Shadow bias and shadow acne

Shadow acne is a pattern of dark stripes or dots on surfaces that should be fully lit. Shadow bias is the setting that fixes it, by pushing the shadow test away from the surface. Too much bias causes peter-panning: shadows that start a short distance from their object, so it seems to float. Each engine's bias settings steer between the two.

Shadow maps in general are covered on real-time lighting and cascaded shadow maps.

#Why acne happens

A shadow map stores, for each texel, how far the nearest surface is from the light. When the scene is drawn, each pixel compares its own distance against that stored value.

Microsoft's explanation: acne "occurs when the shadow map quantizes the depth over an entire texel. When the shader compares an actual depth against this value, it is as likely to be self-shadowed as it is to be unshadowed" (Microsoft Learn: Common Techniques to Improve Shadow Depth Maps). Precision makes it worse: the stored and computed depths are so close that "precision errors cause the depth test to erroneously fail" (Microsoft Learn: Common Techniques to Improve Shadow Depth Maps).

Three side-view panels. Left, too little bias: a sloped surface with orange stair-steps for stored shadow depth crossing above and below it, and a row of black stripes underneath labelled acne. Middle, balanced bias: a box on the ground with its shadow starting right at its base. Right, too much bias: the same box with its shadow starting a short distance away, the gap outlined in orange, labelled peter-panning. — Too little bias and lit surfaces shadow themselves; too much and shadows detach. The usable range lies between the two.Diagram by the handbook author

#Why peter-panning happens

The name "derives from a children's book character whose shadow became detached", and the artefact "makes objects with missing shadows appear to be detached from and to float above the surface" (Microsoft Learn: Common Techniques to Improve Shadow Depth Maps). It "results when the depth offset used is too large" (Microsoft Learn: Common Techniques to Improve Shadow Depth Maps).

#The settings in each engine

Engine	Settings	What the docs say
Godot	Bias, Normal Bias per light	Too low and "self-shadowing occurs"; too high and "shadows separate from the casters" or "appear misaligned" (Godot Engine documentation)
Unity	Bias, Normal Bias, Near Plane per light	Bias controls "the distance at which shadows are pushed away from the light"; Normal Bias shrinks casters "along the surface normal" (Unity Manual)
Unreal	Shadow Bias (and related bias settings) per light	Shadow Bias "Controls how accurate self shadowing of whole scene shadows from this light are"; 0 shows many artefacts, higher values make objects look like they float, and the API suggests about 0.5 as a balance (Unreal Engine API Reference)

Depth bias moves the comparison along the light's direction. Normal bias moves it along the surface normal; Godot says it "can be used to fix special cases of self-shadowing when objects are perpendicular to the light" (Godot Engine documentation: 3D lights and shadows). Slope-scaled bias applies "a large bias to a polygon that is viewed edge-on to the light direction, but not applying any bias to a polygon facing the light directly" (Microsoft Learn: Common Techniques to Improve Shadow Depth Maps).

Different viewsWhich bias should you raise first?

Normal bias first (Godot). "In general, increasing Shadow Normal Bias is preferred over increasing Shadow Bias", because it "does not cause as much peter-panning" yet "can still resolve most shadow acne issues" (Godot Engine documentation: 3D lights and shadows).

Careful with normal bias (Unity). Unity warns that "setting the Normal Bias value too high makes the shadow appear too narrow", and that Normal Bias "can cause an unwanted effect called light bleeding" (Unity Manual: Shadow troubleshooting). Godot agrees that it can make shadows "appear thinner for certain objects" (Godot Engine documentation: 3D lights and shadows).

Tune by eye. Unity notes: "It is generally easier to gauge the right value by eye rather than attempting to calculate it" (Unity Manual: Shadow troubleshooting).

In practice: start from defaults, raise normal bias in small steps until acne disappears, then add depth bias only if needed. Check thin objects (railings, poles, leaves) after every change.

#Fixes other than bias

Tighter shadow ranges. "The more closely together the planes are, the more precise the values in the depth buffer" (Microsoft Learn: Common Techniques to Improve Shadow Depth Maps). Unity's shadow pancaking reduces "the range of the light space used when rendering the shadow map", which "leads to an increased precision in the shadow map, reducing shadow acne" (Unity Manual: Shadow troubleshooting). See cascaded shadow maps.

Geometry with backs. This fix is the environment artist's responsibility. Microsoft advises: "Narrow objects such as walls should have backs even if they are never visible. This will increase the depth disparity" (Microsoft Learn: Common Techniques to Improve Shadow Depth Maps). Without them, "Peter Panning and shadow gaps near the base of objects such as walls are more likely" (Microsoft Learn: Common Techniques to Improve Shadow Depth Maps). Unity's workaround for normal-bias light bleeding is similar: set the renderer's Cast Shadows to Two Sided, though it "can be more resource-intensive" (Unity Manual: Shadow troubleshooting).

real-time lighting · cascaded shadow maps · shadows and virtual shadow maps · light leaking · light and shadow · modular kits · performance and profiling

#Further reading and watching

Common Techniques to Improve Shadow Depth Maps (Microsoft)

the classic explanation of acne, peter-panning, slope-scaled bias and near/far planes. Advanced, free.

WebAdvancedFree

Shadow troubleshooting (Unity Manual)

acne, peter-panning, light bleeding and shadow pancaking in Unity. Intermediate, free.

DocsIntermediateFree

3D lights and shadows (Godot documentation)

Godot's bias and normal bias advice, with images of both artefacts. Intermediate, free.

WebIntermediateFree

ShadowBias (Unreal Engine API)

Epic's description of the per-light bias. Intermediate, free.

DocsIntermediateFree

References

Microsoft · Common Techniques to Improve Shadow Depth Maps · Microsoft Learn (Win32 / DirectX technical articles) · docs · accessed 2026-10-21

Godot Engine contributors · 3D lights and shadows · Godot Engine documentation (source) · docs · accessed 2026-10-21

Unity Technologies · Light component reference · Unity Manual · docs · accessed 2026-10-21

Epic Games · ULightComponent::ShadowBias · Unreal Engine API Reference (4.27) · docs · accessed 2026-10-21

Unity Technologies · Shadow troubleshooting · Unity Manual · docs · accessed 2026-10-21