mental ray Paint Shaders

Version 3.9

Version 3.9.0.9
July 05 2010


Mental ray car paint shader

Contents

Paint Shaders

The paint related shaders comes from the paint library. The declaration of the shaders and Phenomena can be found in the file paint.mi. To use the shaders, the declaration file must be inluded and the library linked:

link "paint.so"
$include "paint.mi"

Car paint has several peculiar characteristics. On top of the cars bodywork is a thin layer of pigment. The properties of this layer is such that the actual perceived color shifts depending on the viewing angle as well as the incident angle of the incoming light.

Within this layer tiny metal flakes are suspended. The flakes reflect light and can be seen glittering on a sunny day, due to individual flakes reflecting sunlight directly at the observer.

On top of this is a clearcoat layer, which can be more or less reflective and more or less glossy, depending on the quality of the layer and any added wax coating. Most notably, this layer tends to exhibit a pronounced Fresnel effect, reflecting more light at glancing angles.


Structure of car paint

Car paint Phenomenon

The mi_car_paint_phen should be applied as a surface shader in a material. The shader supports:

mi_car_paint_phen
declare phenomenon "mi_car_paint_phen" 
(
    color       "ambient",
    color       "base_color",
    color       "edge_color",
    scalar      "edge_color_bias",
    color       "lit_color",
    scalar      "lit_color_bias",
    scalar      "diffuse_weight",
    scalar      "diffuse_bias",
    color       "flake_color",
    scalar      "flake_weight",
    scalar      "flake_reflect",
    scalar      "flake_exp",
    scalar      "flake_density",
    scalar      "flake_decay",
    scalar      "flake_strength",
    scalar      "flake_scale",
    color       "spec",         
    scalar      "spec_weight",      
    scalar      "spec_exp",         
    color       "spec_sec",
    scalar      "spec_sec_weight",  
    scalar      "spec_sec_exp",     
    boolean     "spec_glazing",
    color       "reflection_color",
    scalar      "edge_factor",    
    scalar      "reflection_edge_weight",
    scalar      "reflection_base_weight",
    integer     "samples",
    scalar      "glossy_spread",
    scalar      "max_distance",
    boolean     "single_env_sample",
    color       "dirt_color",
    scalar      "dirt_weight",
    scalar      "irradiance_weight",
    scalar      "global_weight",
    integer     "mode",
    array light "lights"
)

Diffuse Parameters

ambient
is the ambient light component. Note that this parameter is treated differently to the ambient/ambience parameter pair of many other base shaders in that it is influenced by the other diffuse color parameters following, and hence represents incoming light, rather than the objects "ambient color".

base_color
is the base diffuse color of the material.
edge_color
is the color seen at glancing angles (i.e. edges) which tend to appear much darker. For really deep metallic paints seen on sports cars it tends to be almost black.
edge_color_bias
defines the falloff rate of the color towards the edge. The useful range is 0.0 to approximately 10.0, where the value 0.0 turns the effect off. Higher values makes the edge region narrower, lower values makes it wider.

Color shift due to view angle, shifting between a red base_color and a blue edge_color
(atypical colors chosen for demonstration purposes only) with varying edge_bias
lit_color
is the color seen in the area facing the lightsource.
lit_color_bias
defines the falloff rate of the color towards the light. The useful range is 0.0 to approximately 10.0, where the value 0.0 turns the effect off. Higher values makes the colored region facing the light smaller/narrower, lower values makes it larger/wider.

Color shift due to view angle, shifting between a red base_color and a green lit_color
(atypical colors chosen for demonstration purposes only) with varying lit_bias
diffuse_weight
controls the overall level of the diffuse parameters.
diffuse_bias
modifies the falloff of the diffuse shading. The useful range is approximately 0.5 to 2.0, where 1.0 represent standard lambertian shading, higher values pushes the diffuse peak towards the light source, and lower values flattens the diffuse peak.
irradiance_weight
sets the influence of indirect light (photons and final gathering) on the surface. It is internally divided by PI, i.e. a value of 1.0 means the standard 1.0/PI weight.

Specular Parameters

spec
is the color of the primary specular highlight.
spec_weight
is a scalar multiplier.
spec_exp
is the Phong exponent.
spec_sec
is the color of the secondary specular highlight.
spec_sec_weight
is a scalar multiplier.
spec_sec_exp
is the Phong exponent.
spec_glazing
enables a special mode on the primary specular highlight called glazing. By threasholding the specular highlight, it makes the surface appear more polished and shiny. For nice sportscars with a lot of wax, turn glazing on. For beat up cars in the junkyard, turn it off.

Left to right: Flake specularity only, standard specularity, "glazing" mode enabled, and finally; "glazing" mode specularity with flakes

Flake Parameters

flake_color
is the color (reflectivity) of the flakes, which is generally white.
flake_weight
is a scalar multiplier for the above color.
flake_reflect
defines the amount of ray traced reflection in the flakes, which allows glittery reflections of e.g. an HDRI environment. The value of 0.0 turns the effect off. The effect should generally be very subtle and a value of 0.1 is often enough. The final intensity of reflections also depends on flake_color and flake_weight.
flake_exp
is the Phong specular exponent for the flakes.
flake_density
sets the density of the flakes. The useful range is from 0.1 to approximately 10.0, where lower values indicate less dense flakes and higher values indicates denser flakes.
Since flakes are inherently small, they can easily introduce rendering artifacts if their visual density becomes significantly smaller than a pixel. If the oversampling of the rendering is set high, small flakes may also potentially trigger massive oversampling and hence very long render times needlessly, since the averaging caused by the oversampling will essentially cancel out the flake effect. To avoid this the parameter flake_decay exists.
flake_decay
sets a distance at which the influence of flakes fade out. A value of 0.0 disables fading. Any positive value causes the flake_weight to be modulated such that is reaches zero at this distance.

Flakes at different distances with no flake decay. The furthest flakes may potentially cause flicker in animations,
or trigger unnecessary oversampling and long render times (here rendered with low oversampling for illustrative purposes).

Using flake decay. The flake strength diminishes by distance. The same intentionally low oversampling as in the previous image has been used.
flake_strength
sets the difference between the orientation of the flakes. The useful range is 0.0 to 1.0 where 0.0 means that all flakes are parallel to the surface and higher values means the orientation of flakes are more and more varied.
flake_scale
is the size of the flakes. The procedural texture is calculated in object space, and will hence follow the object. Keep in mind that the scale is therefore influenced by any scale transformation on the object instance.

Reflection Parameters

reflection_color
sets the color of the reflections in the clearcoat layer, generally white.

Clearcoat tends to reflect more at glancing angles (edges).

edge_factor
defines the "narrowness" of this edge.
reflection_edge_weight
controls the reflective strength at the edge (generally 1.0).
reflection_base_weight
defines the reflective strength at facing angles (generally low, 0.1 - 0.3).

Optionally, clearcoat layers may be glossy.

samples parameter sets the bumber of glossy reflection rays traced. The value of 0 disables glossiness.
glossy_spread
sets the amount of glossiness. Cars are generally near-mirrors so this value should be kept small.

The glossy reflections are created with the help of the shader mib_glossy_reflection. Therefore, the parameters max_distance (limiting the reach of reflective rays) and single_env_sample (optimizing lookup of environment maps) are exposed parameters of that shader. Only a subset of the parameters are exposed. If more are required, one can either build a custom shading graph or create ones own variation of the mi_car_paint_phen Phenomenon.

Dirt Parameters


Real cars are rarely clean. Here showing the dirt layer (hand painted dirt placement map), including a bump map applied in the dirty regions.

A simple lambertian dirt layer covers the underlaying paint and clearcoat layers.

dirt_color
is the color of said dirt.
dirt_weight
the amount of dirt, which would probably be connected to a texture shader to get variations in the dirt across the surface. If dirt_weight is 0.0 no dirt is added.

Advanced Parameters

irradiance_weight
sets the influence of indirect light (photons and final gathering) on the surface. It is internally divided by PI, i.e. a value of 1.0 means the standard 1.0/PI weight.
global_weight
is a global tuning parameter affecting the entire diffuse, flake and specular subsystems. It does not affect reflections or dirt.
mode
is the light mode.
lights
the light list.

Metallic Paint

The mi_metallic_paint is shader is used to facilitate the rendering of metallic paint. However, it only takes care of the pigmentation and the flakes, not the very important clearcoat reflections. To accomplish the full effect, it needs to be combined with a reflection shader (i.e. connecting it to the base_material parameter of mib_glossy_reflection). It also needs a bump shader for the flakes (generally mi_bump_flakes).

To get all these connections made automatically one can use the ready-made Phenomenon mi_car_paint_phen, which also supports a dirt layer. See the above documentation, which has slightly more detailed documentation of the parameters with example images.

The shader supports:

mi_metallic_paint
declare shader
color "mi_metallic_paint" (
    color       "ambient"           default 0 0 0 1,
    color       "base_color"        default 0.8 0.1 0.0 1,
    color       "edge_color"        default 0.0 0.0 0.0,
    scalar      "edge_color_bias"   default 1.0,
    color       "lit_color"         default 0.6 0.0 0.2,
    scalar      "lit_color_bias"    default 8.0,
    scalar      "diffuse_weight"    default 1.0,
    scalar      "diffuse_bias"      default 1.5,
    scalar      "irradiance_weight" default 1.0,
    color       "spec"              default 1 1 1 1,
    scalar      "spec_weight"       default 0.2,
    scalar      "spec_exp"          default 60.0,
    color       "spec_sec"          default 1 1 1 1,
    scalar      "spec_sec_weight"   default 0.3,
    scalar      "spec_sec_exp"      default 25.0,
    boolean     "spec_glazing"      default on,
    color       "flake_color"       default 1.0 1.0 1.0 1,
    scalar      "flake_weight"      default 1.0,
    scalar      "flake_reflect"     default 0.0,
    scalar      "flake_exp"         default 45.0,
    scalar      "flake_decay"       default 0.0,
    shader      "flake_bump",
    scalar      "global_weight"     default 1.0,
    integer     "mode"              default 3,
    array light "lights"        
)
version 2
apply material
end declare 

Diffuse Parameters

ambient
is the ambient light component. Note that this parameter is treated differently to the ambient/ambience parameter pair of many other base shaders in that it is influenced by the other diffuse color parameters following, and hence represents incoming light, rather than the objects "ambient color".
base_color
is the base diffuse color of the material.
edge_color
is the color seen at glancing angles (i.e. edges) which tend to appear much darker. For really deep metallic paints seen on sports cars it tends to be almost black.
edge_color_bias
defines the falloff rate of the color towards the edge. The useful range is 0.0 to approximately 10.0, where the value 0.0 turns the effect off. Higher values makes the edge region narrower, lower values makes it wider.
lit_color
is the color seen in the area facing the lightsource.
lit_color_bias
defines the falloff rate of the color towards the light. The useful range is 0.0 to approximately 10.0, where the value 0.0 turns the effect off. Higher values makes the colored region facing the light smaller/narrower, lower values makes it larger/wider.
diffuse_weight
allows to tune the overall level of the diffuse parameters.
diffuse_bias
modifies the falloff of the diffuse shading. The useful range is approximately 0.5 to 2.0, where 1.0 represent standard lambertian shading, higher values pushes the diffuse peak towards the light source, and lower values flattens the diffuse peak.
irradiance_weight
sets the influence of indirect light (photons and final gathering) on the surface. It is internally divided by PI, i.e. a value of 1.0 means the standard 1.0/PI weight.

Specular Parameters

spec
is the color of the primary specular highlight.
spec_weight
is a scalar multiplier.
spec_exp
is the Phong exponent.
spec_sec
is the color of the secondary specular highlight.
spec_sec_weight
is a scalar multiplier.
spec_sec_exp
is the Phong exponent.
spec_glazing
enables a special mode on the primary specular highlight called glazing. By threasholding the specular highlight, it makes the surface appear more polished and shiny. For nice sportscars with a lot of wax, turn glazing on. For beat up cars in the junkyard, turn it off.

Flake Parameters

flake_color
is the color (reflectivity) of the flakes, which is generally white.
flake_weight
is a scalar multiplier for the above color.
flake_reflect
defines the amount of ray traced reflection in the flakes, which allows glittery reflections of e.g. an HDRI environment. The value of 0.0 turns the effect off. The effect should generally be very subtle and a value of 0.1 is often enough. The final intensity of reflections also depends on flake_color and flake_weight.
flake_exp
is the Phong specular exponent for the flakes.
flake_bump
is the actual flake bump shader used. The shader mi_bump_flakes exists for this purpose, but any shader that modifies the normal vector (e.g. mib_passthrough_bump_map) can be used.
The shader put in the flake_bump may also return a color, which will be the color (intensity) of the flake, or it may leave the color unmodified.

Since flakes are inherently small, they can easily introduce rendering artifacts if their visual density becomes significantly smaller than a pixel. To avoid this the parameter flake_decay exists. It sets a distance at which the influence of flakes fade out. A value of 0.0 disables fading. Any positive value causes the flake_weight to be modulated such that it reaches zero at this distance.

Additional Parameters

global_weight
is a global tuning parameter that is a global multiplier to the output of the shader.
mode
is the light mode.
lights
the light list.

Flake Bump Shader

This shader is designed to create a bump map with the appearance of small individual flakes at slightly different orientations. It does so by slightly modifying the current normal vector based on a procedurally generated flake texture. It also returns a color that indicates the "intensity" for that flake.

mi_bump_flakes
declare shader "mi_bump_flakes" (
    scalar      "flake_density"     default 0.5,
    scalar      "flake_strength"    default 0.8,
    scalar      "flake_scale"       default 0.2,
)
version 1
apply texture
end declare
flake_density
only affects the returned color, not the amount of change to the normal vector. The useful range is from 0.1 to approximately 10.0, where lower values indicate less dense flakes (more flakes are given low color values near black) and higher values indicates denser flakes.
flake_strength
indicates the amount of normal vector perturbations. Useful range is 0.0 (which disables the effect) to 1.0. For low values, flakes are nearly parallell, for higher values, the difference between each flakes orientation is larger.
flake_scale
is the size of the flakes. The procedural texture is calculated in object space, and will hence follow the object. Keep in mind that the scale is therefore influenced by any scale transformation on the object instance.


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