mental ray Paint Shaders

Version 3.7.1.23
June 26 2008

Car Paint Phenomenon
Metallic Paint Shader
Flake Bump Shader

Introduction

The car paint related shaders are contained in the paint library. The declaration of the shaders and phenomena are provided in the file paint.mi. To use the shaders, the declaration file must be included and the library linked:

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

Example images rendered with mental ray car paint shader

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 clear-coat 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 or mi_car_paint_phen_x should be applied as a surface shader in a material. The latter variant performs the same calculations but returns the individual components before compositing into the final color as separate channels. The phenomenon supports:

Diffuse reflection in the pigmentation layer, with selectable color shift due to observer as well as incident light angle.
Specular highlights from light sources in the metallic flakes
Optional ray traced reflections in the metallic flakes.
Specular highlights from light sources in the clear-coat (with optional "glazing" mode).
Includes a clear-coat layer with selectable mirror or glossy reflections and specular highlights with an optional "glazing" mode.
Includes a lambertian dirt layer, for that "unwashed" look.

The phenomena have the following declarations:

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"
    )

mi_car_paint_phen_x

    declare phenomenon
    struct {
        color   "result",         # main output

        color   "ambient_result",
        color   "ambient_raw",
        color   "ambient_level",

        color   "diffuse_result",
        color   "diffuse_raw",
        color   "diffuse_level",

        color   "indirect_result",
        color   "indirect_raw",
        color   "indirect_level",

        color   "spec1_result",
        color   "spec1_raw",
        color   "spec1_level",

        color   "spec2_result",
        color   "spec2_raw",
        color   "spec2_level",

        color   "flake_result",
        color   "flake_raw",
        color   "flake_level",

        color   "flake_refl_result",
        color   "flake_refl_raw",
        color   "flake_refl_level",

        vector  "flake_normal",

        color   "dirt_result",
        color   "dirt_raw",
        color   "dirt_level",

        color   "reflection_result",
        color   "reflection_raw",
        color   "reflection_level"
    } "mi_car_paint_phen_x" (
        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 light source.
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; The overall level of the diffuse parameters can be tuned with the diffuse_weight parameter.
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 spec_weight spec_exp: the color of the primary specular highlight and a scalar multiplier for the contribution of the effect, and the Phong exponent of the highlight.
spec_sec spec_sec_weight spec_sec_exp: the color of the secondary specular highlight, a scalar multiplier and the Phong exponent.
spec_glazing: enables a special mode on the primary specular highlight called glazing. By thresholding the specular highlight, it makes the surface appear more polished and shiny. For nice sports cars 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. 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 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 illustration).

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

Clear-coat tends to reflect more at glancing angles (edges). Optionally, clear-coat layers may be glossy.

reflection_color: sets the color of the reflections in the clear-coat layer, generally white.
edge_factor: defines the "narrowness" of the edge.
reflection_edge_weight: the reflective strength at the edge (generally 1.0).
reflection_base_weight: the reflective strength at facing angles (generally low, 0.1 - 0.3).
samples: sets the number of glossy reflection rays traced. A 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 base shader mib_glossy_reflections and thus the parameters max_distance (limiting the reach of reflective rays) and single_env_sample (optimizing lookup of environment maps) are exposed parameters of the shader mib_glossy_reflection. 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

A simple lambertian dirt layer covers the underlying paint and clear-coat layers.

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

dirt_color: is the color of said dirt, and
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 optional direct lights for this shader.

Metallic Paint

This 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 clear-coat 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, like mi_bump_flakes).

For convenience, phenomena like mi_car_paint_phen are available which also support a dirt layer. Its documentation has slightly more detailed description of the parameters with example images.

The shader supports:

Diffuse reflection in the pigmentation layer, with selectable color shift due to observer as well as incident light angle.
Specular highlights from light sources in the metallic flakes.
Specular highlights from light sources in the clear-coat (with optional "glazing" mode).
Optional ray traced reflections in the metallic flakes.

The shader comes in two variants, as mi_metallic_paint which returns a single final composited color, and mi_metallic_paint_x which additionally returns separate channels for the individual components before compositing.

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

mi_metallic_paint_x

  declare shader
      struct {
          color   "result",

          color   "ambient_result",
          color   "ambient_raw",
          color   "ambient_level",
 
          color   "diffuse_result",
          color   "diffuse_raw",
          color   "diffuse_level",
 
          color   "indirect_result",
          color   "indirect_raw",
          color   "indirect_level",
 
          color   "spec1_result",
          color   "spec1_raw",
          color   "spec1_level",
 
          color   "spec2_result",
          color   "spec2_raw",
          color   "spec2_level",
 
          color   "flake_result",
          color   "flake_raw",
          color   "flake_level",
 
          color   "flake_refl_result",
          color   "flake_refl_raw",
          color   "flake_refl_level",
 
          vector  "flake_normal"
      } "mi_metallic_paint_x" (
          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 light source.
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: scale the overall diffuse shading.
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: a scalar multiplier and
spec_exp: the Phong exponent.
spec_sec: is the color of the secondary specular highlight,
spec_sec_weight: a scalar multiplier and
spec_sec_exp: the Phong exponent.
spec_glazing: enables a special mode on the primary specular highlight called glazing. By thresholding the specular highlight, it makes the surface appear more polished and shiny. For nice sport scars 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.; 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.
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.

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 optional direct lights for this shader.

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 parallel, 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.

mental ray Paint Shaders

Table of Contents

Introduction

Car Paint Phenomenon

mi_car_paint_phen

mi_car_paint_phen_x

Diffuse parameters

Specular parameters

Flake parameters

Reflection parameters

Dirt parameters

Advanced parameters

Metallic Paint

mi_metallic_paint

mi_metallic_paint_x

Diffuse parameters

Specular parameters

Flake parameters

Additional parameters

Flake Bump Shader

mi_bump_flakes