Left: "no" refraction. Right: refraction.
Below are some examples of different refractions.
Please don't mind the shadows. They are not realistic (a problem
inherent to the ray-tracing theory). To make the materials comparable I've
made them colourless. You'll also see names with the images so you can
relate better to the refraction index.
Please study the images. Compare and look for the
differences, see how the lines behind get distorted. You can use these
images for reference!
After you've had a good look at the images, I'll
explain what the refraction index does in light bending.
Block
Block refraction: explanation
The bending of light determines what you see on
the faces of a transparent object (quite obvious, right?).
I'll be using the ray-tracing method: a ray starting
from your eye to determine what you're seeing.
2.2 — Refraction index = 1
In figure 2.2 you see a block
with refraction index 1. Rays pass right through it. This can only happen
in vacuum (space) of course, but for our demonstration this shows the difference
better. Even the air around us bends light! (refraction index = 1.003).
In this case our eye will see a piece of the green
block through the front surface.
Try to figure out what you will see in the top
surface. This should be very easy ;-).
2.3 — High refraction
Figure 2.3 shows a highly refractive block.
The refraction is so high that we see the bottom surface of the block squashed
to just a strip. And through the front surface we can even see the
red ball on the other side!
Try to figure out what you will see in this top
surface.
Compare this refraction construction with the computer
rendered blocks above.
A surface can show:
-
a pure reflection,
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a pure refraction,
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reflection(s) and refraction combined.
The internal reflections make it difficult to determine
what's reflection and what's refraction. However, using this information
should enable you to discern between them.
Drop shape
Again: Please don't mind the shadows. They
are not realistic (a problem inherent to the ray-tracing theory).
Drop shape refraction: explanation
figure 2.4 — Reflections of highlights
| Figure 2.4 shows how you
get to see the double highlights in transparent objects:
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The primary highlight is the one reflected off the
outer surface.
-
The secondary highlight is the one that is reflected
off the inner surface of the object. Due to refraction this highlight may
not be where you would expect it.
Logically if you have a really weird transparent glob
(some multi-orbed object) you could also have more than two highlights.
It all depends on how the light is refracted inside and how it gets out
of the object.
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Fresnel effect
This is an effect for transparent materials introduced
by the French physicist Augustin Jean Fresnel (pronounced fra nel).
Basically the effect results in the following:
The more perpendicular you look at a surface,
the more transparent it is. If you'd look at 90° on a surface, it should
be transparent (at the maximum level of the material). If you'd look to
the surface at an angle of e.g. 10° it should just only reflect or
glance off surrounding light. Often you can see the Fresnel effect on open
sea.
The image illustrates the effect.
figure 2.5 — viewing angles on
a surface.
figure 2.6 — demo Fresnel effect
In figure 2.6 you see a drawing using the Fresnel
effect. You can see stick in front go through the water. The reflection
may be a bit confusing though. The underwater part of distant stick should
not be visible. Only a reflection on the water shows. You may also notice
that the water in front is darker. That is because that part is transparent
(it's darker underwater).
Notice also that the underwater part of the stick
in front is distorted due to refraction.
— • End
of Part 2 • —
Conclusion
This concludes transparency. I have shown you several
aspects to keep in mind when drawing transparent objects. Transparency
alone is not enough to make a material look real. You should add reflective
properties to it as well.
You have noticed that drawing refractions in nigh
impossible, but...
here's the big eye-opener:
Because it is so difficult to comprehend and imagine
refractions you usually do not have to construct them. Just
fake them. Nobody will be able to verify that what you've drawn is correct
or false! :-D
Using the method at the beginning (orb)
will often suffice. If you'd make a roundish bottle, it's perfect. If the
bottle is cylindrical use the highlights as in a reflective cylinder with
the inverse shading shown with the transparent orb.
Only when it is too obvious you should pay attention
to refraction, e.g. in case of a magnifying glass. For these specialistic
purposes I have provided the reference images above which you can draw
from.
Anyhow, don't feel daunted by this tutorial. It's
meant as a guide and a reference. Make your drawings, compare them with
this information or photographs and see what you can learn from it.
And have fun while at it. That's the best way
to learn drawing!
To chapters:
-
Introduction
-
Fast metal
-
Understanding and refining
reflections
-
Transparency
 Back to the FARP main page.
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