This discussion covers Blender 2.79. I assume you are comfortable with the user interface, know how to model, and have some understanding of the node editor and creating materials in Blender using cycles.
I posted a render of a flashlight to a Facebook group. The most challenging feedback was that the glass looked wrong. I have tried to improve my glass material, but I’m lost. Here is my story.
I made a photograph of a fountain glass in front of a window, sitting on lazy Susan, lit from the side by a 10000 lux full spectrum lamp, at night. Because I took the photo, I know the camera sensor, focal length, focal point, and exposure time.
I then made correct scale models of the lamp, the window, the glass and the lazy susan, placing each in the positions they occupy in the photograph.
Next, I positioned a blender camera in the same position as the real camera.
The objects are precisely modeled, even the thickness of the glass is accurate. Notice that the light is out of the field of view, but is reflected in the window.
I added the coke label, a dark wood texture to the table and a light wood texture to the lazy susan. Wood textures are from Poliigon. I selected wood colors that matched the objects, but didn’t model the several varieties of wood that make up the lazy susan.
I did not add dirt on the lazy susan or smears on the glass. I planned to get the materials and lighting right and then add those. This is where I hit snags. I rendered the scene using a studio lighting set up #16 HDRI from the Zbgy studio lighting pack.
(Render details: I left the camera at the default 35mm setting, since this matched the image closely enough. I set a focal point at the bottom center of the glass and an Aperture of f1.8. I enabled Filmic color management with default settings, and denoiser with default settings. The resolution is 768 x 1024 @ 100%. For the test render, clamping was set to 1 for both direct and indirect. I used 512 samples.)
The first problem: lighting
I disabled world lighting and enabled an area lamp. The lamp was set to a blackbody temperature of 5500K and sized to match the light source of 23cm x 10cm diffuser. I calculated the strength by converting lux to wt/m^2 and setting the strength. The image was far too dark. So I tuned the strength using the lazy susan as a guide. The light is somewhat brighter than in the photograph. In short, the scene lighting does not precisely match the real world.
There are a few modelling discrepancies. The focal length isn’t precisely right, so the perspective distortion of the bottom of the glasses don’t match. The model glass has a thinner base than the original. There is only one Coke logo on the class and it is a slightly different size and position than in the original. The position of the lamp doesn’t seem to precisely match. It is not reflected in the window, nor is it reflected in the same place on the glass. [Note: I fixed some of this before doing the final render. It is left as a reader to determine which parts.]
Modeling Glass in Blender
Taking these into account, we arrive at the second issue, the crux of this post: The glass shader. How can we improve it?
There are many things that the basic glass shader doesn’t do and there are many kinds of glass. I had collected three different glass shaders from various sources and rendered the scene with exactly the same setup, but with each of the glass shaders. As you can see, each shader is focused on particular aspects of glass and the renders are different from each other and from the basic glass shader.
There are two approaches to attempting photo realistic glass in Blender. The first augments the glass shader to add features it is missing, or replaces the glass with a refractive shader.. A wide range of tutorials on this can be found by a Google search.
The second is to fake it. An excellent discussion of this approach can be found in Alexandrov’s Create Realistic Ice and Seriously Awesome Refraction. If you would like to explore this approach I recommend subscribing to Gleb Alexandrov on youtube and perhaps visiting his site Creative Shrimp.
Augmenting the basic glass shader.
How light interacts with a surface
I’ll keep the physics to a minimum, but there are certain terms that are useful. Here is the obligatory diagram
When light strikes a surface, it is split into up to four parts. It can be
- absorbed: Any surface will absorb light. Absorption creates the color of a surface because not all colors of light are absorbed the same amount.
- reflected: Some part of the light will be reflected from the object. When we see a surface, we are seeing the light that is reflected by it.
- scattered: The rougher the object, the more light will be scattered
- transmitted: If the object is transparent, some percentage of the light will pass through it. The light will bend as it enters the object and bend again as it leaves the object. The parameter that describes this is the Index of Refraction, IOR for short.
There is a problem with Blender’s model of light that makes modeling some aspects of light impossible. Blender’s nodes that use the IOR do not take into account that different wavelengths of light will have different IOR values at the same interface. This is called dispersion and is why you can’t model a prism creating a spectrum in Blender.
How blender models this behavior.
Blender introduced the principled shader in 2.79. Much of what I discuss next can now be modeled by a single principled shader. I am going to use the older method because I think the separate nodes make it easier to understand what is going on. I’m also going to ignore texture for now.
Blender divides reflection and scattering into two shaders. The diffuse shader models the scattering effect while the glossy shader models the reflection effect. Andrew Price of Blender Guru likes to describe materials as metal or non-metal, which he calls dielectric. Metals are perfectly reflective while dielectric materials are not. But even metals can be rough so Blender provides a mix shader so that you can model a material with both properties.
Absorption is handled by the value of the color. The material absorbs all light, except for light of the specified color. It absorbs some of the specified color as well. A light color will reflect more of the light than a dark color of the same hue.
This model doesn’t handle transmission, however. For transparent material, we can switch to the glass shader. The 2.79 Blender manual says
The GlassBSDF node is used to add a Glass-like shader mixing refraction and reflection at grazing angles.
and goes on to add
With caustics disabled, glass will miss shadows, and with filter glossy they might be too soft. We can make a glass shader that will use a Glass BSDF when viewed directly, and a Transparent BSDF when viewed indirectly.
and provides the glass shader most often shown in tutorials
The glass shader handles reflection and refraction but will generate caustics. The thing to know about caustics is that Blender cycles renderer does a poor job with them and people often disable them in the render properties to avoid the noise they add to a scene.
The problem is that disabling caustics means that the glass won’t pass shadows from other objects. The transparent shader compensates for this.
The trick is how the glass and transparent shaders are mixed. In reality they aren’t. Either one or the other is selected by the nodes driving the Fac input of the mix shader. The math node, set to maximum will return either a 0 or a 1 depending on the output from the light path. When the fac is 0, the glass shader is used exclusively. when it is 1, the transparent shader is.
The manual has this to say about the light path node
Ray types can be divided into four categories:
- Camera: the ray comes straight from the camera.
- Reflection: the ray is generated by a reflection off a surface.
- Transmission: the ray is generated by a transmission through a surface.
- Shadow: the ray is used for (transparent) shadows.
Reflection and transmission rays can further have these properties:
- Diffuse: the ray is generated by a diffuse reflection or transmission (translucency).
- Glossy: the ray is generated by a glossy specular reflection or transmission.
- Singular: the ray is generated by a perfectly sharp reflection or transmission.
The Light Path node can be used to find out the type of ray the shading is being computed for.
Oddly enough, disabling caustics and using this material makes the render less photo realistic in some ways. Making shadow rays transparent causes the render to lose the shadows created by the glass itself.
A different way?
The glass shader has is suitable for many situation, but it doesn’t provide the controls needed for all cases. There is another approach to modeling glass materials. An alternative approach adopted by a lot of shaders is to mix a glossy node and a refractive node, using the Fresnel effect to control the mix
There are many ways to adapt the basic shader. For example, Mike Hicks and Alexander Bräunig use the glass shader instead of the refractive shader. The penfinity shader avoids the Fresnel node and instead uses the Layer weight node, mixing Fresnel and Facing outputs. The variation I chose for the render is from Stephen Woods. It mixes the Fresnel and Facing output, but used two copies of the Layer Weight node, allowing the blend to be set separately for both, making it more suitable for my final render.
But wait, there’s more.
There are two pieces of glass in the scene, the window and the drinking glass. While they appear to both be made from the same kind of glass, it turns out that different aspects of glass are important in each, so I used two different glass materials.
Rendering glass requires close attention to color management, sampling, and light path parameters. While these are outside the scope of this post, I will mention in passing that glass scenes tend to be noisy and require high sample counts to render well. All of the images in this post were rendered using 512 samples, except the last. At 4096 samples, it still has some noise.
What clamping you do, if any, especially direct clamping, has a major impact.
The number of bounces may have to be increased, since light passing through or reflecting from glass may affect how other objects look.
Blender Cycles is not very good at caustics, but they may be necessary. Carefully decide whether to enable them or not.
There is more than one way to model glass in Blender. You have to work around the shortcoming of the glass node if you use it. If you have multiple glass objects in a scene you may need different glass materials for each, even if they all seem to be made from the same glass.
It is not possible to model photorealistic glass in Blender, but using a range of techniques will allow you to approximate it in a way that is appropriate to your scene.
In response to comments on Facebook, I changed the model to more accurately represent the way typical glasses are constructed and redid the render with Wood’s glass. I also rendered the model using the principled glass shader, for completeness.