acdream/src/AcDream.App/Studio/PanelFbo.cs

using System;
using System.Numerics;
using AcDream.App.Rendering.Wb;
using AcDream.App.UI;
using Silk.NET.OpenGL;

namespace AcDream.App.Studio;

/// <summary>
/// Renders a <see cref="UiHost"/> into an off-screen FBO each frame and
/// returns the color texture handle for display in ImGui.
///
/// <para>Pattern lifted verbatim from <see cref="AcDream.App.Rendering.PaperdollViewportRenderer"/>:
/// RGBA8 color texture + Depth24Stencil8 renderbuffer, lazily (re)created on
/// size change. The entire 2-D UI pass is sealed in a <see cref="GLStateScope"/>
/// so it cannot disturb the surrounding ImGui GL state.</para>
///
/// <para>FBO origin is bottom-left (GL convention). The caller must flip V when
/// displaying the texture in ImGui (pass uv0=(0,1), uv1=(1,0) to ImGui.Image)
/// so the image appears right-side-up in ImGui's top-left coordinate system.</para>
/// </summary>
public sealed unsafe class PanelFbo : IDisposable
{
    private readonly GL _gl;

    // Off-screen target — lazily (re)created when the requested size changes.
    private uint _fbo;
    private uint _colorTex;
    private uint _depthRbo;
    private int  _fbW;
    private int  _fbH;

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

    /// <summary>
    /// Render <paramref name="host"/> (a full <see cref="UiHost"/> draw pass) into a
    /// private FBO at <paramref name="width"/> × <paramref name="height"/> pixels.
    /// Returns the GL color texture handle (0 on failure). The texture is valid until
    /// the next call to <see cref="Render"/> with a different size, or until <see cref="Dispose"/>.
    /// </summary>
    public uint Render(int width, int height, UiHost host)
    {
        if (width <= 0 || height <= 0 || host is null) return 0u;

        EnsureFramebuffer(width, height);
        if (_fbo == 0) return 0u;

        // Seal the entire pass: GLStateScope saves + restores every GL state the
        // UI draw touches (viewport, blend, FBO binding, etc.) so ImGui's own state
        // — set up by BeginFrame and expected intact by Render — is untouched.
        using var scope = new GLStateScope(_gl);

        _gl.BindFramebuffer(FramebufferTarget.Framebuffer, _fbo);
        _gl.Viewport(0, 0, (uint)width, (uint)height);
        _gl.Disable(EnableCap.ScissorTest);
        _gl.ClearColor(0.18f, 0.18f, 0.18f, 1f);   // opaque dark-grey canvas background (the FBO IS the canvas)
        _gl.ClearDepth(1.0);
        _gl.DepthMask(true);
        _gl.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);

        host.Draw(new Vector2(width, height));

        // FBO stays bound here; GLStateScope.Dispose() restores the previous binding.
        return _colorTex;
    }

    /// <summary>
    /// Read the FBO color attachment back to CPU as a flat RGBA8 byte array.
    /// Must be called AFTER <see cref="Render"/> for the same <paramref name="width"/> and
    /// <paramref name="height"/> (so the FBO exists and is the right size).
    ///
    /// <para>FBO origin is bottom-left (GL convention). The caller is responsible for
    /// flipping rows vertically before saving as a top-left-origin image format (PNG).</para>
    ///
    /// <para>Returns an empty array when the FBO is not ready.</para>
    /// </summary>
    public unsafe byte[] ReadColorRgba(int width, int height)
    {
        if (_fbo == 0 || width <= 0 || height <= 0) return Array.Empty<byte>();

        int byteCount = width * height * 4;
        var buf = new byte[byteCount];

        _gl.BindFramebuffer(FramebufferTarget.Framebuffer, _fbo);
        fixed (byte* p = buf)
        {
            _gl.ReadPixels(0, 0, (uint)width, (uint)height,
                PixelFormat.Rgba, PixelType.UnsignedByte, p);
        }
        _gl.BindFramebuffer(FramebufferTarget.Framebuffer, 0);

        return buf;
    }

    // ── FBO lifecycle (mirrors PaperdollViewportRenderer.EnsureFramebuffer) ──────

    private void EnsureFramebuffer(int width, int height)
    {
        if (_fbo != 0 && width == _fbW && height == _fbH) return;
        DeleteFramebuffer();

        _fbW = width;
        _fbH = height;

        _colorTex = _gl.GenTexture();
        _gl.BindTexture(TextureTarget.Texture2D, _colorTex);
        _gl.TexImage2D(TextureTarget.Texture2D, 0, InternalFormat.Rgba8,
            (uint)width, (uint)height, 0,
            PixelFormat.Rgba, PixelType.UnsignedByte, (void*)0);
        _gl.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureMinFilter,
            (int)TextureMinFilter.Linear);
        _gl.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureMagFilter,
            (int)TextureMinFilter.Linear);
        _gl.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureWrapS,
            (int)TextureWrapMode.ClampToEdge);
        _gl.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureWrapT,
            (int)TextureWrapMode.ClampToEdge);
        _gl.BindTexture(TextureTarget.Texture2D, 0);

        _depthRbo = _gl.GenRenderbuffer();
        _gl.BindRenderbuffer(RenderbufferTarget.Renderbuffer, _depthRbo);
        _gl.RenderbufferStorage(RenderbufferTarget.Renderbuffer,
            InternalFormat.Depth24Stencil8, (uint)width, (uint)height);
        _gl.BindRenderbuffer(RenderbufferTarget.Renderbuffer, 0);

        _fbo = _gl.GenFramebuffer();
        _gl.BindFramebuffer(FramebufferTarget.Framebuffer, _fbo);
        _gl.FramebufferTexture2D(FramebufferTarget.Framebuffer,
            FramebufferAttachment.ColorAttachment0,
            TextureTarget.Texture2D, _colorTex, 0);
        _gl.FramebufferRenderbuffer(FramebufferTarget.Framebuffer,
            FramebufferAttachment.DepthStencilAttachment,
            RenderbufferTarget.Renderbuffer, _depthRbo);

        var status = _gl.CheckFramebufferStatus(FramebufferTarget.Framebuffer);
        _gl.BindFramebuffer(FramebufferTarget.Framebuffer, 0);
        if (status != GLEnum.FramebufferComplete)
        {
            Console.WriteLine($"[studio] PanelFbo incomplete: {status} ({width}x{height})");
            DeleteFramebuffer();
        }
    }

    private void DeleteFramebuffer()
    {
        if (_fbo      != 0) { _gl.DeleteFramebuffer(_fbo);         _fbo      = 0; }
        if (_colorTex != 0) { _gl.DeleteTexture(_colorTex);        _colorTex = 0; }
        if (_depthRbo != 0) { _gl.DeleteRenderbuffer(_depthRbo);   _depthRbo = 0; }
        _fbW = _fbH = 0;
    }

    public void Dispose() => DeleteFramebuffer();
}