Shader "Custom/ExternalViewportShader"
{
    Properties
    {
        //_MainTex ("External Texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" "Queue"="Geometry" }
        LOD 100

        Pass
        {
            // GLSLPROGRAM block is required for raw GLSL in Unity
            GLSLPROGRAM

            // 1. Enable the EGL Image External extension for Android
            //#extension GL_OES_EGL_image_external_essl3 : require

            // Unity 6 predefines VERTEX and FRAGMENT during separate compilation passes
            #ifdef VERTEX
            
            // Unity 6 uses built-in matrices but they must be declared or used via Unity-specific naming
            // In raw GLSL, we use the standard attribute/varying naming convention
            varying vec2 v_texcoord;

            void main()
            {
                // gl_ModelViewProjectionMatrix is still available in Unity's GLSL compatibility layer
                gl_Position = gl_Vertex;
                v_texcoord = gl_MultiTexCoord0.xy;
            }
            #endif

            #ifdef FRAGMENT
            
            // 2. Use samplerExternalOES for AHardwareBuffer data
            //uniform samplerExternalOES _MainTex;
            varying vec2 v_texcoord;

            void main()
            {
                // 3. For Unity 6, use textureExternal for maximum compatibility 
                // on GLES3 devices to ensure the compiler handles the conversion properly.
                // texture2D(s, coord) also works, but textureExternal is the modern Unity GLSL recommendation.
                gl_FragColor = vec4(1, 0, 0, 1);//texture2D(_MainTex, v_texcoord);
            }
            #endif

            ENDGLSL
        }
    }
    Fallback "Unlit/Texture"
}