Tutorial 114: PBR Materials with PbrEffect and SkinnedPbrEffect

CNA Tutorials  ·  3D Content

PbrEffect and SkinnedPbrEffect implement glTF 2.0’s metallic-roughness material model against the specification’s own reference BRDF: a GGX normal distribution, Smith-Schlick-GGX visibility, and Schlick Fresnel. They are CNAEXT effects with the familiar stock-effect workflow — construct one, set properties, bind it, draw.

They are also unambiguously CNAEXT. Real XNA 4.0 predates the physically-based content pipeline entirely; there is no XNA or FNA equivalent to mirror, so nothing here is an attempt at API fidelity. The CNAEXT marker sits on the members that have no XNA equivalent rather than on the class declaration, and none of it depends on a build option. Tutorial 115 explains what the marker means for your build.

The material surface

Each of the seven texture slots comes as a getter, a raw-pointer setter and an owning setter that keeps the texture alive. The five core slots use the original property names; the two KHR_materials_specular slots carry the EXT suffix.

The metallic-roughness map uses glTF’s own packing — green is roughness, blue is metallic. The occlusion map is read from the red channel, where 1 is fully lit and 0 fully occluded.

The scalar and vector factors multiply into those maps, and work perfectly well on their own when a slot is empty:

Putting it together, a hand-built brushed-metal material:

#include "Microsoft/Xna/Framework/Graphics/PbrEffect.hpp"

using namespace Microsoft::Xna::Framework;
using namespace Microsoft::Xna::Framework::Graphics;

PbrEffect effect(device);

effect.setTextureProperty(&albedo);
effect.setNormalMapProperty(&normal);
effect.setMetallicRoughnessMapProperty(&metalRough);

effect.setMetallicFactorProperty(1.0f);
effect.setRoughnessFactorProperty(0.35f);
effect.setEmissiveFactorProperty(Vector3::Zero);

effect.setWorldProperty(world);
effect.setViewProperty(view);
effect.setProjectionProperty(projection);
effect.EnableDefaultLighting();

Use the SetOwned* variants when the effect should outlive your own reference to the texture — which is exactly what the model importers do, since the textures they decode have no other owner:

effect.SetOwnedNormalMap(std::shared_ptr<Texture2D>(std::move(decodedNormalMap)));

Alpha, sidedness and colour space

Beyond the maps and factors, the effect carries the rest of a glTF material’s state. Some of it is carried rather than applied: it travels with the material, and your draw code acts on it.

Lights, shadows and image-based lighting

PbrEffect implements IEffectMatrices, IEffectFog and IEffectLights, so its lighting and fog surface is the one you already know from BasicEffect: DirectionalLight0 through DirectionalLight2, setAmbientLightColorProperty(), EnableDefaultLighting(), and the four fog properties. Since alpha.1 it also implements IShadowReceiverEXT and accepts an ImageBasedLightEXT; those two are the new part of this section.

DirectionalLight& key = effect.getDirectionalLight0Property();
key.setEnabledProperty(true);
key.setDirectionProperty(Vector3::Normalize(Vector3(-0.5f, -1.0f, -0.3f)));
key.setDiffuseColorProperty(Vector3(1.0f, 0.96f, 0.9f));

effect.setAmbientLightColorProperty(Vector3(0.05f, 0.06f, 0.08f));

Without an environment — the default, and the only option on a renderer that ignores one — a metal reflects the analytic lights and the ambient term and nothing else, so a mirror-smooth metal sphere in an empty scene looks dark and flat rather than wrong. Give rough metals a little ambient and a fill light and they read correctly.

Image-based lighting

Both effects have setImageBasedLightEXT(const ImageBasedLightEXT&) and getImageBasedLightEXT(). The struct carries the three split-sum products — an Irradiance cube for diffuse light, a PrefilteredSpecular cube whose mip level is roughness, and a BrdfLut table — plus PrefilteredMipCount and an Intensity. IsValidEXT() is true only when all three textures are set; an invalid bundle is treated as no environment. The struct holds raw pointers, so the textures it names must outlive the effect’s draws.

#include "Microsoft/Xna/Framework/Graphics/ImageBasedLightEXT.hpp"

ImageBasedLightEXT ibl;
ibl.Irradiance          = irradianceCube;      // TextureCube*
ibl.PrefilteredSpecular = prefilteredCube;     // TextureCube*, mip chain = roughness ramp
ibl.BrdfLut             = brdfTable;           // Texture2D*
ibl.PrefilteredMipCount = 5;
ibl.Intensity           = 1.0f;

if (device.SupportsImageBasedLightingEXT())    // see the renderer table below
    effect.setImageBasedLightEXT(ibl);

CNA does not generate these textures. Bake the irradiance cube, the prefiltered specular cube (one mip level per roughness step) and the BRDF lookup table with a tool of your choice or your own render passes, and load them as ordinary TextureCube and Texture2D content; ImageBasedLightEXT and the effect side that consumes them are always compiled.

Shadows

IShadowReceiverEXT is the always-compiled receiving half of shadows, implemented by BasicEffect, SkinnedEffect, PbrEffect and SkinnedPbrEffect: setShadowMapEXT, setLightViewProjectionEXT, setShadowsEnabledEXT, setShadowDepthBiasEXT, setShadowFilterRadiusEXT, setShadowCascadesEXT (a ShadowCascadeStateEXT of up to four cascades) and setPunctualLightEXT (a PunctualLightEXT point or spot light with a cube or 2D shadow). CNA does not generate the maps: they come from a depth pass you write yourself, and Tutorial 59 shows both the pass and how to hand its result to these receivers. A shadow attenuates the direct lights only; the environment term is untouched.

Shadow sampling is honoured only where GraphicsDevice::SupportsShadowSamplingEXT() is true. Everywhere else the state is accepted and ignored, which gives an unshadowed image and no error.

One divergence from the other stock effects: setLightingEnabledProperty(false) is refused. A physically-based material with no lighting has no defined meaning, so the effect mirrors the same constraint real XNA’s SkinnedEffect already imposes.

SkinnedPbrEffect

SkinnedPbrEffect is the whole material surface above plus SkinnedEffect’s bone API — identical names, identical limits:

Which means an AnimationPlayer drives it exactly as it drives a SkinnedEffect:

pbrSkinned.SetBoneTransforms(player.GetSkinTransforms());

See Tutorial 112 for the player, and Tutorial 57 for the bone limits in detail.

How the importer decides

You will usually not construct these effects yourself — the glTF importer picks one per mesh part. The choice follows the material model, not the presence of a particular texture, and this has held since alpha.1.

PBR takes priority over the legacy dual-texture approximation. Vertex colour multiplies the PBR base colour in both rigid and skinned layouts; it no longer changes the selected material model.

Once the importer has chosen, the maps and factors are wired in for you, including KHR_materials_emissive_strength, whose multiplier is folded into the emissive factor before the emissive texture is applied. To adjust an imported material at runtime, cast and set. PbrEffect and SkinnedPbrEffect are separate classes, so cast for both (and note that a loop written for BasicEffect skips imported glTF parts entirely):

for (ModelMesh* mesh : model.getMeshesProperty()) {
    for (ModelMeshPart* part : mesh->getMeshPartsProperty()) {
        if (auto* pbr = dynamic_cast<PbrEffect*>(part->getEffectProperty())) {
            pbr->setRoughnessFactorProperty(0.8f);   // scuff the whole surface
        } else if (auto* skinned = dynamic_cast<SkinnedPbrEffect*>(part->getEffectProperty())) {
            skinned->setRoughnessFactorProperty(0.8f);
        }
    }
}

Factor-only and material state

The same rule covers a primitive with no material object: glTF defines that case as its default metallic-roughness material, so CNA creates the corresponding PBR effect rather than silently substituting a different lighting model.

Renderer coverage

Six renderer implementation families — every family that draws pixels — implement the universal PBR draw parameters, and two renderers draw nothing. This table lists all 10 public identities; the last column is the separate question of shadow and environment sampling:

These are implementation families as much as identities: EasyGL alone serves three of the identities above. Each renderer reports its own answer as PbrEffectShading and SkinnedPbrEffectShading in its capability profile (Tutorial 133); query it and test the concrete build rather than assuming a renderer is visually equivalent to another. The first row is pinned by CNA’s glTF PBR fallback-policy tests, whose inventory covers every renderer family that consumes PBR maps. That is a statement about code paths, not about pixels compared against a reference renderer.

The Windows Direct3D workflow (d3d-windows-ci.yml) is manually dispatched at this snapshot, so the DIRECTX9 entries rest on the code and on Wine/DXVK or cross-build evidence — not on an automatic or physical Windows run. See Tutorial 72 for the single- and multi-renderer modes, and verify PBR on the concrete renderer and driver you intend to ship.

Per-map UV channels and transforms

glTF lets every texture reference pick its own TEXCOORD set and apply an independent KHR_texture_transform. The importer carries the first two distinct authored sets into renderer-facing UV0/UV1, records a selector for each of the five core maps and both specular maps (setTextureCoordinateSetEXTProperty(slot, set)), and transports every map’s scale/rotation/translation independently (setTextureTransformEXTProperty(slot, TextureTransformEXT{Offset, Scale, Rotation})).

A material requesting a third distinct authored set exceeds the two-channel vertex layouts. CNA falls that map back to packed channel 0 and adds a stable diagnostic to Model::getGltfImportReportEXTProperty(); both direct glTF and converter-generated .cnj models expose the report.

Where to go next

• Tutorial 59: Shadow Mapping — render a shadow map and feed it to a BasicEffect or PbrEffect.
• Tutorial 115: CNAEXT — what the marker on these effects actually does.
• Tutorial 112: Skeletal Animation from glTF — driving SkinnedPbrEffect.
• Tutorial 37: Multiple Lights — the three-light convention in depth.
• Effects reference and the CNAEXT extensions reference