Pbr руководство от allegorithmic том 1

PBR_Guide_Vol.1

PBR_Guide_Vol.1

PBR_Guide_Vol.1

PBR_Guide_Vol.1

User Manual: Pdf

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Tutorial 3djobs.ru

Последние новости о CG-индустрии, 3D-программном обеспечении, уроках, видеороликах, анимации, обзорах, продуктах VFX и многом другом.

Свет и Материя: Теория физически-корректного рендеринга и шейдинга

http://perevodvsem.ru/threads/the-comprehensive-pbr-guide-volume-1-the-theory-of-physically-based-rendering.635/

Это первая из двух частей это руководства, в которой сфокусируется на теории физически корректного рендеринга и шейдинга.

Это важно для нас, чтобы понимать, как лучи света взаимодействуют с поверхностью вещества, потому что наша работа как текстурщиков, при создании для создания текстуры, описать-воплотить эти свойства. Текстуры и материалы, которые мы создаём взаимодействуют со светом в наших виртуальных мирах и большее понимание того, как свет ведёт себя, способствует лучшему виду наших текстур. Мы начнём с рассмотрения луча сета и продвинемся до определения основных факторов для PBR.

Это руководство собрано Весом МакДермоттом параллельно протестировано экспертами в этой области

Инструкция по работе с PBR от Allegorithmic Том 2.

Свет и материя: Практические советы по созданию PBR текстур (5-я редакция)

http://perevodvsem.ru/threads/the-comprehensive-pbr-guide-volume-2-practical-guidelines-for-pbr-texturing.712/

Во второй Части мы обсудим практическое применение создаваемых нами PBR текстур, а так же получим набор основополагающих принципов, основанных на полученных знаниях из первой части руководства

Мы начнём с переосмысление PBR визуализации с художественной точки зрения. Далее поговорим о основополагающих рабочих процессах и принципах отображения «металистичности»/шероховатости и блеска/отражения для создания физически корректных текстурных карт.

Это руководство собрано Весом МакДермоттом параллельно протестировано экспертами в этой области

Книги перевел Helluvallyric

The Comprehensive PBR Guideby Allegorithmic — vol. 1Light and Matter : The theory of Physically-Based Rendering and Shading Cover by Gaëtan Lassagne, written by Wes McDermott

Table of Contents• Light Rays — 2• Absorption and Scattering (Transparency and Translucency) — 3• Diffuse and Specular Reflection — 4 Microfacet Theory — 5• Color — 6• BRDF — 6• Energy Conservation — 7• Fresnel Effect — 7 F0 (Fresnel Reflectance at 0 Degrees) — 8• Conductors and Insulators (Metals and Non Metal) — 9 Metals — 9 Non-Metals — 10• Linear Space Rendering — 11• The Key Factors — 11• References — 12Page 1 Technical edit by: Cyrille Damez and Nicolas Wirrmann vol. 1 — The theory of Physically-Based Rendering and Shading

Light and MatterThe theory of Physically-Based Rendering and ShadingLight is a complex phenomenon as it can exhibit properties of both a wave and a particle. As a result, different modelshave been created to describe the behavior of light. As texture artists, we are interested in the Light Ray Model as itdescribes the interaction of light and matter. It’s important for us to understand how light rays interact with surfacematter because our job is to create textures that describe a surface. The textures and materials we author interactwith light in our virtual worlds and the more we understand about how light behaves, the better our textures will look.In this guide, we will discuss the theory behind the physics through which physically-based rendering models arebased upon. We will start with a light ray and work up to defining the key factors for PBR.Light RaysThe Light Ray Model states that a light ray has the When a light ray hits a surface, either or possibly both oftrajectory of a straight line in homogeneous transparent these things can happen:media such as air. The Light Ray Model also says that theray will behave in a predictable manner when 1. The light ray is reflected off the surface and travelsencountering surfaces such as opaque objects or passing in a different direction. It follows the Law of Reflection,through a different medium such as air to water. This which states that the Angle of Reflection is equal to themakes it possible to visualize the path the light ray will Angle of Incidence (Reflected Light).follow as it moves from a starting point to where iteventually changes into another form of energy such as 2. The light ray passes from one medium to another inheat. the trajectory of a straight line (Refracted Light).The light ray that hits a surface is called the Incident Ray At this point, we can state that light rays split into twoand the angle that at which it hits is called the Angle of directions: reflection and refraction. At the surface, theIncidence as shown in figure 01. ray is either reflected or refracted and it can beA light ray is incident on a plane interface between two eventually absorbed by either medium. However,media. absorption doesn’t occur at the surface.Figure 01 Page 2 vol.1 — The theory of Physically-Based Rendering and Shading

Absorption and Scattering (Transparency and Translucency)When traveling in an inhomogeneous medium or hopefully clean, you can open your eyes and see at atranslucent material, light can be absorbed or scattered: fairly good distance through the clear water. However,1. With absorption, the light intensity decreases as it is let’s imagine that same pool hasn’t been cleaned in achanged into another form of energy (usually heat), and while and the water is dirty. The dirt particles scatter theits color changes as the amount of light absorbed light and thus make the clarity of the water much lower.depends on the wavelength, but the direction of the ray The further light travels in such a medium/material, thedoesn’t change. more it is absorbed and/or scattered. Therefore, object2. With scattering, the ray direction is changed randomly, thickness plays a large role in how much the light isthe amount of deviation depending on the material. absorbed or scattered. A thickness map can be used toScattering randomizes light direction but the intensity describe object thickness to the shader as shown in figuredoesn’t change. An ear is a good example. The ear is thin 02.(absorption is low), so you can see the scattered lightpenetrating out of the back of the ear. If there is no Object thickness plays a large rolescattering and the absorption is low, rays can pass in how much the light is absorbeddirectly through the surface such as with glass. For or scatteredexample, if you are swimming in a pool, which isFigure 02Diffuse and Specular Reflection Rougher surfaces will have larger and dimmer looking highlights. Smoother surfaces will keep specularSpecular reflection is light that has been reflected at the reflections focused, which can appear to look brighter orsurface, as we discussed above in the Light Ray section. more intense when looked at from the proper angle.The light ray is reflected off the surface and travels in a However, the same total amount of light is reflected indifferent direction. It follows the Law of Reflection, which both cases as shown in figure 03.states that on a perfectly planar surface the Angle of Diffuse reflection is light that has been refracted. TheReflection is equal to the Angle of Incidence. However, it light ray passes from one medium to another and isis important to note that most surfaces are irregular and scattered multiple times inside the object. Then it isthat the reflected direction will therefore vary randomlybased on the surface roughness. This changes lightdirection, but the light intensity remains constant.Page 3 vol. 1 — The theory of Physically-Based Rendering and Shading

refracted again out of the object making its way back to sense, does not take surface roughness into account, butthe original medium at approximately the same point there are diffuse reflection models that do such as Oren-where it went through the first time as shown in figure Nayar.04. Materials that have both high scattering but lowDiffuse materials are fairly absorbent, meaning that if the absorption are sometimes referred to as «participatingrefracted light travels for too long in that material, it has media» or «translucent materials». Examples of these area good chance of being completely absorbed. This means smoke, milk, skin, jade and marble. Rendering of thethat if the light ever comes out of that material, it has latter three may be possible with the additional modelingprobably not traveled very far from the point of entry. of subsurface scattering where the difference betweenThat’s why the distance between the entry and exit the ingoing and outgoing point of the light ray is nopoints can be neglected. The Lambertian model, which is longer neglected. Accurate rendering of medium withusually used for diffuse reflection in a traditional shading highly varying and very low scattering and absorption like smoke or fog may require even more expensive methods such as Monte Carlo simulations. Rougher surfaces will have larger and dimmer looking highlightsFigure 03Figure 04 Page 4 vol.1 — The theory of Physically-Based Rendering and Shading

Microfacet TheoryIn theory, both diffuse andspecular reflection are dependenton the surface irregularitieswhere the light rays intersect. Inpractice though, the effect ofroughness on diffuse reflection ismuch less visible because of thescattering happening inside thematerial. As a result, theoutgoing direction of the ray isfairly independent of surfaceroughness and the incidentdirection. The most commonmodel for diffuse reflection(Lambertian) completely neglectsit.In this document, we havereferred to these surfaceirregularities as surfaceroughness. Actually, it is often Figure 05referred to by several names the microsurface normal and the half normal are equalsuch as roughness, smoothness, glossiness or micro- will contribute as some will be blocked by shadowingsurface, depending on the PBR workflow in use, but they (light direction) or masking (view direction) as isdescribe the same aspect of a surface, which is sub-texel illustrated in figure 05.geometric detail. The surface irregularities at a microscopic level cause light diffusion. For example, blurred reflections are dueThese surface irregularities are authored in the roughness to scattered light rays. The rays are not reflected inor glossiness map depending on the workflow you are parallel so we perceive the specular reflection as blurredusing. A physically-based BRDF is based on the as shown in figure 06.microfacet theory which supposes that a surface iscomposed of small-scaled planar detail surfaces of The surface irregularities at avarying orientation called microfacets. Each of these microscopic level cause lightsmall planes reflects light in a single direction based on diffusionits normal as shown in figure 05.Micro-facets whose surface normal is oriented exactlyhalfway between the light direction and view direction willreflect visible light. However, not all microfacets wherePage 5 Figure 06 vol. 1 — The theory of Physically-Based Rendering and Shading

ColorThe color of a surface(which is to say the colorthat we see) is due towhich wavelengths areemitted by the light source,which are absorbed by theobject and which othersare reflected bothspecularly and diffusely.The remaining reflectedwavelengths are what wesee as color.For example, the skin of anapple mostly reflects redlight. Only the redwavelengths are scatteredback outside the apple skinand the others are Figure 07absorbed by it as shown in figure 07.It also has bright specular highlights the same color as Substance PBR shaders use thethe light source because with materials like the skin of an GGX microfacet distributionapple that are not electrical conductors (dielectrics),specular reflection is almost independent of wavelength.Therefore, for such materials the specular reflection isnever colored. We will discuss more about the differenttype of materials (metals and dielectrics) in later sections.BRDF rays can be considered as reversals of each other without affecting the outcome of the BRDF.A Bidirectional Reflectance Distribution Function (BRDF) The BRDF used by Substance’s PBR shaders is based onsimply put is a function that describes the reflectance Disney’s «principled» reflectance model, which is based onproperties of a surface. In computer graphics, there are the GGX microfacet distribution. GGX provides one of thedifferent BRDF models some of which are not physically better solutions in terms of specular distribution in that itplausible. For a BRDF to be physically plausible, it must has a shorter peak in the highlight and a longer tail in thebe energy conserving and exhibit reciprocity. Forreciprocity, I am referring to the Helmholtz Reciprocity falloff, which is to say that it looksprinciple, which states that incoming and outgoing light more realistic as shown in figure 08. Object GGX provides one of the better solutions in terms of specular distributionFigure 08 vol.1 — The theory of Physically-Based Rendering and Shading Page 6

Energy ConservationEnergy Conservation plays a vital role in physically-based rendering solutions. It states that the total amount of lightre-emitted by a surface (reflected and scattered back) is less than the total amount it received. In other words, thelight reflected off the surface will never be more intense than it was before it hit the surface. As artists, we don’t haveto worry about controlling Energy Conservation. This is one of the nice aspects of PBR in that energy conservation isalways enforced by the shader. It’s part of the physically-based model and it allows us to focus more on art ratherthan physics.Fresnel EffectThe Fresnel reflection factor also plays a vital role in Fresnel is not something that we control in PBR as we didphysically-based shading as a coefficient of the BRDF. in traditional shading. Again, this is another physicsThe Fresnel Effect as observed by French physicist aspect that is handled for us by the PBR shader. When itAugustin-Jean Fresnel states that the amount of light you comes to viewing a surface at a grazing incidence, allsee reflected from a surface depends on the viewing smoothed surfaces will become a nearly 100% reflectorangle at which you perceive it. at a 90 degree angle of incidence.For example, think of a pool of water. If you look straight For rough surfaces, reflectance will become increasinglydown, perpendicular to the water surface, you can see specular but we won’t approach 100% speculardown to the bottom. Viewing the water surface in this reflection. What matters then is the angle between themanner would be at zero degrees or normal incidence, normal of each microfacet and the light, not the anglenormal being the surface normal. Now, if you look at the between the normal of the «macrosurface» and the light.pool of water at a grazing incidence, more parallel to the Because the light rays are dispersed into differentwater surface, you will see that the specular reflections directions, the reflection appears softer or dimmer. Whaton the water surface become more intense and you may you get at a macroscopic level is a bit like the average ofnot be able to see below the surface of the water at all. all the Fresnel effect you would have for the microfacets.Figure 09 For rough surfaces, reflectance will become increasingly specular but we won’t approach 100% specular reflectionPage 7 vol. 1 — The theory of Physically-Based Rendering and Shading

F0 (Fresnel Reflectance at 0 Degrees)When light hits a surface straight on or perpendicularly (0 roughness the actual changes in value can be hard todegree angle), there is a percentage of that light that is see. However, there is a difference in the values. Inreflected back as specular. Using the Index of Refraction figure 11, you can see a chart that shows the F0 ranges(IOR) for a surface, you can derive the amount that is for both metal and non-metal materials.reflected back and this is referred to as Notice that the ranges for non-metals do not deviateF0 (Fresnel 0) as shown in figure 09. from each other drastically. Gemstones are an exceptionThe amount of light that is refracted as they have higher values. We will discuss F0 as itinto the surface is referred to a 1-F0. specifically relates to conductors and insulators a bit later.The F0 range for most commondielectrics will be from 0.02 — 0.05 andfor conductors the F0 range will be0.5-1.0. Thus, the reflectivity of asurface is determined by the refractiveindex as shown in the followingequation from Sebastien Lagarde’s»Feeding a Physically-based ShadingModel» blog post as shown in figure 10.It is the F0 reflectance value that weare concerned with in regards toauthoring our textures. Non-metals(dielectrics/insulators) will have agreyscale value and metals (conductors)will have an RGB value. With regards toPBR and from an artistic interpretation Figure 10of reflectance, we can state that for acommon smooth dielectric surface, F0 will reflectbetween 2% and 5% of light and 100% at grazing anglesas was shown in figure 09.The dielectric (non-metal) reflectance values don’tactually change very drastically. In fact, when altered byFigure 11 vol.1 — The theory of Physically-Based Rendering and Shading Page 8

Conductors and Insulators (Metals and Non-Metals)When creating materials for PBR, I find it helpful to think in terms of metal or non-metal. I simply ask myself if thesurface is metal or not. If it is, I follow one set of guidelines and if it’s not, I follow another. This can be a rathersimplistic approach as some materials may not fall into these categories such as metalloids, but in the overall processof creating materials, distinguishing between metal and non-metal is a good approach and metalloids are an exception.To set up guidelines for materials, we first must understand what we are trying to create. With PBR, we can look atthe properties of metals (conductors) and non-metals (insulators) to derive this set of guidelines. Refracted light is absorbed, the color tint of metals come from the reflected light and thus in our maps, we don’t give metals a diffuse colorMetalsMetals (conductors) are good conductors of heat and top of the raw metal. Only the raw metal exposed fromelectricity. Simply put, the electric field in conducting chipped away paint is treated as metal. The same goesmetals is zero and when an incoming light wave made of for dirt on the metal or any matter that obscures the rawelectric and magnetic fields hits the surface, it is partially metal.reflected and all the refractedlight is absorbed. The reflectance I stated above that I always ask myself if a material is avalue for polished metal is going metal or not. However, to be more precise, the questionto be high at a range of about should also inquire the state of the metal such as is it70-100% reflective as shown in painted, rusted or covered in dirt/grease. The materialfigure 12. will be treated as dielectric if it is not raw metal and there could be some blending between metal and non-metalSome metals absorb light at depending on the weathering.different wavelengths. Forexample, gold absorbs blue light Weathering elements can play aat the high-frequency end of the large role in the reflective state ofvisible spectrum so it appears metalyellow as a result. However,since the refracted light isabsorbed, the color tint ofmetals come from the reflectedlight and thus in our maps, wedon’t give metals a diffuse color.For example, in the specular/gloss workflow, raw metal is setto black in the diffuse map andthe reflectance value is a tintedcolor value in the specular map. Figure 12With metals, the reflectancevalue will be RGB and can be tinted. Since we areworking within a physically-based model, we need to usereal-world measured values for the metal reflectance inour maps.Another important aspect with metals in terms oftexturing is that metal can corrode. This means thatweathering elements can play a large role in thereflective state of metal. If the metal rusts for example,this changes the reflective state of the metal and thecorroded areas are then treated as a dielectric material asshown in figure 13.Also, metal that is painted is not treated like a metal butrather a dielectric as well. The paint acts as a layer onPage 9 vol. 1 — The theory of Physically-Based Rendering and Shading

Figure 13Non-MetalsNon-metals (insulators/dielectrics) Figure 14are poor conductors of electricity.The refracted light is scattered and/or absorbed (often re-emerging fromthe surface) and thus they reflect amuch smaller amount of light thanmetals and will have an albedo color.We stated earlier that the value forcommon dielectrics would be around2-5% based on the F0 as computedby the index of refraction. Thesevalues are contained within thelinear range of 0.017-0.067 (40-75sRGB) as shown in figure 14. Withthe exception of gemstones, mostdielectrics will not be greater than4%.Just as with metals, we need to use real-world measured The value for common dielectrics isvalues, but it can be difficult to find an IOR for other around 2-5% based on the F0 asmaterials that are not transparent. However, the value computed by the Index ofbetween most common dielectric materials doesn’t Refraction (IOR)change drastically, so we can utilize a few guidelines tofollow in terms of reflectance values, which we will coverin volume two. vol.1 — The theory of Physically-Based Rendering and Shading Page 10

Linear Space RenderingLinear space rendering can take up an entire article all on its own. So, we won’t go in-depth into the specifics.However, the important takeaway is that computations are calculated in linear space.Simply put, linear space rendering provides correct math for lighting calculations. It’s about creating an environmentthat allows light to behave as it does in the real world. In linear space, the gamma is 1.0. However, for this to lookcorrect to our eyes, the linear gamma needs to be shifted. Gamma-encoded space (sRGB) compensates for imagesthat are displayed on a computer screen. The value of the image is adjusted for display.When computing color values and performing operations on colors, all computations should be performed in linearspace. A simple way to look at it is that if an image is to be displayed in the render such as base color or diffuse, thenthese maps need to be set as sRGB. What happens in Substance is that if the image is tagged as sRGB, it will beconverted to linear for calculations and then set back to sRGB for display. However, when you store mathematicalvalues that purely denote surface attributes in a texture such as roughness or metallic, then these maps must be setas linear.Substance handles the conversion between linear/sRGB space for inputs automatically as well gamma-correction onthe computed result in the rendered viewport. As the artist, you don’t need to worry about the internal working oflinear-space computations and conversions in the Substance pipeline. When using Substance materials via theSubstance Integration plugin, the conversions for linear space are also handled automatically.However, it’s important to understand the process, as when Substance maps are utilized as exported bitmaps and notSubstance materials, you may need to manually handle the conversions depending on the renderer you are using. Youneed to know that base color/diffuse maps are sRGB and the rest are linear. When using Substance materials via the Substance Integration plugin, the conversions for linear space are also handled automatically.Key FactorsNow that we have explored the basic theory behind the physics, we can derive some key factors for PBR. 1. Energy Conservation. A reflected ray is never brighter than the value it had when it first hit the surface. Energy Conservation is handled by the shader. 2. Fresnel. The BRDF is handled by the shader. The F0 reflectance value has minimal change for most common dielectrics and falls within a range of 2% — 5%. The F0 for metals is a high value ranging from 70-100%. 3. Specular intensity is controlled through the BRDF, roughness or glossiness map and the F0 reflectance value. 4. Lighting calculations are computed in linear space. All maps that have gamma-encoded values such as base color or diffuse are usually converted by the shader to linear, but you may have to make sure that the conversion is properly done by checking the appropriate option when importing the image in your game engine or renderer. Maps that describe surface attributes such as roughness, glossiness, metallic and height should be set to be interpreted as linear.Page 11 vol. 1 — The theory of Physically-Based Rendering and Shading

References1. Physically-Based Shading at Disney Brent Burley, Walt Disney Animation Studios. https://disney-animation.s3.amazonaws.com/library/s2012_pbs_disney_brdf_notes_v2.pdf2. Microfacet Models for Refraction through Rough Surfaces http://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.pdf3. Feeding a Physically-Based Shading Model by Sebastien Lagarde http://seblagarde.wordpress.com/2011/08/17/feeding-a-physical-based-lighting-mode/4. An Introduction to BRDF Models by Daniël Jimenez Kwast http://hmi.ewi.utwente.nl/verslagen/capita-selecta/CS-Jimenez-Kwast-Daniel.pdf Allegorithmic develops the new generation of 3D texturing software: Substance Painter, Substance Designer and Bitmap2Material. With most AAA game studios using these tools, Substance has become the standard for creating next-generation PBR (Physically Based Rendering) assets. For more information on Substance, please visit our website at www.allegorithmic.comvol.1 — The theory of Physically-Based Rendering and Shading Page 12