Writing mental ray shaders: Color Balance

Goal

A simple shader that combines the gain and offset shader

Methodology

1. create a color parameter called base

2. create a color parameter called gain

3. create a color parameter called offset

4. create a shader called "color_gain"

5. create a shader called "color_offset"

6. create a function where result = base * gain

7. create a function where result = result + offset

Mi File

declare shader
 color "color_balance" (
  color "base"  default 1 1 1,
  color "gain" default 1 1 1,
  color "offset" default 0 0 0,
 )
end declare

C File

#include "shader.h"

DLLEXPORT

struct color_balance {
 miColor base;
 miColor gain;
 miColor offset;
 };

DLLEXPORT

miBoolean color_balance (miColor *result, miState *state, struct color_balance *params) {
 miColor *base = mi_eval_color(&params->base);
 miColor *gain = mi_eval_color(&params->gain);
 miColor *offset = mi_eval_color(&params->offset);
 result->r = base->r * gain->r;
 result->g = base->g * gain->g;
 result->b = base->b * gain->b;
 result->r += offset->r;
 result->g += offset->g;
 result->b += offset->b;
 return miTRUE;
 }
 

Writing mental ray shaders: Color Offset

Goal

A simple shader that takes a base color and adds another color.

Methodology

1. create a color parameter called base

2. create a color parameter called offset

3. create a shader called "color_offset"

4. create a function where result = base + offset

Mi File

declare shader
 color "color_offset" (
  color "base"  default 1 1 1,
  color "offset" default 0 0 0
 )
 apply material
end declare

C File

#include "shader.h"

DLLEXPORT

struct color_offset {
 miColor base;
 miColor offset;
};

DLLEXPORT

miBoolean color_offset (miColor *result, miState *state, struct color_offset *params) {
 miColor *base = mi_eval_color(&params->base);
 miColor *offset = mi_eval_color(&params->offset);
 result->r = base->r + offset->r;
 result->g = base->g + offset->g;
 result->b = base->b + offset->b;
 return miTRUE;
 }

Writing mental ray shaders: Color Gain

Goal

A simple shader that takes a base color and multiplies by another color.

Methodology

1. create a color parameter called base

2. create a color parameter called gain

3. create a shader called "color_gain"

4. create a function where result = base * gain

Mi File

declare shader
 color "color_gain" (
  color "base" default 1 1 1,
  color "gain" default 1 1 1
 )
 version 1
 apply material
end declare

C File

#include "shader.h"

DLLEXPORT

struct color_gain {
 miColor base;
 miColor gain;
 };

DLLEXPORT

miBoolean color_gain (miColor *result, miState *state, struct color_gain *params) {
 miColor *base = mi_eval_color(&params->base);
 miColor *gain = mi_eval_color(&params->gain);
 result->r = base->r * gain->r;
 result->g = base->g * gain->g;
 result->b = base->b * gain->b;
 return miTRUE;
 }

Phenomenon Shaders: Droplets

Intro

__________________________________________________________

The end user of a phenomenon shader are usually lighters in a pipeline environment so as a shader artist/developer, it would be helpful to think in terms of how a lighter will use this shader. The phenomenon does not replace the shader graph. However, it does simplify the user interface of the shader graph into something that is easier for the lighter. This is also currently the pipeline direction Renderman Studio 3 is going. Our previous RMS pipelines exposes the full shader tree to the lighter, which is 1. unwieldy to the lighter 2. unwieldy to the hardware. But unlike renderman, there are no performance gains in mentalray. 

I will be making two mental ray phenomenon shaders, first will be the water droplet shader, and the second will be the rim shader, which will be exercise. I can use these phenomenons later on in my fruit shader series. Other resources for making phenomenon shaders can be found here,

• renderWiki phenomenon tutorial
• djx blog phenomenizer tutorial
• mentalImages forum mod bart on color array inputs and evaluation

Key Points
__________________________________________________________

• Splines, color ramps, maya layered shader/texter and any other nodes with arrays will NOT work in phenomenon
• Use MentalRay equivalent nodes when possible
• Phenomenizer does not work with maya nodes

Determining User Settings

__________________________________________________________

First, a review my shader graph.

Overview

Detailed Droplet Tree

Here are the settings I have deemed important enough for customizability.

small droplet

• cell size
• density
• spottiness

large droplet

• cell size
• density

mask

• time
• frequency

mask_remap

• v1 position
• v2 position

bump

• bump depth

mia material

• diffuse weight
• reflection color

Reformatting the Shader Tree

__________________________________________________________

Looking at my shader graph, notice there are several remapHSV nodes in there, these are going to be difficult to implement in a phenomenon. As getting array inputs(the spline graph) has not been proven to work(yet?). However, I will just work around it for now, substituting remapHSV with a contrast node.  

Replace Remap with Contrast

Here, I just tweak the contrast node to mimic what the remaphsv node is doing. The other remaphsv nodes define the crossectional shape of the droplet by customizing the gradient falloff of the noise map.

Reworking and Testing the droplet

By removing these remaphsv nodes, the bump would look slightly off, but only noticeable in extreme closeups. I retweaked threshhold level of the leather noise node to match the values of my original. Hence, I will add the threshhold controls to my droplet UI. A quick way to test is just use surface shaders or for mental ray nodes, a mib_lambert. I also removed the layered texture node and replaced it with a mib_color_mix. As seen above, a quick comparison of each node and the final texture output. 

Cleaned up

Writing the Phenomenon

__________________________________________________________

I will begin implementing the above network into a phenomenon. The first thing to do,

declare phenomenon
 color "droplet_phen" (
 )
 version 1
 apply material
end declare

This is the basic phenomenon declaration. Next, declare the root node, which should be the end result node.

shader "dropletSurface" "maya_surfaceshader" (
  "outColor" = "mia_material1.outValue",
  "outTransparency" 0. 0. 0.,
  "outMatteOpacity" 1. 1. 1.,
  "outGlowColor" 0. 0. 0.
  )

 shader "shadingEngine" "maya_shadingengine" (
  "surfaceShader" = "dropletSurface.outColor",
  "cutAwayOpacity" 0.,
  "alphaMode" 0
  )

 root = "shadingEngine"

The root node is named shadingEngine, which is a maya node with the name "maya_shadingEngine". Why am I using maya_shadingEngine? It's explained here, which states,

"All maya built in mentalray shaders does not use a simple “color” as an output but a “struct”, a user defined output structure. Because root expects a color, it does not know how to evaluate this thing and this causes this error:...

...One shader that provides an color output instead of a struct is the shading group shader."

So here is the basic structure of a phenomenon shader, each node is defined as

shader "shader_instance_name" "mayabase_name"

followed by its variable properties. To find out the exact name and variables of a node, check inside the mentalray/include directory of the maya installation and find the following files,

• mayabase.mi - maya nodes
• base.mi  - mentalray nodes
• architecture.mi - mia
• subsurface.mi - misss

Unlike the aforementioned mi files, the type of each property does not need to be declared, we are only assigning values to the property. 

Before I start importing all the nodes into my phenomenon file, I declare the the settings that allows the user to access the shader.

color "droplet_phen" (
  scalar "diffuse_weight", #:default 0.2
  color "reflection_color", #:default 0.9 0.9 0.9
  scalar "sm_droplet_cell_size", #:default 0.125 shortname "smdr_size"
  scalar "sm_droplet_density", #:default 0.3 shortname "smdr_density"
  scalar "sm_droplet_spottyness", #:default 0 shortname "smdr_spot"
  scalar "lg_droplet_cell_size", #:default 0.6 shortname "lgdr_size"
  scalar "lg_droplet_density", #:default 0.1 shortname "lgdr_density"
  scalar "mask_frequency", #:default 13
  scalar "mask_time",  #:default 0
  vector "mask_bias",  #:default 0.6 0.6 0.6
  scalar "bump_depth",  #:default 0.5
  transform "sm_droplet_placement",
  transform "lg_droplet_placement",
  transform "mask_placement"
  )

The three transform variables does not have a default value as it will differ from scene to scene. I left it open so I can manually connect a place3dtexture node into the transform slot. I will then link the interface to the three 3d textures I will be using later on. One can specify default values of a transform variable, its a 4x4 matrix, separated by commas(IIRC).

From the tip to the root,  import the nodes into the phenomenon file.

shader "small_droplets" "maya_leather" (
  "cellColor" 1. 1. 1.,
  "creaseColor" 0. 0. 0.,
  "cellSize" = interface "sm_droplet_cell_size",
  "density" = interface "sm_droplet_density",
  "spottyness" = interface "sm_droplet_spottyness",
  "randomness" 1,
  "threshold" 0.6,
  "creases" 0,
  "filter" 0,
  "filterSize" 0. 0. 0.,
  "filterOffset" 0,
#  "blend",
  "wrap" 1,
  "invert" 0,
  "alphaIsLuminance" 0,
  "colorGain" 1. 1. 1.,
  "colorOffset" 0. 0. 0.,
  "alphaGain" 1,
  "alphaOffset" 0,
  "defaultColor" 0.5 0.5 0.5,
  "placementMatrix" = interface "sm_droplet_placement",
  "local" 0
  )

There are basically four things I'm doing from here on out,

1. create a new node from the corresponding node in the shader tree
2. copy settings from the original shader tree into the phenomenon
3. assign variables to the interface
4. assign variables to the output of another node

Take a look at the following,

shader "mask_contrast" "maya_contrast" (
  "value" = "small_droplet_mask.outColor",
  "contrast" 100. 100. 100.,
  "bias" = interface "mask_bias"
  )

1. I created a contrast node, and named it "mask_contrast"
2. I set the contrast to a 100 in  every channel(I just need one actually)
3. I assigned the bias to an interface item
4. I assigned the input of contrast to be the noise map

Every node is a repeat of the above process. Once every node in the original shader tree is represented in the phenomenon file, load and test the shaders. It is very likely to encounter syntax errors, as the input and output names can be quite confusing. Testing the mi file node by node is a good way to debug the phenomenon. Testing and building the phenomenon is similar to how a shader is built and tested in hypershade. Below is the full phenomenon file.

Droplet UI

Final Droplet Phenomenon

__________________________________________________________

declare phenomenon
 color "droplet_phen" (
  scalar "diffuse_weight", #:default 0.2
  color "reflection_color", #:default 0.9 0.9 0.9
  scalar "sm_droplet_cell_size", #:default 0.125 shortname "smdr_size"
  scalar "sm_droplet_density", #:default 0.3 shortname "smdr_density"
  scalar "sm_droplet_spottyness", #:default 0 shortname "smdr_spot"
  scalar "lg_droplet_cell_size", #:default 0.6 shortname "lgdr_size"
  scalar "lg_droplet_density", #:default 0.1 shortname "lgdr_density"
  scalar "mask_frequency", #:default 13
  scalar "mask_time",  #:default 0
  vector "mask_bias",  #:default 0.6 0.6 0.6
  scalar "bump_depth",  #:default 0.5
  transform "sm_droplet_placement",
  transform "lg_droplet_placement",
  transform "mask_placement"
  )
 
 shader "small_droplets" "maya_leather" (
  "cellColor" 1. 1. 1.,
  "creaseColor" 0. 0. 0.,
  "cellSize" = interface "sm_droplet_cell_size",
  "density" = interface "sm_droplet_density",
  "spottyness" = interface "sm_droplet_spottyness",
  "randomness" 1,
  "threshold" 0.6,
  "creases" 0,
  "filter" 0,
  "filterSize" 0. 0. 0.,
  "filterOffset" 0,
#  "blend",
  "wrap" 1,
  "invert" 0,
  "alphaIsLuminance" 0,
  "colorGain" 1. 1. 1.,
  "colorOffset" 0. 0. 0.,
  "alphaGain" 1,
  "alphaOffset" 0,
  "defaultColor" 0.5 0.5 0.5,
  "placementMatrix" = interface "sm_droplet_placement",
  "local" 0
  )

 shader "large_droplets" "maya_leather" (
  "cellColor" 1. 1. 1.,
  "creaseColor" 0. 0. 0.,
  "cellSize" = interface "lg_droplet_cell_size",
  "density" = interface "lg_droplet_density",
  "spottyness" 0.1,
  "randomness" 1,
  "threshold"0.80,
  "creases" 0,
  "filter" 0,
  "filterSize" 0. 0. 0.,
  "filterOffset" 0,
#  "blend",
  "wrap" 1,
  "invert" 0,
  "alphaIsLuminance" 0,
  "colorGain" 1. 1. 1.,
  "colorOffset" 0. 0. 0.,
  "alphaGain" 1,
  "alphaOffset" 0,
  "defaultColor" 0.5 0.5 0.5,
  "placementMatrix" = interface "lg_droplet_placement",
  "local" 0
  )

 shader "small_droplet_mask" "maya_volumenoise" (
  "amplitude" 1,
  "ratio" 0.809,
  "threshold" 0,
  "scale" 1. 1. 1.,
  "origin" 0. 0. 0.,
  "depthMax" 1,
  "frequency" = interface "mask_frequency",
  "frequencyRatio" 7.252,
  "inflection" 0,
  "time" = interface "mask_time",
  "noiseType" 3,
#  "density",
#  "spottyness",
#  "sizeRand",
#  "randomness",
#  "falloff",
#  "numWaves",
#  "implode",
#  "implodeCenter",
  "filter" 0,
  "filterSize" 0. 0. 0.,
  "filterOffset" 0,
#  "blend",
  "wrap" 1,
  "invert" 0,
  "alphaIsLuminance" 0,
  "colorGain" 1. 1. 1.,
  "colorOffset" 0. 0. 0.,
  "alphaGain" 1,
  "alphaOffset" 0,
  "defaultColor" 0.5 0.5 0.5,
  "placementMatrix" = interface "mask_placement",
  "local" 0
  )

 shader "mask_contrast" "maya_contrast" (
  "value" = "small_droplet_mask.outColor",
  "contrast" 100. 100. 100.,
  "bias" = interface "mask_bias"
  )

 shader "mib_color_mix1" "mib_color_mix" (
  "num" 2,
  "mode_0" 2,
  "mode_1" 0,
  "mode_2" 0,
  "mode_3" 0,
  "mode_4" 0,
  "mode_5" 0,
  "mode_6" 0,
  "mode_7" 0,
  "weight_0" = "mask_contrast.outValueX",
  "weight_1" 1,
  "weight_2" 1,
  "weight_3" 1,
  "weight_4" 1,
  "weight_5" 1,
  "weight_6" 1,
  "weight_7" 1,
  "color_0" = "small_droplets.outColor",
  "color_1" = "large_droplets.outColor",
  "color_2" 0. 0. 0.,
  "color_3" 0. 0. 0.,
  "color_4" 0. 0. 0.,
  "color_5" 0. 0. 0.,
  "color_6" 0. 0. 0.,
  "color_7" 0. 0. 0.,
  "color_base" 0. 0. 0.
  )

 shader "bump3d1" "maya_bump3d" (
  "normalCamera" 0. 0. 1.,
  "bumpValue" = "mib_color_mix1.outColorR",
  "bumpDepth" = interface "bump_depth",
  "bumpFilter" 1.,
  "bumpFilterOffset" 0.,
  "tangentUCamera" 1. 0. 0.,
  "tangentVCamera" 0. 1. 0.
  )

 shader "misss_set_normal1" "misss_set_normal" (
                "normal" = "bump3d1.outNormal",
                "space" 1,
                "add" 0
      )

    shader "mia_material1" "mia_material" (
     "diffuse_weight" = interface "diffuse_weight",
     "diffuse" = "mib_color_mix1.outColor",
     "diffuse_roughness" 0.,
     "reflectivity" = "mib_color_mix1.outColorR",
     "refl_color" = interface "reflection_color",
     "refl_gloss" 1.,
     "refl_gloss_samples" 8,
     "refl_interpolate" off,
     "refl_hl_only" off,
     "refl_is_metal" off,
     "transparency" 0.,
     "refr_color" = "mib_color_mix1.outColor",
     "refr_gloss" 1.,
     "refr_ior" 1.52,
     "refr_gloss_samples" 8,
     "refr_interpolate" off,
     "refr_translucency" off,
     "refr_trans_color" 0.7 0.6 0.5 1.,
     "refr_trans_weight" 0.5,
     "anisotropy" 1.,
     "anisotropy_rotation" 0.,
     "anisotropy_channel" -1,
     "brdf_fresnel" off,
     "brdf_0_degree_refl" 0.2,
     "brdf_90_degree_refl" 1.,
     "brdf_curve" 5.,
     "brdf_conserve_energy" on,
     "intr_grid_density" 2,
     "intr_refl_samples" 2,
     "intr_refl_ddist_on" off,
     "intr_refl_ddist" 0.,
     "intr_refr_samples" 2,
     "single_env_sample" off,
     "refl_falloff_on" off,
     "refl_falloff_dist" 0.,
     "refl_falloff_color_on" off,
     "refl_falloff_color" 0. 0. 0. 1.,
     "refl_depth" 5,
     "refl_cutoff" 0.01,
     "refr_falloff_on" off,
     "refr_falloff_dist" 0.,
     "refr_falloff_color_on" off,
     "refr_falloff_color" 0. 0. 0. 1.,
     "refr_depth" 5,
     "refr_cutoff" 0.01,
     "indirect_multiplier" 1.,
     "fg_quality" 1.,
     "fg_quality_w" 1.,
     "ao_on" off,
     "ao_samples" 16,
     "ao_distance" 10.,
     "ao_dark" 0.2 0.2 0.2 1.,
     "ao_ambient" 0. 0. 0. 1.,
     "ao_do_details" on,
     "thin_walled" off,
     "no_visible_area_hl" on,
     "skip_inside_refl" on,
     "do_refractive_caustics" off,
     "backface_cull" off,
     "propagate_alpha" off,
     "hl_vs_refl_balance" 1.,
     "cutout_opacity" 1.,
     "additional_color" 0. 0. 0. 1.,
     "no_diffuse_bump" off,
     "bump" = "misss_set_normal1.normal",
     "mode" 4,
     "lights" []
     )



 shader "dropletSurface" "maya_surfaceshader" (
  "outColor" = "mia_material1.outValue",
  "outTransparency" 0. 0. 0.,
  "outMatteOpacity" 1. 1. 1.,
  "outGlowColor" 0. 0. 0.
  )

 shader "shadingEngine" "maya_shadingengine" (
  "surfaceShader" = "dropletSurface.outColor",
  "cutAwayOpacity" 0.,
  "alphaMode" 0
  )

 root = "shadingEngine"
   version 1
   apply texture, material

end declare

Making an Orange Shader, Shader Exercises

Intro

__________________________________________________________

I'll be making a procedural orange shader, that looks good at closeups. So this is an exercise in procedural noise layering. The main take away here,

• identifying noise layers/general analysis
• chaining bump maps

I always start out with references, so here are some,

source

This pretty much has all the properties an orange in detail. I want an extremely red orange, so I will push it even further than the above image.

I can quickly outline some bump noise patterns I can see here,

• high frequency, ~0.4mm to 0.8mm, these are the cellular bumps and color at extreme close up
• medium frequency, there are some dimples ~0.5mm to 1mm, but their profile is such that its more visible
• low frequency, large scale knobbiness of the orange skin, some browning skin patterns

Some notes on the color, sss, and reflection

• high frequency leathery skin thats even more pronounced in SSS
• high glossy specularity with color at the end of the falloff
• to determine the average SSS, look at the light side to dark side fall off. Even 

Bump

__________________________________________________________

From my noise pattern outline, i quickly created 3 3d textures,

Bump Sub Tree

Low, Med, High Freq Map Settings

I used leather 3d texture maps for the high and med freqeuncy maps and a solid fractal 3d map for the low frequency map.

High Frequency Pattern

Med. Freq. Pattern 1

Med. Freq. value correct 1

Med. Freq. Pattern 2

Med. Freq. value correct 2

Low Frequency Pattern

Final bump output

 I tweaked each individual bump pattern separately, determining that the high frequency patterns are small bump, while the medium frequency pattern are sharp holes, and the low frequency patterns are bumps as well. The geometry provided have built in low frequency noise as well. I chained each bumps' outnormal to the next bumps' normal camera to achieve the final bump output.

Diffuse & SSS

__________________________________________________________

diffuse, spec, bump

I''ll work on the diffuse and sss at the same time, but first, I duplicated  the orange and scaled it up a little.

The reason being, I need to take a look at the SSS falloff, and for that I need to see how far the SSS penetrates into the shadow side of the orange. Don't worry about how the bump looks so strange, after adding SSS the high contrast washes out. 

2.0 epidermal radius

For the above render, the epidermal radius is set at 2.0, which washes out any shadows, so i'll set it down to 0.2.

0.2 epidermal radius

This gives the shadow a softer edge and the light penetration is consistent with my reference. 

8.0 subdermal radius

The subdermal scatter looks like what I want to achieve, albeit not as intense as I am showing it here. If you shine a flash light, hold it close to the orange, you will discover that at high intensities, the light travels quite far inside the meat, and the skin has these little holes that light penetrates in and out of. Or the holes might not be holes but variations in the thickness of the orange peel. 

2.0 subdermal radius

Looks about the same, still too far inside the shadows

1.0 subdermal radius

Now, for the diffuse color, using a ramp with its built in HSV noise is good enough if the UV seams are hidden well enough. However, the models provided, and a lot of the times, UVs are an afterthought. As a shader artists, 3d procedural textures, custom coded or procedural texture trees(hypershade, slim, mental mill) are preferred.

diffuse on surface shader

Final Shading Tree

Final Render

Shader TD and the look development process explained

I found this gem in the CGtalk forums, couldn't have explained it better.

"Well, when you have multiple lighters working on the same sequence, you ideally don't want them to have to tweak the shaders to get the look that they are after. Not only does this take more time (and sometimes break the pipeline if all the shader parameters aren't promoted up to a settable/animatable level), but it also makes it difficult to keep the look consistent among several artists. 

Generally a reference light setup will be made for the show, or for each sequence in a film. The texture artists and look dev artists will make sure the objects look good under those lighting conditions. Then if needed, several variations of the object and/or its shader parameters will be published out for use by the lighting artist. Likewise, a lead lighting artist/TD will generally set up the overall lighting scheme for the set or sequence, and publish that out as well. Then it is up to the individual lighting artists to take the set lighting preset and the surfacing preset, and tweak the lights as needed on a per-shot basis.

Lighters shouldn't be tweaking textures, just like they shouldn't be tweaking geometry. If any texture or shader or geometry problems show up, they should be kicked back as retakes to the appropriate departments. Likewise, lighters shouldn't be tweaking shaders either. Lighters should only be concentrating on the lighting (and often compositing) of the scene. 

Now, I'm not saying that in practice many lighters don't wind up tweaking everything under the sun in order to get their shots to work. They do. They just shouldn't have to. It is messy, often hard to reproduce across multiple shots, and generally a waste of their time."

-MDuffy

Link

Making a Grape Shader

Intro
__________________________________________________________


A little about what i'll be trying to achieve with this tutorial,
1. create a grape shader(and other various shaders)
2. create a grape phenomenon shader
3. create presets for the grape shader and adapted to use with cherries.


Ok, so we want to make some delicious fruity shaders, seeing theres an available and free fruit platter over at 3drender.com's lighting challenge we'll just start with that. Next up, we'll need to gather some references,

Image 1
HSV range of grapes

Image 2

Image 3

Image 4

Images 5

Images 6

Image 7

There are several varieties of grapes, with hue variations between purple and green. I have decided to proceed with the red/maroon grapes.


From the reference we can start determining the number of layers and shading component this shader would require.

The Basics

__________________________________________________________

• diffuse
• reflection
• specular
• sss
• rim

1. in closer examination, the grape skin has some white discoloration to it.

2. the reflection is from the water, and there is a glossy reflection/spec from the skin

3. two layers, the skin and the meat

4. heavy rim effect, combine with reflection fresnel effect






1.Diffuse
__________________________________________________________
One thing to remember is that since we will be adding SSS the diffuse component should be darker than what appears in the photo. Try peeling a grape skin, and flatten it out on the table, that should be the diffuse color component(and even darker than that for effect). There are some white discoloration on the grape skin, from what i read they are molds?. I'll add the effect, but tone it down, so it still looks real, but more appealing.

Diffuse Component Sub Tree

Diffuse Component(on surface shader)

For the white mold i used two 3d textures, a 3d noise and multiply it by a crater to break up the pattern. And for the base, I use a ramp as a solid color, and add small bit of HSV color noise. Breaks up the monotony of solid colors. Add the white mold to the base, but set the alpha of the mold layer to something low. 


Key Settings: 

• Base color
• White mold alpha

2. Reflection
__________________________________________________________

Look closely at the reference, particularly images 2 and 6. In image 2, the reflection intensity are higher due to the water. In image 6, we can see the base glossy reflection/specularity. We should also consider whether the white mold has any effect on base glossy reflection/specularity.  Before you start, get a hdr map in there, there are three ways to do this, 

• Image Based Lighting under render globals
• mib_lookup_spherical connected to the environment slot in the Shading Goup
• if theres a slot in the node

I use the second method as its more flexible.

Reflection and Water Reference

I'm aiming for the glossy base reflection of the grape skin first. So I'll put in a mib_glossy_reflection.

Glossy Reflection

Here we have glossy reflection at its default settings(environment color at white), the environment color is really just the intensity(really terrible that Mental Ray lacks consistency across the board). Notice how the area light disappeared, and seems to be a blackhole, not good. Most of the time mib_glossy_reflection will work for me, however, i need to tweak the surface shading normals(ie bump) on my reflection, so i'll use the mia material instead. 

Base MIA Glossy Reflection

For MIA glossy reflection, i set the BDRF curve to 1.0, and 90 degree refl intensity to 0. As I don't want the rim/fresnel effect here. It still shows, however, but neglible. Tweak the glossiness and intensity and turn off everything else. 


Next, for the water, we need to analyse the noise pattern, scale, and frequency of the droplets. So some observations,

• droplets are ~0.5mm to ~2mm in diameter
• droplets occur more on the top, and less on the bottom
• a single large droplet on the bottom, on some grapes.

Droplet Tree

Lrg and Sml Droplet Settings

Mask Settings

I will use the leather 3d texture node for both small and large droplets, and a volume noise for masking out areas for small droplets. Note the remap HSV, both large and small droplet uses the same curve, the curve defines the profile of droplet. 

Droplet bumps

Scale  Y

I'm testing with a lambert shader. Since we're using a 3d texture, connect the map to a 3d bump > misss_normal > mia bump slot. The large drops are too round, so I will scale the Y axis of the placed3d node to simulate the gravity effect on the droplet. I scaled the small droplets the same way, but less. 

Mask

Reflection + Refraction

Base + Droplet

Value Correct

I will turn off the diffuse component and turn on reflection and refraction now. I mapped the noise to an HSV remap to get the contrast and value for reflection and refraction. I corrected the final output with a HSV remap as the hottest highlights are blowing out, and the dark side reflections aren't showing up. 

3. SSS

__________________________________________________________

Looking at the references, we can determine some properties,

• grape skin, should be similar to the diffuse layer, but brighter.
• the meat is yellowish, with vein patterns in them
• the back scattering is yellowish as well, to give that very translucent look

Shading network and settings.

Theres plenty of SSS tutorials out there, so i'll just throw out the settings. I'm using the fast_SSS shader here. The diffuse color goes into the diffuse color slot here as well. Kind of skipping on detail methodology here, but the basic shading principles still apply here, Add noise to everything, and add noise to the noise. 


4. Rim
__________________________________________________________

Rim Tree and Settings

There are light side rims and the dark side rims. Using a sampler info node > ramp(v coord) > lambert(color), i can create the light side rim. I would first invert the ramp, then tweak the ramp for the rim thickness i desire.

the dark side rim would be like this,


A. sampler info > ramp > surface shader(color)
B. sampler info > ramp > lambert(color)


A-B = dark side rim, so use a layered texture and subtract it.


Notes: for B, i would set diffuse to  a value between 1 and 3 to get a thin dark side rim. but anything greater than 1 i would add a Remap HSV and set the high saturation level to inverse of diffuse level. so if diffuse is 3, saturation would be 0.333.

B. sampler info > ramp > lambert(color) > Remap HSV > layered texture.


5. Combining everything
__________________________________________________________

Final Tree

Extreme Closeup

Close Up

Final View

The droplet doesn't hold up that well under extreme closeups, but at that distance, the droplets becomes the focus, and a particle on surface + blobby solution will be better suited for our hero droplet. 


Just before going for the final render, I turned on Final Gather, and I turned on indirect lighting for the lightmap as well. I went back and forth and tweaked the settings, the basic setup is there, however, the difficult part is balancing every aspect of the shader tree and coming to a conclusion on how YOU want it to look like. Sometimes, its easier to just go for a photorealistic look, but sometimes  it looks better placing emphasis on several aspects. In this instance, I upped the SSS, specifically the back SSS, and left the reflection on relatively high.