When creating a space using Unity's Universal Render Pipeline (URP), it is very common to place a Post-process Volume for each area in the scene (indoors, outdoors, caves, etc.) and switch the screen color (color grading) and atmosphere (fog, bloom, etc.). However, you may encounter bugs such as ``Even when you move the area, the post-processing effect does not switch smoothly, and the color tone of the screen changes suddenly the moment you cross the border'' or ``No effect is applied even if you enter the volume collider.''

In this article, we will clarify the reasons why the blending and switching of Post Processing Volumes within the same scene does not work as intended, and explain in detail the solution approach to achieve natural transitions.

For beginners: How would you describe this problem in real life?

Let's compare this volume blending problem to a real-life "room air conditioner (cooling/heating)".

Suppose there is a cold, air-conditioned blue room (Room A) and a heated red room (Room B), which are connected by a short hallway with no doors.

Normally, as you move from room A through the hallway to room B, the air naturally becomes progressively lukewarm and eventually "gradually blends" into warmer air. However, when the blend setting (Blend Distance) is set to 0, it is the same as an extremely unnatural phenomenon that cannot occur in reality, such as ``There is an invisible transparent insulation wall at the border of the room, and the moment you cross the border even by a millimeter, the temperature instantly changes from 15 degrees to 30 degrees.''

Furthermore, the camera's Volume Trigger (sensor) setting is missing, which is the same as ``The player's body (camera) moves to room B while leaving the thermometer (sensor) in room A.'' No matter where the body moves, the sensor remains in the first room, so the temperature the player feels (screen effect) remains cold in room A. To switch naturally, it is necessary to create a range where the boundaries blend (Blend Distance) and to have a temperature sensor (Trigger) in the camera itself.

Problem symptoms and specific phenomena

If volume blending is not working properly, the following behavior will occur:

  • The moment a character passes through the boundary line of the trigger collider that separates the area, the screen exposure and color change instantly without warning, giving the player a strong sense of visual discomfort.
  • Even if the camera enters the Local Volume placed on the scene, the Global Volume settings still take priority and area-specific effects are not reflected.
  • In areas where multiple volumes overlap, the Priority setting is ignored, causing the screen to flicker irregularly and effects to be combined in unintended combinations.

Possible cause: Why does blending fail?

The causes of volume switching problems are mainly summarized in the following three points.

  1. Blend Distance (blend start distance) not set: If the Volume component's Blend Distance is set to 0, the effect contribution rate (Weight) will instantly switch from 0 to 1 inside and outside the collider, so smooth interpolation will not occur.
  2. Volume Trigger omitted: If Volume Trigger in Universal Additional Camera Data on the camera object side is empty (None), the system will not be able to determine which object's coordinates should be used as a reference for local volume collider judgment (intrusion judgment), and the judgment will not be updated correctly.
  3. Collider Layer Setting Conflict: The collider set on the volume does not match the camera's collision detection layer, or the collider is not set to Is Trigger, resulting in an intrusion not being detected.

Specific solutions and approaches

The steps to resolve this issue and create a smooth blend transition are as follows.

1. Appropriate setting of Blend Distance

Open the Volume component of the area you want to transition and adjust the value of the Blend Distance property. For example, if you want the blend to start gradually when the player approaches `3.0` meters before the boundary line, set the value to `3.0`. 

[Volume Inspector Settings]
- Mode: Local
- Blend Distance: 3.0  <-- ここを境界線のサイズに合わせて拡張する
- Priority: 1.0 (親となるGlobalより高い値にする)

2. Self-assignment of camera to Volume Trigger

Select the Main Camera in your scene and in the Inspector, look for the Volume Framework ➔ Trigger property under Universal Additional Camera Data (at the bottom of the camera component). Assign the camera's own Transform (Main Camera object itself) here by dragging and dropping. 

[Camera Inspector - Volume Framework]
- Trigger: Main Camera (Transform)  <-- カメラ自身をトリガーの基準点として登録

Confirmation checklist useful in practice

This is a checklist if something goes wrong when setting up post-processing for multiple areas.

Confirmation items Standards/measures
Check Is Trigger Is Is Trigger of the collider (such as Box Collider) attached to the Local Volume enabled?
Priority order Is the Priority value of the Volume of the narrow area you want to prioritize higher than the base Global Volume?
Camera Post Processing Is the Post Processing checkbox of the Main Camera component itself set to ON?

Summary

URP's post-process volumes can easily create cinematic screen transitions by linking them correctly. If you experience unnatural blinking or switching when switching, first reconsider securing the color mixing area using Blend Distance and attaching Volume Trigger to the camera. By simply following these steps, you will be able to express the changes in the atmosphere of the screen extremely smoothly as you move between areas.