When incorporating a real-time 3D character model into a UI screen (3D preview of the shop, player status screen, character lineup before the start of a battle, etc.), the most common approach in Unity development is to write the camera image to a "RenderTexture" and display it using the uGUI's "RawImage" component. However, when implementing this, color inconsistency problems occur in many projects, such as ``The colors of the 3D model are too dark and sinking in,'' ``The shadows are crushed to black and the complexion looks bad,'' and ``The highlights are unnaturally bright.''** In this article, we will explain how this drawing bug caused by the mismatch between Linear space and Gamma space works, and the steps to resolve it to completely synchronize the colors.

1. Root cause: Double processing of gamma correction in linear color space

In modern high-quality 3D game development, Linear is commonly selected for Color Space in Unity's Project Settings > Player > Other Settings to ensure physically correct light calculations. In this linear space, the transmission of textures and render buffer colors follows the following conversion process.

  1. Importing textures: When an sRGB (gamma-corrected, human-natural image) texture is loaded, it is automatically degamma-ed (linearized: a process that makes the colors numerically correct) before being passed to the shader.
  2. Calculation in shader: Performs light reflection and blending calculations in linear space (light additions such as 2x and 0.5x are calculated correctly here).
  3. Screen output: When finally outputting to a monitor, "gamma correction (sRGB encoding)" is applied again to offset the monitor's gamma characteristics (physical characteristics that display dark images).

The bug occurs when an intermediate buffer called "RenderTexture" is inserted in this flow.

When dynamically generating a RenderTexture from a script or asset, this buffer is created as a ``Linear buffer'' by default. When a sub camera draws a 3D model for this linear RenderTexture and assigns it to a RawImage without post-processing, the default UI shader of the RawImage interprets it as ``the input texture is a normal sRGB texture (= data that has not yet been gamma corrected)''. As a result, the UI shader tries to perform an additional linear transformation when drawing, resulting in color values ​​being double linearly transformed (to a power), making them extremely dark, and shadows being crushed to black.

2. Post-processing (Tonemapping) double application problem

Another cause of color corruption is the repeated application of post-processing effects. To make the 3D model look nice, enable post-processing of the preview sub camera and write `Tonemapping` and `Color Grading` to RenderTexture. However, the UI canvas (UI Canvas) where the RawImage with this RenderTexture pasted exists is drawn by the main camera. If `Post-processing` is enabled on the main camera side as well, and the UI layer is included in the post-processing application mask (Volume Mask), a second Tonemapping will be performed on the main camera side on top of the RenderTexture image to which Tonemapping has already been applied. **

This results in blown out highlights (contrast collapse due to double application of tonemaps) and color bugs where the overall hue shifts unnaturally to yellow or blue.

3. Solution steps to build a correct and clear 3D preview UI

In order to draw a 3D preview UI with 100% color accuracy, it is necessary to optimize the texture format and set the camera path separation.

① Correct setting of RenderTexture's sRGB flag (C#)

When dynamically creating a RenderTexture in a C# script, be sure to use `RenderTextureDescriptor` instead of the `RenderTexture` constructor and explicitly set the `sRGB` property to `true`. This will cause Unity to mark the created texture as "sRGB (gamma buffer)" and automatically handle the correct color space conversion when the UI shader samples it. 

using UnityEngine;
using UnityEngine.UI;

public class RenderToUiController : MonoBehaviour
{
    public Camera sub3DCamera;
    public RawImage uiRawImage;

    private RenderTexture characterRenderTexture;

    void Awake()
    {
        CreateCorrectRenderTexture();
    }

    void CreateCorrectRenderTexture()
    {
        // 1. Create a RenderTextureDescriptor
 RenderTextureDescriptor desc = new RenderTextureDescriptor(
 512, // width
 512, // height
 RenderTextureFormat.ARGB32, // standard color format with alpha channel
 24 // depth buffer precision (24 bit)
 );

 // 2. ★Super important: Enable sRGB color space (automatically gamma corrected when writing from Linear space)
 desc.sRGB = true; 
 desc.useMipMap = false;
 desc.autoGenerateMips = false;

 // 3. Generate RenderTexture
 characterRenderTexture = new RenderTexture(desc);
 characterRenderTexture.filterMode = FilterMode.Bilinear;
 characterRenderTexture.wrapMode = TextureWrapMode.Clamp;
 characterRenderTexture.Create();

 // 4. Assign to camera and UI
 sub3DCamera.targetTexture = characterRenderTexture;
 uiRawImage.texture = characterRenderTexture;
 }

 void OnDestroy()
 {
 if (characterRenderTexture != null)
 {
 sub3DCamera.targetTexture = null;
 uiRawImage.texture = null;
 characterRenderTexture.Release();
 Destroy(characterRenderTexture);
 }
 }
}

② Complete separation of UI camera and post process

To prevent double application of post process, be sure to perform the following camera settings.

  1. Completely exclude UI layers (e.g. `UI` and `UI3D` layers) from the scope of the "Post-processing" settings of the main camera that draws the main scene. Specifically, uncheck the UI-related layers from the main camera's `Volume Mask`.
  2. If you have added a UI-only Overlay camera for drawing UI to the camera stack, uncheck the "Post-processing" checkbox in the settings of that UI camera. This prevents unnecessary post-effects from overlapping when drawing the UI.