Shadow acne when using real time shadows

[RakagoBVE]

Description

Real time shadows appear smeared onto the objects no matter the settings. Adjusting shadow/normal bias doesn't fix it.

Reproduction

Load any route and train with real time shadows enabled.

Examples

Related information

• Operating system: Windows 11
• Method of control: Keyboard
• Game version: Any with real time shadows implemented. I am using 1.13.0.3

[ginga81]

I was also curious about this but didn't know what it was. I just yesterday, learned it's called "shadow acne" and tried adjusting it.

I changed the shadow bias from the default to 0.0025, and I think that probably eliminated the problem where is the present before.

If it's a problem with the default value, it might be better to find an appropriate value and set it.

[ginga81]

It might be that the system simply discards any faint shadows that are so subtle that they completely disappear, treating them as if they don't exist.

[adfriz]

@RakagoBVE you may tweak the normal bias to 2.00

Note:

i think i have typo that normal bias label, this should be slope scaled bias and its a multiplier for the shadow bias not a constant...

@ginga81 for your case, if there is another shadow acne appearing, tweaking normal bias should be safer, because it should not shift the shadow too much

[ginga81]

Manual adjustment was very difficult.
Is there any way to prevent shadow acne from appearing automatically?
I thought that if I used another software, I wouldn't have any shadow acne even if I didn't do anything.
Do we have no choice but to view the sample objects and sample routes over and over again and adjust them ourselves until we're satisfied?
The shadow acne in my screenshot was the default value.
After displaying it over and over again and adjusting it, I finally arrived at 0.0025, but I thought it was a tedious and difficult task for beginners.
I also learned about shadow acne for the first time this time.
Shadow Bias:0.0025
Normal Bias:2.00
The display has changed with these settings.
At 0.0025, the shadow of the pantograph no longer appears on the car body.
I'm also an amateur, but I'm having trouble finding the right settings.

[adfriz]

in the end its case by case basis...

thats why i open the bias settings so anyone can tweak it if needed.

[RakagoBVE]

@adfriz tweaking normal bias to 2.00 makes the shadows less noticeable but the acne is still there

I also tried ginga's settings and still no dice.

[adfriz]

thats the hard part, shadow bias must not too high or it will look detached. and normal bias must not too low or it will get acne again or get light bleed on shadowed area.

[ginga81]

I wrote 'Are there any automated processing routines to prevent shadow acne in 3DCG?' to ChatGPT

Yes, there are. However, a "universal automated processing method that completely prevents shadow acne in any scene" does not currently exist in real-time CG.

Shadow acne occurs when the depth value used during shadow map generation conflicts with the actual depth value of the geometry surface due to numerical errors. Therefore, many renderers and game engines combine multiple automated corrections.

The following are typical automated processing methods:

1. Slope-Scaled Depth Bias (Most Common)

Automatically adjusts the bias amount considering the angle between the light ray direction and the polygon surface.

Conceptually:

bias = constantBias + slopeScale * tan(theta)
theta = angle between light direction and normal

The more slanted the surface, the larger the bias.

This method is used in many GPU APIs and engines.

DirectX SlopeScaledDepthBias
OpenGL polygon offset
Unreal Engine Shadow Bias settings
Unity Bias / Normal Bias
2. Normal Offset Bias

This method slightly pushes vertices and pixels in the direction of the normal to determine shadows.

offsetPosition =
position + normal * bias

Advantages

Easier to suppress acne on planes

Disadvantages

If too large, Peter Panning (the phenomenon of shadows appearing floating) occurs

Almost standard in recent real-time engines.

3. Receiver Plane Depth Bias

This method calculates the gradient of the light-receiving surface to estimate the necessary bias.

Conceptually

bias ≈ depthGradient × texelSize

Features

Adapts to shadow map resolution

Low scene dependency

Often used in high-quality shadow implementations.

4. Automatic Texel Size Linked Bias

Commonly seen in recent engines.

bias = k * shadowTexelWorldSize

In other words,

The bias is automatically determined from the shadow map resolution, cascade size, and light range.

This alone can significantly reduce acne.

5. Combining with PCF

Sampling of surrounding texels during shadow detection.

PCF (Percentage Closer Filtering)
shadow =
average(compare(depth_i))

This makes minor acne and jagged edges less noticeable.

However, this is not a fundamental solution.

6. Moment Shadow Maps
VSM
EVSM
MSM

These are methods such as:

By changing the depth comparison itself,

they have the advantages of being more resistant to acne and reducing bias dependence.

However,

they introduce other problems such as light bleeding and increased memory consumption.


7. Automatic Processing for Ray Tracing

Ray-traced shadows typically use the following ray offset:

rayOrigin =

hitPoint + normal * epsilon

This is a standard process to avoid "Shadow Terminator" and "Self Intersection."

It's almost essential for RTX renderers.

Standard routines in actual engines

Current game engines usually combine:

Constant Bias

Slope Scale Bias

Normal Offset

Texel Size Linked Correction

PCF

For example, in pseudocode:

float bias =

constBias +

slopeScale * (1.0 - dot(N, L));

bias += shadowTexelWorldSize * texelFactor;

shadowCoord.z ​​-= bias;


Is complete automation possible?

Difficult.

This is because the optimal bias depends on:

Shadow map resolution

Light projection range

Polygon size

Cascade position

Normal map strength

Therefore, in practice,

Automatic calculation is the basic approach,

Leaving a small amount of Bias slider adjustment for artists is the most common design.

In recent high-quality implementations, the combination of "Receiver Plane Depth Bias + Normal Offset + Texel Size linked correction" is used as a shadow acne countermeasure with significantly less manual adjustment.

Written by it, 'the combination of "Receiver Plane Depth Bias + Normal Offset + Texel Size linked correction" is used as a shadow acne countermeasure with significantly less manual adjustment.'
If achieve this, but does it seem difficult?

[RakagoBVE]

After trying many configurations the acne is still present - the only difference is that when you use high value for bias the objects don't cast shadows on themselves anymore, which hides the issue. The acne itself is always present...

[adfriz]

to be honest, one of the problems why most trains or object in openbve got shadow acne is because the mesh or object has thin polygon. this is the best explanation https://computergraphics.stackexchange.com/questions/2192/cause-of-shadow-acne

good example is side view of your trains, if my guess right... it has a wide rectangle with attached image texture, either with Face or Face2 command and if you view it perpendicular it shouldn't have any thickness.

one trick to make it less obvious is use larger shadow resolution and use bigger shadow cascade