Translucency: Guide to Semi-Transparent Materials

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Introduction

Welcome to a tutorial on objects that transmit light! This tutorial will briefly cover the science behind these kinds of materials, teach you how to analyze and visualize your illustrations, and walk you through three different examples. For our final section, we will put your new skills to the test on a creature design! I hope you enjoy, feel free to leave questions in the comments section.

What is Translucency?

You may skip this section if you like, but it will help to know what you are actually trying to paint/draw!

Transparency is often used to talk about anything that allows light to pass through it. However, technically speaking, transparency only refers to things you can see shapes through clearly. To clarify, first, lets get a black piece of paper with holes cut out so that light can shine through them as pictured below:

Now imagine a hard piece of plastic that gets more transparent towards the left side and we get the following:

The left side is transparent, you can see the outline of the shape clearly and there is very little color change. As you go right, the shape becomes less clear and the color is changed to a more saturated red. On the third leaf, you can no longer make out the shape at all and there is just a bright red glow. Finally, even this glow is blocked out and the material is completely opaque. These semi-transparent materials are called translucent, and they can affect the images that pass through them, often making them fuzzy or extremely discolored. The left leaf (total transparency) is only possible in objects that have specific molecular structures such as crystals or liquids and are very pure so that nothing inside interferes. It is important to note that these are still capable of changing the color of the light that enters (think about yellow oil or blue water), but this is due to the molecules themselves, not their structure. Relatively few substances are actually like this however--in the real world, most are impure and have structures full of defects. This causes light to interact more with the particles inside objects. Below is a rough illustration of the molecular structure of a transparent solid, one with impurities, and one with structural defects:

The last two have particles that SCATTER the light in multiple directions, which forces it to interact more with the particles inside resulting in a strong color change. Additionally, the total amount of light that is able to continue is reduced and less focused because some of it was scattered away! As these "imperfections" get denser/more common, the object becomes more opaque. To picture how this would look in real life, picture a glass cylindar and a blue plastic cylinder. If a light shines directly down on them, all we would see in the glass is the reflection. On the plastic, however, we would "see" the light beam shining through the material because light is being scattered towards you!

Though complicated, this is a very important subject to learn because there is literally never a moment something translucent isn't close by! You may be familiar with things such as juice, glass, plastics, and jello, but translucent materials include your own skin and blood, the air you breath (think atmospheric perspective), plant leaves, and even more.

Surfaces: Form and Texture

The key to understanding translucent objects is to study their surfaces. Their forms, textures, and the position of light sources all affect the overall appearance because they affect how light interacts with them. Once you can estimate how light will travel within the object, there are some simple rules you can follow to illustrate them.

Form: Reflection and Refraction

NOTE: Since we aren't as concerned about surface reflections and refraction of images in this tutorial, we won't be covering how to reflect/refract secondary light sources. There are many tutorials that cover this subject much better than I could anyways!

In objects with any kind of transparency, put simply, light changes direction at the surfaces by either bouncing off or entering the object. When light hits the surface above a certain angle, it will continue inside the object at a different angle (this is called refraction). If light hits a surface below a certain angle, the light cannot enter and bounces off at the same angle (this is called reflection). Below you can see the yellow light refracts and the blue light reflects. The red line indicates the angle that this will change.

This is rather easy to predict on objects with flat surfaces like the rectangular prisms below. The top prism is reflecting sunlight, which falls in parallel lines to the Earth. This makes our jobs easy as all lines will have the same angles when they hit the same surface. The bottom prism is reflecting a common light source like lightbulbs which emanate light in a sphere around themselves. Because of this, we need to trace several different light rays to determine where light will travel. The uppermost one is nearly parallel to the surface it hits and will reflect. The middle 2 rays both hit the surface at a great enough angle to refract into the object. The right one will hit the bottom surface at an angle that will let it reflect again towards the eye! The last ray will reflect off the right surface and hit the ground.

It is important to take note of this: light can reflect inside objects as well. In objects with two parallel surfaces that are close together, light can continuously bounce around inside until it reaches a perpendicular surface that allows it to refract out.

Round objects, which we see the most in nature, are a little more complicated: because the angles of surfaces are constantly changing, light coming from the same source can be refracted and reflected in vastly different directions. This causes effects like magnification and distortion. Additionally, given two mirrored surfaces, as we often find in containers and layers that wrap around other objects (skin for instance!), the same effect of continuously reflected light can occur with an interesting difference. Since the surface can curve from perpendicular to parrallel to perpendicular in direction to the original light source, some light rays can enter one part of the surface, bounce around inside, and then exit another part of the same surface!

NOTE: In translucent objects, this is what commonly results in subsurface scattering.

In the below image, if we take two hollow spheres, one of glass and the other of dyed plastic, and a light source, we can determine what we will see in different positions.

By tracing a few rays (bright yellow), we can see there are regions of the sphere that will reflect light (green), bounce light around (teal), regions that wont have much light interaction (orange), and a region that llight will be relatively unaffected as it passes through (yellow). From this, we can detemine how these spheres will look from the position of the top eye and the bottom eye.
The top eye will be able to see a reflection on the left side of both spheres, and will also be able to see the light passing through the subsurface of the plastic sphere as it gets scattered. Finally, it will also see the light passing through the opposite side of the plastic sphere (the yellow region).
The bottom eye won't be able to see any reflections, but will be able to see the subsurface scattered light. In the glass this will be a darker teal and in the plastic it will be a dark and saturated blue. Finally, we can also take note of the orange regions that result in a dark ring in both spheres surrounding the brightest part, a filtered view of the light source itself. The center of this circle in the plastic sphere will be the least saturated as this is the least affected light coming from the source. It is also extremely helpful in different situations to think of it like this: the lit portion is shining through the unlit portion. So we are technically seeing the lit section THROUGH the unlit side which will scatter the color.

You can also practice this method in reverse:
1. Imagine a line from your eye directly into the illustration at a certain point on a surface of the object.
2. Estimate the angle that line will bounce or enter from on the surface you are looking at.
3. Color in a region of the object that will have about the same angle for your reference.
4. Does the line hit a light source? Would the line pass through an interior surface? If it were to bounce off a second interior surface, where would it go? Does the line bounce directly towards the light source(s)?
Imagining this will help you understand what image your viewer would actually see at different angles across each surface. None of this has to be perfect! As long as you make assumptions that feel correct, you will be able to achieve the correct effect! It may take (a lot...) of practice, but soon it will be easier to do instinctively.

Finally, we must consider one more possibility: a translucent material covering an opaque one. A perfect example of this is skin. We have yellowish-white cells that are able to transmit light. Additional pigment called melanin affects this color and creates different tones of tan (or freckles). Just under this skin is our blood and muscles, which, for simplicity, we will consider to be an opaque subsurface. When light shines on our skin, some enters it and bounces between the tannish surface of our skin and the red subsurface resulting in subsurface scattering that steadily turns the light redder. As you can see below (not to scale!).

You may be thinking, "Wait! That doesn't look like skin!" and you'd be right! This is why we have to consider texture in our process. As you will see below, the texture of the object can affect how well you can see scattered light.

Texture

Due to their nature, translucent objects tend to have three common surface textures:
• Smooth--any liquid, glass, plastic (mostly), and polished crystals will have surfaces that are almost completely free of bumps.
• Striated--because of the way crystals grow and break, they sometimes have parallel lines running across the surface. You may occasionally see this on other materials but it is more likely to be man-made or circular.
• Rough--this is most common on near-opaque objects such as skin or plant tissue due to their cellular make up. Otherwise, damaged materials or crystal with a complex surface may have this texture.

There are many more, but these will serve our purposes just fine. We have already assumed a smooth surface when we covered the forms, so now we need only to consider striated and rough surfaces:

Striated surfaces reflect light from the source more periodically. Because of this, less light will be able to enter the object itself and reflected light will make the surface appear more solid. Remember that reflected light will always overpower interior scattering.

Rough surfaces give the object a solid appearance as light isn't reflected as regularly and bounces off much more like it would off an opaque object. For this reason, it is easier to visualize this with a beam of light hitting the surface. We can assume that majority of it will bounce off the surface in all directions and the rest will enter the surface.

Compared to other textures, much less light is able to enter these surfaces and, as such, this scattered light is not often visible on the lit portion of the object, only apparent on the edge of shadow or through the other side. Additionally, because these are usually made up of cells, the interior is comparatively dense which causes the most extreme color shift and an inability to penetrate very far if the object is thick.

So what happened to our skin example earlier? We simply diidn't add surface lighting! This light is so bright that scattered light only has a chance of being visible at the edges of the shadow. This also happens to be along the routes of continuous reflection which makes it the most saturated and red. If we only render the top surface at first, we would see this:

Here, the shadow is actually darker than the subsurface light. So, if we erase a bit of this layer...

Now that looks more like skin! It is because of this effect that we can follow these general rules for shading translucent objects: if it is completely opaque, you will shade in a straight line down the color wheel without changing saturation. If it is translucent, then you should adjust the saturation near the edge of the light as in the middle example below. In special cases, like skin, there may be a subsurface that is a different color. In this example, we need to increase saturation AND adjust the hue towards the color of the subsurface (in this example, yellowish green).

RULES (Skip to here if you prefer a quick overview)

So what do we do with all of this information? Using what we know, we can make practical decisions about illustrating these objects. The ones I personally follow are as follows:

1. Reflections and surface lighting will always overpower interior light from scattering.
2. Surface lighting will only occur if the surface has a texture that isn't smooth. Reflections will only occur on surfaces that aren't rough.
3. If a lit surface is overlapped by an unlit surface (consider the plastic sphere example), the lit surface will be visible but scattered through the unlit surface. This only applies if the thickness and opaqueness are small.
4. The less light travels through the object, the brighter and closer it will be to the original light color. Pay attention to the parts that light will shine striaght through.
5. The more light travels through the object, the dimmer and more saturated it will be. Pay attention to the parts that will be reflected through long paths.
6. The major paths for light will glow and light up the surrounding material.
7. Planes parallel but on the opposite side to planes being lit will receive the most transmitted light. Again, pay attention to where light will shine straight through.
8. If there is a subsurface, you can assume light will bounce along the edge of the object until it becomes visible where the light source no longer hits the surface--the edge of the shadow.
9. The distance light will travel through a translucent object depends on how intense the light source is and how dense/opaque the material is--if it is thicker, it'll be darker and vice versa.
10. Always render the lowest surface first. You can use layer effects for the overlapping materials.
11. Corners "gather" light--light will bounce around more often in tight spaces and so will often light up corners where it is the tightest. Make corners brighter!
12. Use dark/midtone backgrounds! The most common use of translucent objects are in dark spaces where their effects are most obvious, so it helps to plan for this.

13. PLAN PLAN PLAN!!!! Use your sketching layers to mark out where light will shine on the object, think about the planes and which ones are parallel, draw out the paths light will take, and anything else that might help you render it!

Projects

We are going to start with three simple projects, a crystal lit by fire, a potion in an auburn bottle lit by a point light, and a crystal skull lit by the sun.

NOTE: I am going to be using a painterly method with the water color brushes (they are amazing), but you can apply this to different styles that involve linework as well.

First, start by sketching out your designs. I prefer to use white on a dark background so it is easier to imagine the final product.

Second, for this project at least, I want you to analyze the planes of your designs! Use a different color for each plane, it doesn't have to be accurate but it will help you out.

Finally, for one last analysis, check where the light is going to hit the upper surface of your objects and draw rough lines on the paths that light rays are most likely to take. For example, on my skull, they are mostly going to go straight through, but there are going to be some travelling along the edges and exiting where the surface is no longer lit.

For the rendering, I tend to use the following process:
1. Sketch/Plan
2. Flat Colors
3. Gentle Glow--add a faint glow along major light paths, brighter nearer the source.
4. Detailed Glow--analyze the planes that will emit the most light. Also pay attention to any overlapping surfaces, etc.
5. Edges--since corners gather more light, I will make these brighter. Also, if I feel like there are more spots that need less saturated light, I add these in.
6. Surface Light--make sure you know what surface texture your object has and add in reflections or surface lighting if relevant. Make sure these overpower any scattered light glows.
7. Polish--I personally tend to duplicate the surface lighting layer and add it as a Soft Light layer on top. This improves the saturation and allows me to go in and adjust the saturation of scattered light as needed.

You can see this process take place in this skull. 1. Sketch 2. Flat color: a midtone of the blue I want. 3. A faint glow along the path the light will take the most. 4. Adding detailed glowing in the back of the eyes, through the nose, cheek bones, and along some of the teeth/chin, all of which are roughly parallel and opposite planes from those of the back of the head. Note that I am using very saturated colors for this part and I reduce the brightness of planes that aren't completely parallel. 5. Edges/corners get a little brighter which helps get rid of the sketch lines without making it look weird, though I honestly may have added to much. 6. I wanted the surface to be just a little rough, so I added the highlight and some surface lighting. 7. I made saturation adjustments across the whole thing after adding a duplicate of the surface lighting layer and set it to soft light.

For this one, you can see the steps I took, but I worked very quickly with 1.-4., as I knew I wanted a striated surface which, when lit from the front, would block most of the glow from being seen. I also kept the interior lighting darker to make up for this surface, just for practice. For 6, I drew a reflection of a fire on the most direct surface and then did the same with less intensity on the other three surfaces that face it. Then, to show the striations, I erased parts of this layer with parallel lines across each surface.

Once again, the same process, but this time, I have to take note of an overlapping surface! I decide to tackle this issue on the detailed glow step (4). Since the liquid is sloshing around, I made part of it slide up the glass and overlap part of the surface that is lit by the light. This would be lit the brightest when it is perpendicular to the light rays, so I started with a bright curved edge and made a curved gradient towards the edge of the liquid. I adjusted this appearance with the edges layer (especially on the edges of the surface to make the difference between air and glass more apparent) and added a desaturated glow on the bottom of the liquid since this will recieve the most light with the least interaction. Finally, because this is a smooth surface, I only added highlights and then polished the saturation!

This process was a little more complicated. First, I started with flat colors, drawing a shape of the bottle underneath the liquid. Then, I partially erased the center because this area will produce the least light/saturation. Then I outlined the thickest part of the glass and popped in a cork. To make this look a little more realistic, I went ahead and added some red lighting under the liquid to the thick part. Next, I duplicated the bottom layer, moved it above the liquid, and set it to multiply. This produced the effect of a dark brown bottle. Finally, I wanted the surface to be scuffed, like it has been through a lot, so I added a somewhat saturated highlight and some minor surface lighting with a textured brush. Finally, I polished it and added some saturated highlights to the screw and back edge of the bottle, making an assumption that some scattered light would make its way behind the bottle. PHEW!

THE ULTIMATE TEST!!!!

It is time to come up with a creature design! I want you to create a creature based on a land animal. It should have an odd colored skin and a jello-like coat that wraps around its body that will be translucent. I suggest you use a texture that is either smooth or at least partially smooth. Feel free to add in bones, organs, or anything you want underneath this layer, just make sure you get to practice working with a subsurface that is a different color. As an example, here is my interpretation of this prompt:

My process was a lot simpler for this one, but was a true test of my knowledge. I sketched it out, did some white line work (that I eventually removed), and then flatted it. I then rendered the blue skin which I wanted to reveal only a few bones (organs are scary...) and added some additional shading. Then I added green over the skin, erased some areas, made some of it more blue, removed some color along the edges followed by a green highlight, and then added the final highlight that made it look very smooth. Finally, I decided I wanted some variation in the tanslucency and added the appearance of some bubbles and stripes under the skin with bright green on a hard light layer.

I hope you enjoyed this tutorial and are able to put your new skills to the test! Good luck with your creature designs and other translucent needs.

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