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

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

namespace AcDream.App.Rendering;

/// <summary>
/// 2D batched quad renderer for text + solid rectangles. Coordinates are in
/// screen pixels with origin top-left, +X right, +Y down. Call
/// <see cref="Begin"/> at the start of a HUD pass, queue geometry via
/// <see cref="DrawString"/> / <see cref="DrawRect"/>, then <see cref="Flush"/>.
///
/// Uses two internal vertex buffers (text and rect) flushed in two draw calls
/// to avoid a per-vertex "use texture" flag. Rects are drawn first so text
/// sits on top of background panels.
/// </summary>
public sealed unsafe class TextRenderer : IDisposable
{
    private const int FloatsPerVertex = 8;   // pos(2) + uv(2) + color(4)

    private readonly GL _gl;
    private readonly Shader _shader;
    private readonly uint _vao;
    private readonly uint _vbo;
    private int _vboCapacityBytes;

    private readonly List<float> _textBuf = new(8192);
    private readonly List<float> _rectBuf = new(1024);
    private int _textVerts;
    private int _rectVerts;
    private Vector2 _screenSize;

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

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

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

        uint stride = FloatsPerVertex * sizeof(float);
        _gl.EnableVertexAttribArray(0);
        _gl.VertexAttribPointer(0, 2, VertexAttribPointerType.Float, false, stride, (void*)0);
        _gl.EnableVertexAttribArray(1);
        _gl.VertexAttribPointer(1, 2, VertexAttribPointerType.Float, false, stride, (void*)(2 * sizeof(float)));
        _gl.EnableVertexAttribArray(2);
        _gl.VertexAttribPointer(2, 4, VertexAttribPointerType.Float, false, stride, (void*)(4 * sizeof(float)));

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

    /// <summary>Begin a HUD pass. Call once per frame before any Draw* calls.</summary>
    public void Begin(Vector2 screenSize)
    {
        _screenSize = screenSize;
        _textBuf.Clear();
        _rectBuf.Clear();
        _textVerts = 0;
        _rectVerts = 0;
    }

    /// <summary>Draw a filled rectangle in screen pixel space.</summary>
    public void DrawRect(float x, float y, float w, float h, Vector4 color)
    {
        AppendQuad(_rectBuf, x, y, w, h, 0, 0, 0, 0, color);
        _rectVerts += 6;
    }

    /// <summary>Draw a 1-pixel-thick outline rect.</summary>
    public void DrawRectOutline(float x, float y, float w, float h, Vector4 color, float thickness = 1f)
    {
        // top, bottom, left, right
        DrawRect(x, y, w, thickness, color);
        DrawRect(x, y + h - thickness, w, thickness, color);
        DrawRect(x, y, thickness, h, color);
        DrawRect(x + w - thickness, y, thickness, h, color);
    }

    /// <summary>
    /// Draw a single line of text at (x,y) where (x,y) is the top-left of the
    /// typographic block. Handles '\n' as a line break.
    /// </summary>
    public void DrawString(BitmapFont font, string text, float x, float y, Vector4 color)
    {
        float cursorX = x;
        // The caller provides top-y; shift to baseline for glyph offset math.
        float baseline = y + font.Ascent;

        for (int i = 0; i < text.Length; i++)
        {
            char c = text[i];
            if (c == '\n')
            {
                cursorX = x;
                baseline += font.LineHeight;
                continue;
            }
            if (!font.TryGetGlyph(c, out var g))
            {
                // Unknown glyph — skip its advance width if '?' exists.
                if (font.TryGetGlyph('?', out var q))
                    cursorX += q.Advance;
                continue;
            }

            float gx = cursorX + g.OffsetX;
            float gy = baseline + g.OffsetY;
            float gw = g.Width;
            float gh = g.Height;

            if (gw > 0 && gh > 0)
            {
                AppendQuad(_textBuf,
                    gx, gy, gw, gh,
                    g.UvMinX, g.UvMinY, g.UvMaxX, g.UvMaxY,
                    color);
                _textVerts += 6;
            }
            cursorX += g.Advance;
        }
    }

    private static void AppendQuad(List<float> buf,
        float x, float y, float w, float h,
        float u0, float v0, float u1, float v1, Vector4 color)
    {
        // Two triangles (6 verts). CCW in pixel space is clockwise in NDC
        // because the vertex shader flips Y, so OpenGL's default front-face
        // is GL_CCW — we rely on cull-face being disabled during HUD pass.
        //   (x, y) ─ (x+w, y)
        //     │         │
        //   (x, y+h) ─ (x+w, y+h)
        //
        // Triangle 1: (x,y) (x+w,y+h) (x+w,y)
        // Triangle 2: (x,y) (x,y+h)   (x+w,y+h)
        void V(float px, float py, float pu, float pv)
        {
            buf.Add(px); buf.Add(py);
            buf.Add(pu); buf.Add(pv);
            buf.Add(color.X); buf.Add(color.Y); buf.Add(color.Z); buf.Add(color.W);
        }
        V(x,     y,     u0, v0);
        V(x + w, y + h, u1, v1);
        V(x + w, y,     u1, v0);
        V(x,     y,     u0, v0);
        V(x,     y + h, u0, v1);
        V(x + w, y + h, u1, v1);
    }

    /// <summary>Upload + draw accumulated rects + text. font may be null if only DrawRect was used.</summary>
    public void Flush(BitmapFont? font)
    {
        if (_textVerts == 0 && _rectVerts == 0) return;

        _shader.Use();
        _shader.SetVec2("uScreenSize", _screenSize);

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

        // Save GL state.
        bool wasDepth = _gl.IsEnabled(EnableCap.DepthTest);
        bool wasBlend = _gl.IsEnabled(EnableCap.Blend);
        bool wasCull  = _gl.IsEnabled(EnableCap.CullFace);
        _gl.Disable(EnableCap.DepthTest);
        _gl.Disable(EnableCap.CullFace);
        _gl.Enable(EnableCap.Blend);
        _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);

        // Untextured rects first — they form panel backgrounds.
        if (_rectVerts > 0)
        {
            _shader.SetInt("uUseTexture", 0);
            UploadBuffer(_rectBuf);
            _gl.DrawArrays(PrimitiveType.Triangles, 0, (uint)_rectVerts);
        }

        // Textured text glyphs.
        if (_textVerts > 0 && font is not null)
        {
            _shader.SetInt("uUseTexture", 1);
            _gl.ActiveTexture(TextureUnit.Texture0);
            _gl.BindTexture(TextureTarget.Texture2D, font.TextureId);
            _shader.SetInt("uTex", 0);
            UploadBuffer(_textBuf);
            _gl.DrawArrays(PrimitiveType.Triangles, 0, (uint)_textVerts);
        }

        // Restore GL state.
        if (!wasBlend) _gl.Disable(EnableCap.Blend);
        if (wasCull)   _gl.Enable(EnableCap.CullFace);
        if (wasDepth)  _gl.Enable(EnableCap.DepthTest);

        _gl.BindVertexArray(0);
    }

    private void UploadBuffer(List<float> buf)
    {
        int bytes = buf.Count * sizeof(float);
        if (bytes == 0) return;

        if (bytes > _vboCapacityBytes)
        {
            fixed (float* p = CollectionsMarshal.AsSpan(buf))
                _gl.BufferData(BufferTargetARB.ArrayBuffer, (nuint)bytes, p, BufferUsageARB.DynamicDraw);
            _vboCapacityBytes = bytes;
        }
        else
        {
            fixed (float* p = CollectionsMarshal.AsSpan(buf))
                _gl.BufferSubData(BufferTargetARB.ArrayBuffer, 0, (nuint)bytes, p);
        }
    }

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