Realistic Skin Shader for XPS (Patreon)

UPDATE!!! Version : 2.0 (RealSkin PBR Preintegrated 2.fx) fixed some black artifacts

A skin shader built on the same pipeline production renderers

and AAA games use for skin: linear space lighting, curvature

driven pre-integrated subsurface scattering, GGX specular and

filmic tonemapping. Best used on characters with normal and specular maps.

This is NOT a tweak of the usual XPS skin shader. It is a

different approach from the ground up. Most homemade skin

shaders are artistic approximations: a global wrap term plus

hand tuned color tints. This one derives the look from the

geometry and from physical material constants.

It also fixes the seams that show up in almost every homemade

XPS skin shader. See section 5.

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1. INSTALLATION

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1) Copy this file:

RealSkin PBR Preintegrated 1.fx

into the "shaders" folder of your XPS install, normally:

...\XNALara_XPS\shaders\

2) Open XPS. If it was already running, close it and reopen it.

3) In the Control Window go to:

Shader > User written shader > RealSkin PBR Preintegrated 1

4) Done. The check mark means it is active.

5)

NOTE: XPS will generate a .fx.txt file next to the shader by

itself. That is the compilation listing. It is normal, leave it

alone.

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2. IS THIS THE RIGHT SHADER FOR YOUR MODEL?

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READ THIS FIRST. It will save you a lot of confusion.

USE IT FOR:

Models with photographic skin textures, a normal map and

ideally a specular map. Realistic characters, scans, ports

from modern games. This is where curvature driven scattering

and GGX specular actually have something to work with.

DO NOT USE IT FOR:

Toon and anime models. Their textures already have the

shading painted into them by the artist. A PBR skin shader

will not add realism to a flat toon texture, it will remove

the look that was baked in. For those models use a toon

shader instead.

The shader is doing its job correctly in both cases. It simply

has nothing to compute on a texture where the light was already

painted by hand.

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3. THE 4 USER PARAMETERS

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Set them in XPS under: Config > User parameters

All of them range from 0.0 to 1.0

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PARAMETER 1 - SMOOTHNESS / WETNESS

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0.0 = dry matte skin (roughness 0.60)

1.0 = wet skin (roughness 0.12)

Recommended: 0.25 - 0.45

In PBR this is ROUGHNESS, not "highlight size". It controls

how wide AND how intense the reflection is at the same time,

because in a real material those two things are not

independent. A rough surface scatters its reflection over a

wide area, which also makes it dimmer. That coupling is why

PBR materials read as real and ad hoc ones do not.

* Normal skin ......... 0.30

* Sweaty / oiled ...... 0.60

* Lips ................ 0.65

* Matte character ..... 0.10

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PARAMETER 2 - SPECULAR REFLECTANCE

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0.0 = physically correct skin F0 (0.028), x1

1.0 = x3, exaggerated photoshoot skin

Recommended: 0.25 - 0.5

At 0 this does NOT turn the highlight off. Real skin always

reflects around 2.8% of the light hitting it head on, and

pretending otherwise is exactly what makes CG skin look like

clay. If you want less shine, raise the roughness with

parameter 1 instead.

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PARAMETER 3 - SUBSURFACE SCATTERING

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0.0 = no scattering, plain Lambert diffuse

1.0 = maximum subsurface diffusion

Recommended: 0.5 - 0.8

This is the flagship parameter of the shader. It drives the

curvature based pre-integrated scattering described in

section 4. Nose, ears, lips, knuckles and fingers will light

up red on their own, while the forehead and chest stay clean.

You do not paint anything and you do not tune anything per

character.

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PARAMETER 4 - SHADOW DARKNESS

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0.0 = natural falloff

1.0 = deep, contrasted, near black shadows

Recommended: 0.4 - 0.7

Applies a power curve (exponent 1.7 to 5.0) to the shading

factor of all three lights. Since that factor runs from 0 to

1, raising it to a power above 1 sinks the shadow side and

leaves the lit side completely untouched: 1 to any power is

still 1. So it darkens and tightens the terminator without

dimming your character.

It is applied BEFORE each light's native "Shadow depth", so

that control still works if you want to lift the shadow back

up on one specific light.

It also acts on Light 3. That matters: Light 3 is half

Lambert fill, so it never drops below 0.5 at the equator and

was leaving a constant veil of light over the whole model

that no "Shadow depth" setting could remove.

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RECOMMENDED STARTING POINT

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Parameter 1 = 0.30

Parameter 2 = 0.35

Parameter 3 = 0.60

Parameter 4 = 0.55

HEADS UP: XPS initializes all 4 parameters to 0. If you just

enabled the shader and everything looks matte and flat, it is

NOT broken, the sliders are simply still at zero.

EXPOSURE is controlled with the light intensity sliders in

XPS, not with a shader parameter. The filmic tonemapping

means you can push those intensities well past 1.0 without

blowing the image out to flat white.

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4. WHAT MAKES THIS DIFFERENT

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* LINEAR SPACE LIGHTING

The diffuse texture and the light colors are converted from

sRGB to linear, ALL the light is accumulated in linear, and

the result is converted back at the very end. This is the

single biggest factor in how the result reads, and almost no

XPS shader does it. Adding lights in sRGB, which is the

normal thing to do, distorts the terminator, dirties the

blend between two differently colored lights and flattens

every gradient.

* CURVATURE DRIVEN PRE-INTEGRATED SCATTERING (Penner)

Instead of one global wrap term applied equally to the whole

body, the shader measures the actual CURVATURE of the mesh at

every pixel using screen space derivatives, and scatters the

light more where the geometry curves more. It also scatters

PER CHANNEL: red travels several times further under the skin

than green and blue, which is why skin shadows go red.

The result comes out of the geometry itself. Thin, curved

parts glow, flat parts do not, automatically.

* GGX SPECULAR INSTEAD OF BLINN-PHONG

GGX has the long tail that skin actually has: the highlight

does not end abruptly, it fades. With Schlick Fresnel over

the physical F0 of skin (0.028) and Smith visibility.

* ENERGY CONSERVATION

Light reflected by the specular layer cannot come back out as

diffuse, so it is subtracted. Without this, shiny skin ends

up looking lit from the inside.

* FILMIC TONEMAPPING (ACES)

Most shaders end in saturate(), which CLIPS at 1.0: strong

highlights become flat white blobs with no gradient. Here a

filmic curve rolls the highlights off smoothly the way a real

camera does. This is a large part of what makes a render look

rendered.

The scene is pre-exposed by 0.6 before the curve, because the

Narkowicz ACES fit expects a scene already exposed with mid

grey near 0.18, while XPS hands over values with mid grey

near 0.5. Without that pre-exposure the curve works in its

upper range and returns MORE light than it receives, which

washes the image out.

* PORE MICRO-OCCLUSION

Derives cavity from the difference between the detail normal

and the geometric normal, and darkens slightly there. Real

depth in pores and wrinkles with no AO map required.

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5. SEAMS, AND WHY THIS SHADER HAS NONE

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If you have tried homemade skin shaders for XPS you have

probably seen a strange line running down the center of the

face, or a hard cut at the neck. There are four common causes.

All four are fixed here. Documented in case you write your own

shaders:

1) NORMAL MAP DECODING

XPS models have MIRRORED UVs down the center of the face and

the body. Tangent handedness flips at the mirror line. If

the normal map is reconstructed incorrectly - ignoring the Z

channel, using a skewed bias, or failing to invert the green

channel as the XPS convention requires - the lighting splits

right there. You must decode all THREE channels and apply

the TangentSpace_R/G/B remap with green negated.

2) VFACE

When a back face is detected, flip ONLY the normal. If you

also flip the tangent and the bitangent you create a hard

discontinuity at the boundary between front and back faces.

3) CullMode = NONE

People add this "to avoid seams" and get the exact opposite:

it draws back faces and shades them inverted. Do not set it.

Let XPS control that render state from its own options.

4) RENDER GROUPS

If you sample the bump or specular texture on a mesh that

does not have one, another mesh's texture stays bound. The

result is a perfectly visible line exactly at the border

between two meshes: face/neck, neck/torso. You have to check

render group tables before sampling.

The subtle part almost nobody catches: the specular MATH is

safe to run on every mesh. The only thing that has to be

filtered by render group is TEXTURE SAMPLING.

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6. LIGHTING TIPS

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* Use all 3 lights. The shader treats Light 3 as soft hemisphere

fill with no specular contribution, specifically for that job.

* Put Light 2 BEHIND the model at low intensity and raise

parameter 3. That backlight triggers the transmission term

and lights up ears, nostrils and fingers in red. It is the

single most convincing effect in the whole shader.

* Enable bump maps in XPS Options, or you lose the pore

micro-occlusion entirely.

* Push light intensity above 1.0 freely. The filmic curve will

compress the highlights instead of clipping them. That is how

you expose, since there is no exposure parameter.

* High parameter 4 (0.6+) with a single strong key light gives

an excellent dramatic portrait look.

* If the character has a specular map, its alpha channel drives

roughness and overrides parameter 1 on that mesh. That is

intentional: the model's own map knows better than a global

slider where the skin is shinier.

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7. TROUBLESHOOTING

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"Everything looks flat and matte when I enable it"

All 4 parameters are at 0, which is how XPS initializes

them. Go to Config > User parameters and enter the

recommended values from section 3.

"It looks worse than the default shader on my model"

Check section 2. If it is a toon or anime model with flat

painted textures, this shader is the wrong tool. Use a toon

shader for those.

"Parameter 1 does nothing on some meshes"

That mesh has a specular map with glossiness in the alpha

channel, and the map takes priority. Intentional.

"The image is too bright / washed out"

Lower the light intensities in XPS and raise parameter 4.

"Too dark"

Raise the light intensities. You can go well above 1.0

safely thanks to the tonemapping.

"The subsurface effect is too strong or almost invisible

everywhere"

Curvature is measured in screen space, so it depends on the

scale of your model. Open the .fx in a text editor and look

for this line:

float curvature = saturate(deltaN / max(deltaP, 0.00001) * 1.5);

Raise or lower that 1.5 to taste. Also note the effect is

naturally stronger in close ups than in wide shots. That is

inherent to screen space curvature, not a bug.

"I still see a seam"

It is almost certainly the MODEL, not the shader. Models

imported from .obj inherit render groups incorrectly in

XPS. Convert the model to .xps / .mesh format and retry.

"Hair or eyelashes look wrong"

This is a SKIN shader. Transparent hair and eyes live in

render groups without shading and the shader passes them

through untouched, but if your model has its hair assigned

to a skin render group it will be shaded as skin.

"XPS throws a compile error"

Make sure you copied the complete .fx and that your XPS

version supports user shaders (11.8.x or newer).

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8. CREDITS AND LICENSE

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Built on XNAaraL's XNALara / XPS shader structure.

The render group tables, the correct TBN construction and the

normal map channel conventions come from dasliebesverbot's (MF)

"Enhanced Shading" shader on DeviantArt, which is the best

public reference in existence on how XPS user shaders actually

work. Full credit to them:

https://www.deviantart.com/dasliebesverbot/journal/

Enhanced-Shading-Custom-shader-957472601

The pre-integrated skin technique is based on Eric Penner's

"Pre-Integrated Skin Shading". The ACES tonemapping curve is

Krzysztof Narkowicz's analytic fit.

LICENSE

Free software, same as the shader it inherits from. You may

use, modify and redistribute it freely. The ONLY condition:

if you modify it, share your modifications freely too.

Free for personal and commercial renders. Credit is always

appreciated but not required.

If you make something nice with it, show me. That is why I am

sharing it.