uniquake/Assets/Scripts/Modules/RenderModule.cs

using System;
using UnityEngine;
using UnityEngine.Rendering;

public partial class RenderModule
{
    private readonly UniQuake uq;

    public RenderModule(UniQuake uniQuake)
    {
        uq = uniQuake;
        BuildCallbacks();
    }

    private float xPos = -8f;

    private int UploadAliasModel(string name, QAliasHeader header, 
        QTriVertex[][] poseVertices, QTriangle[] triangles, QSTVert[] stVertices)
    {
        Debug.Log($"Alias model '{name}' with {header.numVerts} vertices, {header.numTriangles} triangles, {header.numFrames} frame(s)");

        // Massage the input data for easier conversion
        PreprocessMeshData(header, triangles, ref poseVertices, ref stVertices);
        
        // Triangle indices and UVs are the same for all animation frames
        ConvertTriangles(triangles, out var indices);
        ConvertUVs(stVertices, header.skinWidth, header.skinHeight, out var uvs);

        string modelName = System.IO.Path.GetFileNameWithoutExtension(name);
        AliasModel aliasModel = new AliasModel(modelName);
        
        // Identify animation sequences and turn each one into a separate Mesh with a single blend shape animation
        Mesh mesh;
        string animName = null;
        int startFrame = 0;
        for (int frameIdx = 0; frameIdx < header.numFrames; ++frameIdx)
        {
            string frameName = AliasModel.AnimationRegex.Match(header.frames[frameIdx].name).Value;
            if (animName == null)
            {
                animName = frameName;
                continue;
            }

            if (frameName != animName)
            {
                // New animation sequence; convert the previous sequence into a blend shape animation
                mesh = CreateAnimatedMesh(header, poseVertices, indices, uvs, $"{modelName}_{animName}", startFrame, frameIdx);
                aliasModel.AddAnimation(startFrame, mesh);
                
                animName = frameName;
                startFrame = frameIdx;
            }
        }
        
        mesh = CreateAnimatedMesh(header, poseVertices, indices, uvs, $"{modelName}_{animName}", startFrame, header.numFrames);
        aliasModel.AddAnimation(startFrame, mesh);

        var go = new GameObject(modelName);
        go.transform.SetPositionAndRotation(new Vector3(xPos, 0, 0), Quaternion.Euler(-90, 90, 0));

        if (header.numFrames > 1)
        {
            var mr = go.AddComponent<SkinnedMeshRenderer>();
            mr.material = new Material(Shader.Find("Universal Render Pipeline/Simple Lit"));
            mr.sharedMesh = mesh;
            mr.shadowCastingMode = ShadowCastingMode.Off;
            mr.receiveShadows = false;
            mr.lightProbeUsage = LightProbeUsage.Off;
            mr.reflectionProbeUsage = ReflectionProbeUsage.Off;
            
            var animator = go.AddComponent<AliasModelAnimator>();
            animator.aliasModel = aliasModel;
        }
        else
        {
            var mf = go.AddComponent<MeshFilter>();
            mf.sharedMesh = mesh;
            var mr = go.AddComponent<MeshRenderer>();
            mr.material = new Material(Shader.Find("Universal Render Pipeline/Simple Lit"));
            mr.shadowCastingMode = ShadowCastingMode.Off;
            mr.receiveShadows = false;
            mr.lightProbeUsage = LightProbeUsage.Off;
            mr.reflectionProbeUsage = ReflectionProbeUsage.Off;
        }

        xPos += 0.5f;
        return 1;
    }

    /// <summary>
    /// Quake has a bit of a weird outdated mesh setup where skin textures are split into a front side and a back side.
    /// Vertices on the seam between the front and back are used by triangles on both sides, but require a correction
    /// to their UVs when rendering the backside. To handle this properly, we duplicate these vertices and correct the
    /// UVs ahead of time. We do this as a separate pre-process step to keep things simple. 
    /// </summary>
    private static void PreprocessMeshData(QAliasHeader header, QTriangle[] triangles, 
        ref QTriVertex[][] poseVertices, ref QSTVert[] stVerts)
    {
        int newVertCount = 0;

        // First count how many new vertices we need to make, so we can preallocate the required arrays
        for (int triIdx = 0; triIdx < header.numTriangles; ++triIdx)
        {
            if (triangles[triIdx].facesFront != 0)
                continue;

            for (int indIdx = 0; indIdx < 3; ++indIdx)
            {
                int index = triangles[triIdx].vertIndex[indIdx];
                if (stVerts[index].onSeam == 0)
                    continue;
                
                // Back-side vertex on seam, needs to be duplicated and corrected
                ++newVertCount;
            }
        }
        
        if (newVertCount == 0)
            return;
        
        Array.Resize(ref stVerts, header.numVerts + newVertCount);
        for (int frameIdx = 0; frameIdx < header.numFrames; ++frameIdx)
        {
            Array.Resize(ref poseVertices[frameIdx], header.numVerts + newVertCount);
        }

        int newVertIndex = header.numVerts;

        // Now we go over all the triangles again and duplicate the vertices that are on a seam
        for (int triIdx = 0; triIdx < header.numTriangles; ++triIdx)
        {
            if (triangles[triIdx].facesFront != 0)
                continue;

            for (int indIdx = 0; indIdx < 3; ++indIdx)
            {
                int vertIndex = triangles[triIdx].vertIndex[indIdx];
                if (stVerts[vertIndex].onSeam == 0)
                    continue;
                
                // Clone the ST value and correct it to map onto the backside of the skin
                QSTVert stVertCopy = stVerts[vertIndex];
                stVertCopy.s += header.skinWidth / 2;
                stVerts[newVertIndex] = stVertCopy;

                // Clone the vertex position data for all frames
                for (int frameIdx = 0; frameIdx < header.numFrames; ++frameIdx)
                {
                    QTriVertex triVertCopy = poseVertices[frameIdx][vertIndex];
                    poseVertices[frameIdx][newVertIndex] = triVertCopy;
                }

                triangles[triIdx].vertIndex[indIdx] = newVertIndex++;
            }
        }

        header.numVerts += newVertCount;
    }
    
    private static void ConvertVertices(QAliasHeader header, QTriVertex[] triVerts, out Vector3[] vertices, out Vector3[] normals)
    {
        int numVerts = triVerts.Length;
        vertices = new Vector3[numVerts];
        normals = new Vector3[numVerts];

        Vector3 scale = header.scale.ToVector3();
        Vector3 origin = header.scaleOrigin.ToVector3();
        
        for (int i = 0; i < numVerts; ++i)
        {
            vertices[i] = Vector3.Scale(triVerts[i].ToVector3(), scale);
            normals[i] = QLightNormals.Get(triVerts[i].lightNormalIndex);
        }
    }

    private static void ConvertTriangles(QTriangle[] triangles, out ushort[] indices)
    {
        int numTris = triangles.Length;
        indices = new ushort[numTris * 3];
        
        for (int i = 0; i < numTris; ++i)
        {
            // Quake triangles are wound clockwise, so we reverse them here
            indices[i * 3 + 0] = (ushort)triangles[i].vertIndex[2];
            indices[i * 3 + 1] = (ushort)triangles[i].vertIndex[1];
            indices[i * 3 + 2] = (ushort)triangles[i].vertIndex[0];
        }
    }

    private static void ConvertUVs(QSTVert[] stVerts, int skinWidth, int skinHeight, out Vector2[] uvs)
    {
        int numVerts = stVerts.Length;
        uvs = new Vector2[numVerts];

        Vector2 scale = new Vector2(1.0f / skinWidth, 1.0f / skinHeight);
        
        for (int i = 0; i < numVerts; ++i)
        {
            uvs[i] = Vector2.Scale(new Vector2(stVerts[i].s, skinHeight - stVerts[i].t), scale);
        }
    }

    private static Mesh CreateAnimatedMesh(
        QAliasHeader header, QTriVertex[][] poseVertices, ushort[] indices, Vector2[] uvs,
        string animationName, int startFrame, int endFrame)
    {
        ConvertVertices(header, poseVertices[startFrame], out var baseVertices, out var baseNormals);
        
        var mesh = new Mesh { name = animationName };
        mesh.SetVertices(baseVertices);
        mesh.SetNormals(baseNormals);

        CreateBlendShapes(mesh, animationName, baseVertices, baseNormals, header, poseVertices, startFrame, endFrame);
        
        mesh.SetIndices(indices, MeshTopology.Triangles, 0, false);
        mesh.SetUVs(0, uvs);
        mesh.Optimize();
        mesh.RecalculateBounds();
        mesh.UploadMeshData(true);
        return mesh;
    }

    private static void CreateBlendShapes(
        Mesh mesh, string animName, Vector3[] baseVertices, Vector3[] baseNormals,
        QAliasHeader header, QTriVertex[][] poseVertices, int startFrame, int endFrame)
    {
        var deltaVertices = new Vector3[header.numVerts];
        var deltaNormals = new Vector3[header.numVerts];
        
        Vector3 scale = header.scale.ToVector3();
        Vector3 origin = header.scaleOrigin.ToVector3();
        
        int numFrames = endFrame - startFrame;
        
        // Repeat the first frame at the end, so we can smoothly animate the entire cycle and loop the animation without any breaks.
        for (int index = 1; index <= numFrames; ++index)
        {
            int frameIdx = startFrame + index % numFrames;
            var poseVerts = poseVertices[frameIdx];
            
            for (int vertIdx = 0; vertIdx < header.numVerts; ++vertIdx)
            {
                Vector3 vert = Vector3.Scale(poseVerts[vertIdx].ToVector3(), scale);
                deltaVertices[vertIdx] = vert - baseVertices[vertIdx];
                deltaNormals[vertIdx] = QLightNormals.Get(poseVerts[vertIdx].lightNormalIndex) - baseNormals[vertIdx];
            }
            
            mesh.AddBlendShapeFrame(animName, (float)index / numFrames, deltaVertices, deltaNormals, null);
        }
    }
}