--- title: "Transparency and Translucency" canonical: "https://documentation.chaos.com/space/THEORY/116719721/Transparency%20and%20Translucency" format: markdown --- In computer graphics, distinguishing between transparent and translucent objects helps to achieve photorealistic visualizations. This article delves into the basic concepts of light to explain this distinction. Through the help of simple illustrative examples, it also explores the V-Ray features that make the accurate replication of transparent and translucent objects possible. CG Light Basics Light rays behave differently when coming into contact with transparent and translucent objects. To understand this interaction, some fundamental knowledge of light physics is needed. For the purposes of this article, and in the context of ray tracing, we’ll look at light as an electromagnetic radiation propagating along rays in vacuum/air until it reaches matter. A light ray hitting a surface is called an  incident ray , and the angle at which it hits the surface is called the  angle of incidence (see the image on the right) .  Upon hitting a surface, the light ray can interact with it in two ways: it can get  reflected  or transmitted ( refracted ). Reflected rays bounce off the surface, altering their trajectory at an angle equal to the angle of incidence; while transmitted (refracted) rays pass through the surface and bend at an angle, i.e. they change their direction. Once light is refracted, it may get absorbed  and/or  scattered . Absorbed light is the energy that is lost at the interface (the point of the surface hit by light where a ray is either reflected or transmitted). It is transformed into another form of energy, typically heat, and its intensity decreases. When the light goes through the surface and gets refracted and spreads in different directions without changing its intensity, we have scattered rays. Scattering often occurs when light interacts with uneven and heterogeneous media. If there's subsurface scattering, some light may reflect back from the same side of the surface it hit (potentially from a different surface point), some may be completely absorbed, and another portion may transmit and exit from another side of the shaded object. reflected rays refracted ray absorbed rays scattered rays The bending amount of transmitted light is defined by the index of refraction (IOR) . IOR quantifies the speed of light inside a medium, i.e. it shows how much the light slows down inside that medium. For instance, water has an IOR of approximately 1.333, meaning that light travels 1.333 times slower in water than in vacuum/air. Depending on the amount of transmitted light and its propagation, objects can be classified as transparent/refractive, translucent, or opaque (which absorbs and/or reflects all the light and does not allow it to pass through). Although opaqueness is not the focus of this article, it plays a role in classifying objects based on their light interaction behavior. Transparency vs Translucency Transparent Translucent Light behavior light bends and passes directly through light bends and scatters as it passes through Details details of the other side are sharp details of the other side are blurry Transparency A transparent medium is homogeneous. When light passes through a transparent surface, it moves through it continuously in a straight line with practically no scattering. The transparent surfaces look clear or see-through. To achieve the look of a transparent surface control mainly the reflection and refraction parameters of the V-Ray shaders, such as color, IOR, and glossiness. See the examples below for more information. Examples of transparent objects: cellophane, water, glasses, clean air. Note that some instances of thin tinted plastic/glass, or sunglasses can also be considered as transparent objects due to the negligible scattering that appears inside them. In V-Ray, such objects are created as transparent shaders and rendered much more efficiently in this way. All the glass cups are see-through, even those that are tinted in green and pink. In the right pink cup, the back cup's handle is visible, as well as part of the green cup. The glasses are transparent, the nose pads and temples are clearly visible. There is also a small reflection visible in the bottom pair of glasses. Translucency A translucent medium allows only some of the light to directly pass through it. The light rays that pass through are diffused and can bounce and get attenuated multiple times inside the object until they exit. You see fuzzy, unclear images through translucent objects. IOR can vary in a heterogeneous translucent medium. This affects how light travels: light bends when IOR changes gradually, but scatters when IOR changes abruptly. Scattering in different directions is usually non-uniform, often resulting in forward or backscattering, or complex distributions with "spikes" in certain directions. Scattering with front lighting i.e. the light source and the camera are facing the same part of the object Backward Scattering Isotropic Scattering Forward Scattering The orange arrow represents a ray of light going through the volume and the possible scattering directions for the ray. Scattering with back lighting i.e. the light source and the camera are opposite to each other and are facing the opposite parts of the object Backward Scattering Isotropic Scattering Forward Scattering The orange arrow represents a ray of light going through the volume and the possible scattering directions for the ray. You can find examples of scattering here: Smoke Color Rollout In general, most media both scatter and absorb light to some degree which affects the appearance. V-Ray provides numerous methods to recreate translucent objects: from controlling specific parameters of the main VRayMtl, to using specifically designed materials and a combination of materials and textures. See the examples below for more information. Examples of translucent objects: paper, milk, jade, marble. Translucent curtains that are lit by the sunlight from behind. The objects behind the curtains are visible to some extent. Translucent book pages, the letters on the next pages are visible yet not fully distinguishable. Examples The following examples show 3D representations of transparent and translucent objects rendered with V-Ray. The examples show different V-Ray shaders and their parameters used to achieve the effects. Example: Transparent shaders Soap Bubbles Soap bubbles might appear transparent with some tint. Such an effect is achieved by controlling the Thin Film options along with the Reflection , Refraction and Diffuse parameter groups of the versatile VRayMtl. V-Ray also offers ready-made presets for soap bubbles. See the Iridescent surfaces tutorial for more information on how to create transparent soap bubbles. Example: Objects that can be either transparent or translucent Pure glass Pure glass is transparent, it allows light to pass through it. The effect can be achieved using the Refraction and Reflection parameter groups of the VRayMtl. A white or very light gray Refract color makes the material transparent. The glass IOR is usually 1.5-1.6. The Reflection and Refraction Glossiness parameters could be tweaked as well for a more realistic glass appearance. Tinted glass Tinted glass can be either transparent or translucent. Any Refract color different from white and black gives the transparent glass a tint, e.g. a green Refract color gives a green tint. Such glass allows the green light to pass through, while absorbing predominantly the non-green wavelengths. V-Ray also offers a group of parameters that control the level of translucency , if the final goal is to make a translucent glass shader. See the Glass tutorial , to learn how to create standard glass and transparent tinted glass shaders. See the Translucency tutorial , to learn how to create translucent shaders. Example: Translucent shaders There are numerous ways to create translucent shaders with V-Ray. Using the versatile VRayMtl is the most straightforward and common method. This material offers a group of translucency options for achieving the effect of plastic, wax, marble, skin, juice and other translucent liquids. These options control the sub-surface scattering of the light rays. The translucency options must be combined with the Reflection and Refraction options. Plastic As already mentioned, the VRayMtl is perfect for creating plastic. However, achieving a translucent plastic appearance is also possible with the help of the VRayFastSSS2 material. See the Translucent Plastic tutorial for more information. Leaf Depending on the angle of view and the lighting angle, leaves react differently to light. When lit from behind, they look translucent as they disperse the light. In V-Ray, this effect can be easily achieved by enabling the Thin-Walled option of the VRayMtl and tweaking the material's translucency parameters. However, V-Ray also offers an alternative way to simulate leaves’ translucency - by using the VRay2SidedMtl and modifying its Translucency feature. Both approaches are useful for recreating other translucent objects such as fabric curtains, paper or paper lanterns, etc. See the Leaves tutorial to learn how to create a leaf shader. Skin Skin is translucent to some degree, and the effect  can be clearly observed at the thinner parts of the human body, such as ears and finger tips. This effect can be achieved with the help of the VRayAlSurface material. It allows for finer control on the level of sub-surface scattering. See the  VRayAlSurface material page for more details on its parameters. References Here is a list of references about the translucency and transparency comparison: Adobe, The PBR Guide by Allegorithmic - part 1 : Wikipedia, Transparency and translucency :  Naty Hoffman, Background: Physics and Math of Shading , 2013:  https://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf