BSDFs

Materials are attached to shapes either inline or via a named reference. A named BSDF is declared at the top level with an id attribute and referenced from shapes using <ref id="..."/>.

Each material below shows a preview rendered on the standard material-test ball under an environment map (the matpreview scene). Regenerate the swatches with python3 .verify/render_matpreview_gallery.py.

---

Diffuse — type="diffuse"

Lambertian (perfectly diffuse) reflectance. Scatters light equally in all directions above the surface. The surface colour can be specified as a constant RGB value or as a bitmap texture. An optional normal map adds tangent-space surface detail without extra geometry.

Parameters

Name	Type	Default	Description
reflectance	rgb or bitmap	0, 0, 0	Diffuse albedo. Values should be in [0, 1].
normalmap	bitmap	(none)	Tangent-space normal map. RGB image encoded as [0,1] → [-1,1].

Examples

Constant colour:

<bsdf type="diffuse" id="white_wall">
    <rgb name="reflectance" value="0.8, 0.8, 0.8"/>
</bsdf>

Bitmap texture:

<bsdf type="diffuse" id="wood_floor">
    <texture type="bitmap" name="reflectance">
        <string name="filename" value="textures/wood.png"/>
    </texture>
</bsdf>

Texture + normal map:

<bsdf type="diffuse" id="stone">
    <texture type="bitmap" name="reflectance">
        <string name="filename" value="textures/stone_albedo.png"/>
    </texture>
    <texture type="bitmap" name="normalmap">
        <string name="filename" value="textures/stone_normal.png"/>
    </texture>
</bsdf>

Image textures are expected in sRGB space and are linearised on load. UV coordinates come from the mesh; OBJ files supply them via vt entries.

---

Mirror / Conductor — type="mirror" / type="conductor"

A perfect specular reflector (the two type names are equivalent). Reflects every incident ray into the mirror direction with no roughness. This is a delta BSDF — it has no diffuse component and contributes nothing to direct illumination samples.

Parameters

Name	Type	Default	Description
reflectance	rgb	1, 1, 1	Spectral tint applied to reflections. Use 1, 1, 1 for a neutral silver mirror.

Example

<bsdf type="mirror" id="silver">
    <rgb name="reflectance" value="0.9, 0.9, 0.9"/>
</bsdf>

---

Dielectric — type="dielectric" / type="glass"

A smooth dielectric interface (e.g. glass, water, crystal) governed by Fresnel equations. Both reflection and refraction are simulated, including total internal reflection. The surface has no diffuse component.

Parameters

Name	Type	Default	Description
intIOR or ior	float	1.5	Index of refraction of the medium inside the surface. The exterior is assumed to be air (IOR = 1.0).

Common IOR values: glass ≈ 1.5, water ≈ 1.33, diamond ≈ 2.42.

Example

<bsdf type="dielectric" id="glass_sphere">
    <float name="intIOR" value="1.5"/>
</bsdf>

<!-- water surface -->
<bsdf type="glass" id="water">
    <float name="ior" value="1.33"/>
</bsdf>

---

Rough dielectric — type="roughdielectric" / type="roughglass"

A GGX microfacet dielectric: frosted glass that both reflects and refracts through a rough interface. Smaller alpha approaches a smooth dielectric.

Sampling: micro-normals are drawn from the full GGX NDF (Walter et al. 2007), not from the distribution of visible normals (vNDF, Heitz 2018). vNDF sampling never generates back-facing microfacets and collapses the throughput weight to the masking-shadowing ratio G₂/G₁, which would noticeably cut variance here — especially at higher alpha.

Parameters

Name	Type	Default	Description
intIOR or ior	float	1.5	Interior index of refraction (exterior is air).
alpha	float	0.1	GGX roughness parameter.
specularReflectance	rgb	1, 1, 1	Spectral tint applied to the interface.

Example

<bsdf type="roughdielectric" id="frosted_glass">
    <float name="intIOR" value="1.5"/>
    <float name="alpha" value="0.2"/>
</bsdf>

---

Rough conductor — type="roughconductor" / type="roughplastic"

A GGX microfacet BSDF modelling a rough reflective surface. The distribution of microfacet normals follows the GGX (Trowbridge-Reitz) model. Increasing alpha makes the surface appear more matte.

Parameters

Name	Type	Default	Description
eta or reflectance	rgb	0.8, 0.8, 0.8	Spectral reflectance tint.
alpha	float	0.3	GGX roughness parameter. Range (0, 1]: 0.01 is near-mirror, 0.8 is very rough.

Example

<bsdf type="roughconductor" id="brushed_gold">
    <rgb name="eta" value="1.0, 0.77, 0.33"/>
    <float name="alpha" value="0.15"/>
</bsdf>

---

Phong / Blinn — type="phong" / type="blinn" / type="blinn_microfacet"

Classic glossy reflectance models driven by a specular exponent (the cosine-power; higher = sharper highlight). phong and blinn are the energy-normalised lobes; blinn_microfacet uses the Blinn distribution in a microfacet formulation.

phong	blinn	blinn_microfacet

Parameters

Name	Type	Default	Description
reflectance	rgb or bitmap	0.8, 0.8, 0.8	Diffuse/specular base colour.
exponent	float	50	Glossy exponent; larger is shinier.

Example

<bsdf type="blinn" id="glossy_red">
    <rgb name="reflectance" value="0.7, 0.1, 0.1"/>
    <float name="exponent" value="120"/>
</bsdf>

---

Plastic — type="plastic"

A smooth coated diffuse surface: a Fresnel-weighted specular coat over a diffuse substrate.

Parameters

Name	Type	Default	Description
reflectance	rgb or bitmap	0.5, 0.5, 0.5	Diffuse substrate colour.
eta, intIOR, or int_ior	float	1.5	Index of refraction of the dielectric coat.
nonlinear	boolean	false	Enable Mitsuba's coloured internal-scattering model.

Example

<bsdf type="plastic" id="red_plastic">
    <rgb name="reflectance" value="0.8, 0.1, 0.1"/>
    <float name="intIOR" value="1.49"/>
</bsdf>

---

Emitter BSDF — type="emitter" / type="area"

Marks a surface as a light source when declared as a named BSDF. This is equivalent to the inline <emitter type="area"> syntax but allows the emissive material to be reused across multiple shapes.

Parameters

Name	Type	Default	Description
radiance or emission	rgb	1, 1, 1	Emitted radiance.

Example

<bsdf type="emitter" id="ceiling_light">
    <rgb name="radiance" value="10, 8, 6"/>
</bsdf>

<shape type="rectangle">
    <transform name="toWorld">
        <scale x="0.5" y="0.5" z="0.5"/>
        <translate x="0" y="2.5" z="0"/>
        <rotate angle="90" x="1"/>
    </transform>
    <ref id="ceiling_light"/>
</shape>

---

Two-sided wrapper — type="twosided"

A Mitsuba compatibility wrapper. Kestrel's BSDFs are already evaluated on both faces, so a <bsdf type="twosided"> is transparently unwrapped to its inner BSDF — you can leave the wrapper in imported scenes without effect.

<bsdf type="twosided" id="wall">
    <bsdf type="diffuse">
        <rgb name="reflectance" value="0.8, 0.8, 0.8"/>
    </bsdf>
</bsdf>