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

using System.Collections.Generic;
using System.Numerics;
using System.Runtime.InteropServices;
using Silk.NET.OpenGL;

namespace AcDream.App.Rendering;

/// <summary>
/// Minimal GL debug line renderer for visualizing collision shapes,
/// bounding boxes, and other debug geometry. Collect lines each frame
/// via <see cref="AddLine"/> / <see cref="AddCylinder"/>, then call
/// <see cref="Flush"/> to upload + draw them.
///
/// Uses a single shared VBO that's respecialized each frame. Vertex
/// format is (vec3 pos, vec3 color) = 24 bytes per vertex.
/// </summary>
public sealed unsafe class DebugLineRenderer : IDisposable
{
    private readonly GL _gl;
    private readonly Shader _shader;
    private readonly uint _vao;
    private readonly uint _vbo;

    private readonly List<float> _buffer = new(4096);
    private int _vertexCount;
    private int _capacityBytes;

    public DebugLineRenderer(GL gl, string shaderDir)
    {
        _gl = gl;
        _shader = new Shader(gl,
            Path.Combine(shaderDir, "debug_line.vert"),
            Path.Combine(shaderDir, "debug_line.frag"));

        _vao = _gl.GenVertexArray();
        _vbo = _gl.GenBuffer();

        _gl.BindVertexArray(_vao);
        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _vbo);

        // 24-byte stride: vec3 pos + vec3 color
        _gl.EnableVertexAttribArray(0);
        _gl.VertexAttribPointer(0, 3, VertexAttribPointerType.Float, false, 6 * sizeof(float), (void*)0);
        _gl.EnableVertexAttribArray(1);
        _gl.VertexAttribPointer(1, 3, VertexAttribPointerType.Float, false, 6 * sizeof(float), (void*)(3 * sizeof(float)));

        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, 0);
        _gl.BindVertexArray(0);
    }

    /// <summary>Clear accumulated lines. Call at the start of each frame.</summary>
    public void Begin()
    {
        _buffer.Clear();
        _vertexCount = 0;
    }

    public void AddLine(Vector3 a, Vector3 b, Vector3 color)
    {
        _buffer.Add(a.X); _buffer.Add(a.Y); _buffer.Add(a.Z);
        _buffer.Add(color.X); _buffer.Add(color.Y); _buffer.Add(color.Z);
        _buffer.Add(b.X); _buffer.Add(b.Y); _buffer.Add(b.Z);
        _buffer.Add(color.X); _buffer.Add(color.Y); _buffer.Add(color.Z);
        _vertexCount += 2;
    }

    /// <summary>
    /// Draw a cylinder as 2 polygon rings (base + top) connected by 4
    /// vertical line segments at 0/90/180/270 degrees.
    /// </summary>
    public void AddCylinder(Vector3 basePos, float radius, float height, Vector3 color)
    {
        const int segments = 16;
        Vector3 top = basePos + new Vector3(0, 0, height);

        // Ring vertices
        var baseRing = new Vector3[segments];
        var topRing  = new Vector3[segments];
        for (int i = 0; i < segments; i++)
        {
            float theta = i * (MathF.PI * 2f / segments);
            float cx = MathF.Cos(theta) * radius;
            float cy = MathF.Sin(theta) * radius;
            baseRing[i] = new Vector3(basePos.X + cx, basePos.Y + cy, basePos.Z);
            topRing[i]  = new Vector3(top.X    + cx, top.Y    + cy, top.Z);
        }

        // Base ring
        for (int i = 0; i < segments; i++)
            AddLine(baseRing[i], baseRing[(i + 1) % segments], color);
        // Top ring
        for (int i = 0; i < segments; i++)
            AddLine(topRing[i], topRing[(i + 1) % segments], color);
        // 4 vertical connectors
        for (int i = 0; i < 4; i++)
        {
            int idx = i * (segments / 4);
            AddLine(baseRing[idx], topRing[idx], color);
        }
    }

    /// <summary>
    /// Draw an axis-aligned box as 12 edges.
    /// </summary>
    public void AddBox(Vector3 min, Vector3 max, Vector3 color)
    {
        Vector3[] c =
        {
            new(min.X, min.Y, min.Z),
            new(max.X, min.Y, min.Z),
            new(max.X, max.Y, min.Z),
            new(min.X, max.Y, min.Z),
            new(min.X, min.Y, max.Z),
            new(max.X, min.Y, max.Z),
            new(max.X, max.Y, max.Z),
            new(min.X, max.Y, max.Z),
        };
        // Bottom
        AddLine(c[0], c[1], color); AddLine(c[1], c[2], color);
        AddLine(c[2], c[3], color); AddLine(c[3], c[0], color);
        // Top
        AddLine(c[4], c[5], color); AddLine(c[5], c[6], color);
        AddLine(c[6], c[7], color); AddLine(c[7], c[4], color);
        // Verticals
        AddLine(c[0], c[4], color); AddLine(c[1], c[5], color);
        AddLine(c[2], c[6], color); AddLine(c[3], c[7], color);
    }

    /// <summary>Upload + draw all accumulated lines.</summary>
    public void Flush(Matrix4x4 view, Matrix4x4 projection)
    {
        if (_vertexCount == 0) return;

        _shader.Use();
        _shader.SetMatrix4("uView", view);
        _shader.SetMatrix4("uProjection", projection);

        _gl.BindVertexArray(_vao);
        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _vbo);

        int neededBytes = _buffer.Count * sizeof(float);
        if (neededBytes > _capacityBytes)
        {
            fixed (float* ptr = CollectionsMarshal.AsSpan(_buffer))
                _gl.BufferData(BufferTargetARB.ArrayBuffer, (nuint)neededBytes, ptr, BufferUsageARB.DynamicDraw);
            _capacityBytes = neededBytes;
        }
        else
        {
            fixed (float* ptr = CollectionsMarshal.AsSpan(_buffer))
                _gl.BufferSubData(BufferTargetARB.ArrayBuffer, 0, (nuint)neededBytes, ptr);
        }

        // Depth test on so lines get occluded by geometry (but we want them
        // visible through geometry — disable depth test so everything shows).
        bool wasDepthEnabled = _gl.IsEnabled(EnableCap.DepthTest);
        _gl.Disable(EnableCap.DepthTest);

        _gl.DrawArrays(PrimitiveType.Lines, 0, (uint)_vertexCount);

        if (wasDepthEnabled) _gl.Enable(EnableCap.DepthTest);

        _gl.BindVertexArray(0);
    }

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