Every block and "entity" in Chunky is listed as a material. Each material has several properties which determine how light interacts with that material. Material properties for beacon beam segments can be set in the Entities tab, and material properties for every other material can be set in the Materials tab.
The Emittance property of a material is the strength of the light that that material emits. The average color of light emitted from materials which have an emittance value that is greater than 0 is the average of all color values of the texture of that material. Due to path tracing, which is the rendering method that Chunky uses by default, the total light output is dependent on the amount of surface area that a block has. For example, for a given emittance value, a glowstone block outputs a greater amount of light than a torch does.
Calculating material emittance
To calculate the relative emittance value for a block, divide the in-game light level of that block by the total surface area of the block, measured in pixels. Then multiply that value by 102.4. The calculation is represented by the equation:
emittance = (inGameLightLevel/surfaceAreaInPixels)*102.4, which is derived from the equation:
emittance = (inGameLightLevel/15)*(1536/surfaceAreaInPixels). This causes full blocks with an in-game light level of 15, such as glowstone, to have an emittance value of 1. Note that this value is only a simple calculation, and can be adjusted depending on the texture of the block.
The Specular property of a material is the fraction of light reflecting off its surface that reflects as it would reflect off the surface of a mirror. It is measured in a scale of 0 to 1. A material with a specular value of 0 will reflect light diffusely, while a material with a specular value of 1 will reflect light as a mirror would. See this figure for a comparison between different specular values.
One way to make a surface appear wet is to set a small specular value on the material. While physically-correct rendering of wet surfaces is much more complex, this is a decent way to render rainy or wet scenes in Chunky.
The Smoothness of a material is the amount of irregularity in the surface of the texture, which changes the amount of diffusion in the light reflectivity. It is measured in a scale of 0 to 1. A material with a smoothness value of 1 will be perfectly smooth, while a material with a smoothness value of 0 will be perfectly diffuse. See this figure for a comparison between different smoothness values.
Smoothness vs. roughness
Internally, Chunky uses linear roughness for its calculations. Working with roughness is rather hard for people, so LabPBR introduced perceptual smoothness, which is what Chunky uses in the Materials tab. This makes it so that a material with a smoothness value of 0.5 looks twice as smooth as a material with a smoothness value of 0.25 to a human.
You can learn more about perceptual smoothness, including the formula to convert between perceptional smoothness and linear roughness, in the shaderLABS Wiki.
Index of Refraction (IoR)¶
The Index of Refraction (IoR) property of a material is the ratio of the speed of light in a vacuum to the speed of light in that material. This changes how much the light bends when entering and exiting that material. See this figure for a comparison between different IoR values.
The Metalness property of a material is the fraction of light reflecting off its surface that reflects as it would off a mirror, but tinted according to the texture of the material. It is measured in a scale of 0 to 1. A material with a metalness value of 0 will reflect light diffusely, while a material with a metalness value of 1 will reflect light as a mirror would, but tint it according to the texture of that material. See this figure for a comparison between different metalness values, and see this figure for a comparison between metalness and specular properties.
Metalness vs. real world
In the real world, metals reflect light differently than dielectric materials (i.e. non-metals) do. This is what makes them shiny.
While there is no such thing as 50% metalness for a real material, this can be used to approximate dirty metallic surfaces (and for other artistic purposes, of course). For example, by default, Chunky uses metalness values smaller than 1 for oxidized copper blocks.