VoxelEngine/Rendering/DebugOverlay.cs

using OpenTK.Graphics.OpenGL4;
using OpenTK.Mathematics;

namespace VoxelEngine.Rendering;

// Minimal, asset-free debug text. A 5x7 bitmap font is rendered as solid quads
// (one quad per lit font pixel) through a trivial 2D shader. Glyphs are defined
// as visual ASCII-art rows so they can be eyeballed for correctness. Meant for
// low-volume debug HUDs, not general text.
public sealed class DebugOverlay : IDisposable
{
    private const int GlyphW = 5;
    private const int GlyphH = 7;

    private readonly int _vao;
    private readonly int _vbo;
    private readonly Shader _shader;
    private readonly List<float> _verts = new();

    public DebugOverlay()
    {
        this._shader = new Shader("Assets/shaders/debugtext.vert", "Assets/shaders/debugtext.frag");

        this._vao = GL.GenVertexArray();
        GL.BindVertexArray(this._vao);
        this._vbo = GL.GenBuffer();
        GL.BindBuffer(BufferTarget.ArrayBuffer, this._vbo);
        GL.VertexAttribPointer(0, 2, VertexAttribPointerType.Float, false, 2 * sizeof(float), 0);
        GL.EnableVertexAttribArray(0);
        GL.BindVertexArray(0);
    }

    // Draws `lines` stacked top-down in the top-left corner. `scale` is pixels
    // per font pixel; fbWidth/fbHeight are the current framebuffer size, used to
    // map pixel coordinates to NDC. Drawn over the scene (no depth, no culling).
    public void Draw(IReadOnlyList<string> lines, int fbWidth, int fbHeight, int scale = 3)
    {
        if (fbWidth <= 0 || fbHeight <= 0 || lines.Count == 0) return;

        this._verts.Clear();
        const int marginX = 10;
        const int marginY = 10;
        int lineStride = (GlyphH + 2) * scale;
        for (int i = 0; i < lines.Count; i++)
            this.AddString(lines[i], marginX, marginY + i * lineStride, scale, fbWidth, fbHeight);

        if (this._verts.Count == 0) return;

        GL.BindVertexArray(this._vao);
        GL.BindBuffer(BufferTarget.ArrayBuffer, this._vbo);
        GL.BufferData(BufferTarget.ArrayBuffer, this._verts.Count * sizeof(float),
            this._verts.ToArray(), BufferUsageHint.DynamicDraw);

        bool depthWasOn = GL.IsEnabled(EnableCap.DepthTest);
        bool cullWasOn = GL.IsEnabled(EnableCap.CullFace);
        GL.Disable(EnableCap.DepthTest);
        GL.Disable(EnableCap.CullFace);

        this._shader.Use();
        int vertexCount = this._verts.Count / 2;

        // Drop shadow (offset down-right by ~2px) then white text, so it stays
        // legible over sky, grass or stone alike.
        this._shader.SetVector2("u_offset", new Vector2(4f / fbWidth, -4f / fbHeight));
        this._shader.SetVector3("u_color", new Vector3(0f, 0f, 0f));
        GL.DrawArrays(PrimitiveType.Triangles, 0, vertexCount);

        this._shader.SetVector2("u_offset", Vector2.Zero);
        this._shader.SetVector3("u_color", new Vector3(1f, 1f, 1f));
        GL.DrawArrays(PrimitiveType.Triangles, 0, vertexCount);

        GL.BindVertexArray(0);
        if (depthWasOn) GL.Enable(EnableCap.DepthTest);
        if (cullWasOn) GL.Enable(EnableCap.CullFace);
    }

    private void AddString(string text, int pxX, int pxY, int scale, int fbWidth, int fbHeight)
    {
        int cursorX = pxX;
        foreach (char ch in text)
        {
            if (DebugOverlay.Glyphs.TryGetValue(char.ToUpperInvariant(ch), out string[]? rows))
            {
                for (int r = 0; r < GlyphH; r++)
                for (int c = 0; c < GlyphW; c++)
                {
                    if (rows[r][c] != ' ')
                        this.AddQuad(cursorX + c * scale, pxY + r * scale, scale, fbWidth, fbHeight);
                }
            }
            cursorX += (GlyphW + 1) * scale;   // one font-pixel gap between glyphs
        }
    }

    // Append two triangles for the pixel-space square at (px,py) of side `s`.
    // Pixel space is top-left origin, y down; converted to NDC here.
    private void AddQuad(int px, int py, int s, int fbWidth, int fbHeight)
    {
        float x0 = px / (float)fbWidth * 2f - 1f;
        float x1 = (px + s) / (float)fbWidth * 2f - 1f;
        float y0 = 1f - py / (float)fbHeight * 2f;
        float y1 = 1f - (py + s) / (float)fbHeight * 2f;

        this._verts.AddRange(new[]
        {
            x0, y0,  x1, y0,  x1, y1,
            x0, y0,  x1, y1,  x0, y1,
        });
    }

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

    // 5x7 font. Each glyph is 7 rows of 5 columns; non-space = lit pixel.
    private static readonly Dictionary<char, string[]> Glyphs = new()
    {
        ['A'] = new[] { " ### ", "#   #", "#   #", "#####", "#   #", "#   #", "#   #" },
        ['B'] = new[] { "#### ", "#   #", "#   #", "#### ", "#   #", "#   #", "#### " },
        ['C'] = new[] { " ####", "#    ", "#    ", "#    ", "#    ", "#    ", " ####" },
        ['D'] = new[] { "#### ", "#   #", "#   #", "#   #", "#   #", "#   #", "#### " },
        ['E'] = new[] { "#####", "#    ", "#    ", "#### ", "#    ", "#    ", "#####" },
        ['F'] = new[] { "#####", "#    ", "#    ", "#### ", "#    ", "#    ", "#    " },
        ['G'] = new[] { " ####", "#    ", "#    ", "#  ##", "#   #", "#   #", " ####" },
        ['H'] = new[] { "#   #", "#   #", "#   #", "#####", "#   #", "#   #", "#   #" },
        ['I'] = new[] { "#####", "  #  ", "  #  ", "  #  ", "  #  ", "  #  ", "#####" },
        ['J'] = new[] { "#####", "   # ", "   # ", "   # ", "#  # ", "#  # ", " ##  " },
        ['K'] = new[] { "#   #", "#  # ", "# #  ", "##   ", "# #  ", "#  # ", "#   #" },
        ['L'] = new[] { "#    ", "#    ", "#    ", "#    ", "#    ", "#    ", "#####" },
        ['M'] = new[] { "#   #", "## ##", "# # #", "#   #", "#   #", "#   #", "#   #" },
        ['N'] = new[] { "#   #", "##  #", "# # #", "#  ##", "#   #", "#   #", "#   #" },
        ['O'] = new[] { " ### ", "#   #", "#   #", "#   #", "#   #", "#   #", " ### " },
        ['P'] = new[] { "#### ", "#   #", "#   #", "#### ", "#    ", "#    ", "#    " },
        ['Q'] = new[] { " ### ", "#   #", "#   #", "#   #", "# # #", "#  # ", " ## #" },
        ['R'] = new[] { "#### ", "#   #", "#   #", "#### ", "# #  ", "#  # ", "#   #" },
        ['S'] = new[] { " ####", "#    ", "#    ", " ### ", "    #", "    #", "#### " },
        ['T'] = new[] { "#####", "  #  ", "  #  ", "  #  ", "  #  ", "  #  ", "  #  " },
        ['U'] = new[] { "#   #", "#   #", "#   #", "#   #", "#   #", "#   #", " ### " },
        ['V'] = new[] { "#   #", "#   #", "#   #", "#   #", "#   #", " # # ", "  #  " },
        ['W'] = new[] { "#   #", "#   #", "#   #", "# # #", "# # #", "## ##", "#   #" },
        ['X'] = new[] { "#   #", "#   #", " # # ", "  #  ", " # # ", "#   #", "#   #" },
        ['Y'] = new[] { "#   #", "#   #", " # # ", "  #  ", "  #  ", "  #  ", "  #  " },
        ['Z'] = new[] { "#####", "    #", "   # ", "  #  ", " #   ", "#    ", "#####" },
        ['0'] = new[] { " ### ", "#   #", "#  ##", "# # #", "##  #", "#   #", " ### " },
        ['1'] = new[] { "  #  ", " ##  ", "  #  ", "  #  ", "  #  ", "  #  ", "#####" },
        ['2'] = new[] { " ### ", "#   #", "    #", "   # ", "  #  ", " #   ", "#####" },
        ['3'] = new[] { "#####", "   # ", "  #  ", "   # ", "    #", "#   #", " ### " },
        ['4'] = new[] { "   # ", "  ## ", " # # ", "#  # ", "#####", "   # ", "   # " },
        ['5'] = new[] { "#####", "#    ", "#### ", "    #", "    #", "#   #", " ### " },
        ['6'] = new[] { " ### ", "#    ", "#    ", "#### ", "#   #", "#   #", " ### " },
        ['7'] = new[] { "#####", "    #", "   # ", "  #  ", " #   ", " #   ", " #   " },
        ['8'] = new[] { " ### ", "#   #", "#   #", " ### ", "#   #", "#   #", " ### " },
        ['9'] = new[] { " ### ", "#   #", "#   #", " ####", "    #", "    #", " ### " },
        [' '] = new[] { "     ", "     ", "     ", "     ", "     ", "     ", "     " },
        [':'] = new[] { "     ", "  #  ", "  #  ", "     ", "  #  ", "  #  ", "     " },
        [','] = new[] { "     ", "     ", "     ", "     ", "  #  ", "  #  ", " #   " },
        ['-'] = new[] { "     ", "     ", "     ", "#####", "     ", "     ", "     " },
        ['.'] = new[] { "     ", "     ", "     ", "     ", "     ", "  ## ", "  ## " },
        ['/'] = new[] { "    #", "    #", "   # ", "  #  ", " #   ", "#    ", "#    " },
    };
}