Great question! You’re right that Graphics.Blit works for basic 2D texture-to-render-texture copies, but converting an equirectangular (360°) panorama to a cubemap requires more than a direct copy—you need to handle the projection from the spherical panorama to each face of the cube. Let’s walk through how to do this, and address your question about syncing all 6 faces.

Can You Convert a Panorama Texture2D to a Cubemap RenderTexture? #

Absolutely! Here’s a step-by-step approach:

1. Set Up Your Cubemap RenderTexture #

First, create a RenderTexture configured as a cubemap:

RenderTexture cubemapRT = new RenderTexture(1024, 1024, 0, RenderTextureFormat.ARGB32);
cubemapRT.dimension = UnityEngine.Rendering.TextureDimension.Cube;
cubemapRT.enableRandomWrite = false; // Disable unless you need compute shader access
cubemapRT.Create();

2. Use a Custom Shader for Panorama-to-Cubemap Projection #

You’ll need a shader that converts the equirectangular panorama’s UVs to the correct coordinates for each cubemap face. Here’s a simplified example of an equirectangular-to-cubemap shader:

Shader "Custom/EquirectangularToCubemap"
{
    Properties
    {
        _MainTex ("Panorama Texture", 2D) = "white" {}
    }
    SubShader
    {
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
                float3 viewDir : TEXCOORD1;
            };

            sampler2D _MainTex;

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                // Calculate direction from camera to vertex (for cubemap face projection)
                o.viewDir = normalize(UnityObjectToWorldDir(v.vertex));
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                float3 dir = i.viewDir;
                // Convert cubemap direction to equirectangular UVs
                float theta = atan2(dir.z, dir.x);
                float phi = acos(dir.y);
                float2 uv = float2(theta / (2 * UNITY_PI) + 0.5, phi / UNITY_PI);
                return tex2D(_MainTex, uv);
            }
            ENDCG
        }
    }
}

3. Render Each Cubemap Face #

Loop through each of the 6 cubemap faces, bind the face as the active render target, and use Graphics.Blit with your custom shader to render the panorama to that face:

public void ConvertPanoramaToCubemap(Texture2D panoramaTex, RenderTexture cubemapRT)
{
    if (panoramaTex == null || cubemapRT == null || cubemapRT.dimension != UnityEngine.Rendering.TextureDimension.Cube)
        return;

    Material cubemapMaterial = new Material(Shader.Find("Custom/EquirectangularToCubemap"));
    cubemapMaterial.SetTexture("_MainTex", panoramaTex);

    // Loop through all 6 cubemap faces
    for (int face = 0; face < 6; face++)
    {
        // Activate the current cubemap face as render target
        RenderTexture.active = cubemapRT;
        GL.SetRenderTarget(cubemapRT, (CubemapFace)face);

        // Clear the target
        GL.Clear(true, true, Color.black);

        // Blit the panorama to the current cubemap face using the custom shader
        Graphics.Blit(null, cubemapRT, cubemapMaterial, 0);
    }

    RenderTexture.active = null;
    Destroy(cubemapMaterial);
}

Is There a Way to Sync-Copy All 6 Faces at Once? #

Unfortunately, there’s no built-in "one-click" method to copy all 6 faces simultaneously. Each cubemap face requires a separate projection calculation (since each face represents a different view direction from the cube’s center), so you have to process each face individually.

That said, you can optimize this process:

• Batch the rendering: The loop above is efficient enough for most use cases, as each Blit operation is a simple draw call.
• Use compute shaders: For very high-resolution cubemaps, you could use a compute shader to process all 6 faces in parallel on the GPU, but this adds complexity and is only necessary for performance-critical scenarios.

Key Notes #

• Make sure your panorama texture is set to Clamp wrap mode (to avoid UV seams at the edges of the equirectangular image).
• Adjust the shader’s UV calculation if your panorama uses a different format (e.g., a horizontal cubemap strip instead of equirectangular).
• For better quality, you can add mipmap generation to the cubemap RenderTexture by enabling generateMips.

内容的提问来源于stack exchange，提问作者user11458208