acdream/src/AcDream.App/Rendering/FadeOverlay.cs

using System;
using Silk.NET.OpenGL;

namespace AcDream.App.Rendering;

/// <summary>
/// Fullscreen black quad at a given alpha — the teleport fade cover (retail-teleport
/// spec §C, 2026-06-22). Drawn LAST in the frame (over the world + UI) so it covers
/// everything; the <see cref="AcDream.Core.World.TeleportAnimSequencer"/> drives the alpha
/// (opaque-black through the transit, ramping the world back in on WorldFadeIn). The
/// authentic 3D portal-swirl is deferred — this black cover replaces it for now.
///
/// <para>Draws in NDC, so it needs no view/projection. Self-contained GL state
/// (feedback_render_self_contained_gl_state): sets blend + disables depth, restores the
/// frame-global convention (depth test/write on) on exit.</para>
/// </summary>
public sealed class FadeOverlay : IDisposable
{
    private const string VertSrc = @"#version 430 core
layout(location = 0) in vec2 aPos;
void main() { gl_Position = vec4(aPos, 0.0, 1.0); }";

    private const string FragSrc = @"#version 430 core
uniform float uAlpha;
out vec4 FragColor;
void main() { FragColor = vec4(0.0, 0.0, 0.0, uAlpha); }";

    private readonly GL _gl;
    private readonly uint _program;
    private readonly uint _vao;
    private readonly uint _vbo;
    private readonly int _locAlpha;

    // Two triangles covering the whole NDC viewport.
    private static readonly float[] QuadVerts =
    {
        -1f, -1f,  1f, -1f,  1f, 1f,
        -1f, -1f,  1f,  1f, -1f, 1f,
    };

    public FadeOverlay(GL gl)
    {
        _gl = gl ?? throw new ArgumentNullException(nameof(gl));

        uint vs = Compile(ShaderType.VertexShader, VertSrc);
        uint fs = Compile(ShaderType.FragmentShader, FragSrc);
        _program = _gl.CreateProgram();
        _gl.AttachShader(_program, vs);
        _gl.AttachShader(_program, fs);
        _gl.LinkProgram(_program);
        _gl.GetProgram(_program, ProgramPropertyARB.LinkStatus, out int linked);
        if (linked == 0)
            throw new InvalidOperationException($"FadeOverlay link failed: {_gl.GetProgramInfoLog(_program)}");
        _gl.DeleteShader(vs);
        _gl.DeleteShader(fs);

        _locAlpha = _gl.GetUniformLocation(_program, "uAlpha");

        _vao = _gl.GenVertexArray();
        _vbo = _gl.GenBuffer();
        _gl.BindVertexArray(_vao);
        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _vbo);
        unsafe
        {
            fixed (float* v = QuadVerts)
                _gl.BufferData(BufferTargetARB.ArrayBuffer,
                    (nuint)(QuadVerts.Length * sizeof(float)), v, BufferUsageARB.StaticDraw);
            _gl.VertexAttribPointer(0, 2, VertexAttribPointerType.Float, false, 2 * sizeof(float), (void*)0);
        }
        _gl.EnableVertexAttribArray(0);
        _gl.BindVertexArray(0);
        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, 0);
    }

    private uint Compile(ShaderType type, string src)
    {
        uint s = _gl.CreateShader(type);
        _gl.ShaderSource(s, src);
        _gl.CompileShader(s);
        _gl.GetShader(s, ShaderParameterName.CompileStatus, out int ok);
        if (ok == 0)
            throw new InvalidOperationException($"FadeOverlay {type} compile failed: {_gl.GetShaderInfoLog(s)}");
        return s;
    }

    /// <summary>
    /// Draw the fullscreen black cover at <paramref name="alpha"/> (0 = clear → no-op,
    /// 1 = opaque). Must be called LAST in the frame, after the world and UI, so it covers
    /// everything on screen.
    /// </summary>
    public void Draw(float alpha)
    {
        if (alpha <= 0f) return;
        alpha = Math.Clamp(alpha, 0f, 1f);

        // ---- set state (everything this draw depends on) ----
        _gl.Disable(EnableCap.DepthTest);
        _gl.DepthMask(false);
        _gl.Disable(EnableCap.CullFace);
        _gl.Enable(EnableCap.Blend);
        _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);

        _gl.UseProgram(_program);
        _gl.Uniform1(_locAlpha, alpha);
        _gl.BindVertexArray(_vao);
        _gl.DrawArrays(PrimitiveType.Triangles, 0, 6);
        _gl.BindVertexArray(0);
        _gl.UseProgram(0);

        // ---- restore the frame-global convention ----
        _gl.DepthMask(true);
        _gl.Enable(EnableCap.DepthTest);
    }

    public void Dispose()
    {
        _gl.DeleteProgram(_program);
        _gl.DeleteVertexArray(_vao);
        _gl.DeleteBuffer(_vbo);
    }
}