Unity 3D中Tesseract导入异常致OCR调用报错的解决求助

问题概述 #

通过Visual Studio NuGet安装Tesseract后，将相关文件复制到Unity项目Assets/Plugins目录，并放置eng.traineddata资源，运行拍照识别代码时抛出异常：

OCR Error: Exception has been thrown by the target of an invocation.

原示例代码 #

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;
using UnityEngine;
using UnityEngine.Networking;
using UnityEngine.UI;
using Newtonsoft.Json.Linq;
using Tesseract;

public class CaptureCards : MonoBehaviour
{
    public RawImage cameraFeed;
    public Button captureButton;
    public Text cardDetailsText;
    private WebCamTexture webCamTexture;
    private HttpClient httpClient;

    void Start()
    {
        StartCameraFeed();
        httpClient = new HttpClient();
        captureButton.onClick.AddListener(CaptureAndProcess);
    }

    void StartCameraFeed()
    {
        WebCamDevice[] devices = WebCamTexture.devices;
        if (devices.Length == 0)
        {
            Debug.LogError("No camera devices found.");
            return;
        }

        Debug.Log("Available cameras:");
        foreach (var device in devices)
        {
            Debug.Log(device.name);
        }

        string selectedCamera = devices[0].name;
        Debug.Log("Using camera: " + selectedCamera);
        webCamTexture = new WebCamTexture(selectedCamera);
        cameraFeed.texture = webCamTexture;
        cameraFeed.material.mainTexture = webCamTexture;
        webCamTexture.Play();
    }

    async void CaptureAndProcess()
    {
        if (webCamTexture == null || !webCamTexture.isPlaying)
        {
            cardDetailsText.text = "Camera feed not available.";
            return;
        }

        Texture2D captureImage = new Texture2D(webCamTexture.width, webCamTexture.height);
        captureImage.SetPixels(webCamTexture.GetPixels());
        captureImage.Apply();

        string tempPath = Path.Combine(Application.persistentDataPath, "captureCard.png");
        Texture2D preprocessedImage = PreprocessImage(captureImage);
        File.WriteAllBytes(tempPath, preprocessedImage.EncodeToPNG());
        Debug.Log("Image saved at: " + tempPath);

        string cardName = await RecognizeText(tempPath);
        if (!string.IsNullOrEmpty(cardName))
        {
            JObject cardData = await FetchCardDetails(cardName);
            if (cardData != null)
            {
                DisplayCardDetails(cardData);
                SaveCardDetails(cardData);
            }
            else
            {
                cardDetailsText.text = "Card not Found";
            }
        }
        else
        {
            cardDetailsText.text = "Failed to recognize card name";
        }
    }

    Texture2D PreprocessImage(Texture2D original)
    {
        int targetWidth = 800;
        int targetHeight = (int)(original.height * (800.0f / original.width));
        Texture2D resizedImage = new Texture2D(targetWidth, targetHeight);

        for (int y = 0; y < targetHeight; y++)
        {
            for (int x = 0; x < targetWidth; x++)
            {
                Color pixel = original.GetPixelBilinear((float)x / targetWidth, (float)y / targetHeight);
                resizedImage.SetPixel(x, y, pixel);
            }
        }
        resizedImage.Apply();

        Color[] pixels = resizedImage.GetPixels();
        for (int i = 0; i < pixels.Length; i++)
        {
            float gray = pixels[i].grayscale;
            pixels[i] = new Color(gray, gray, gray);
        }
        resizedImage.SetPixels(pixels);
        resizedImage.Apply();

        return resizedImage;
    }

    async Task<string> RecognizeText(string imagePath)
    {
        string resultText = string.Empty;
        string tessDataPath = Path.Combine(Application.streamingAssetsPath, "tessdata");
        Debug.Log($"StreamingAssets Path: {Application.streamingAssetsPath}");
        Debug.Log($"File Exists: {File.Exists(Path.Combine(Application.streamingAssetsPath, "tessdata", "eng.traineddata"))}");

        await Task.Run(() =>
        {
            try
            {
                if (!Directory.Exists(tessDataPath))
                    throw new DirectoryNotFoundException($"Tesseract data directory not found: {tessDataPath}");
                if (!File.Exists(Path.Combine(tessDataPath, "eng.traineddata")))
                    throw new FileNotFoundException("Tesseract trained data file not found (eng.traineddata).");
                if (!File.Exists(imagePath))
                    throw new FileNotFoundException($"Image file not found: {imagePath}");

                using (var engine = new TesseractEngine(tessDataPath, "eng", EngineMode.Default))
                {
                    using (var img = Pix.LoadFromFile(imagePath))
                    {
                        using (var page = engine.Process(img, PageSegMode.SingleBlock))
                        {
                            resultText = page.GetText().Trim();
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                Debug.LogError($"OCR Error: {ex.Message}\n{ex.StackTrace}");
            }
        });

        return resultText;
    }

    async Task<JObject> FetchCardDetails(string cardName)
    {
        try
        {
            string apiUrl = $"https://api.scryfall.com/cards/named?exact={UnityWebRequest.EscapeURL(cardName)}";
            string response = await httpClient.GetStringAsync(apiUrl);
            return JObject.Parse(response);
        }
        catch
        {
            return null;
        }
    }

    void DisplayCardDetails(JObject cardData)
    {
        string name = cardData["name"].ToString();
        string type = cardData["type_line"].ToString();
        string manaCost = cardData["mana_cost"].ToString();
        string description = cardData["oracle_text"].ToString();
        cardDetailsText.text = $"Name: {name}\nType: {type}\nMana Cost: {manaCost}\nDescription: {description}";
    }

    void SaveCardDetails(JObject cardData)
    {
        string savePath = Path.Combine(Application.persistentDataPath, "savedCards.json");
        JArray savedCards = File.Exists(savePath)
            ? JArray.Parse(File.ReadAllText(savePath))
            : new JArray();
        savedCards.Add(cardData);
        File.WriteAllText(savePath, savedCards.ToString());
    }

    void OnDestroy()
    {
        webCamTexture.Stop();
        httpClient.Dispose();
    }
}

解决步骤 #

1. 修正Tesseract库导入方式 #

• 避免直接从NuGet复制文件，Unity对.NET运行时环境有特定要求，建议使用Unity兼容的预编译Tesseract版本，或者通过Unity Package Manager安装适配包
• 手动导入时，需将库文件放置到对应平台目录（如Plugins/x86_64、Plugins/Android），并在Inspector面板设置正确的平台兼容选项（取消勾选无关平台）
• 删除NuGet导入的冗余依赖文件，避免版本冲突

2. 修复tessdata路径与访问权限 #

• 移动端无法直接通过File类访问StreamingAssets目录，需先将eng.traineddata复制到persistentDataPath再使用，修改RecognizeText方法：

  async Task<string> RecognizeText(string imagePath)
  {
      string resultText = string.Empty;
      string targetTessPath = Path.Combine(Application.persistentDataPath, "tessdata");
      string trainedDataPath = Path.Combine(targetTessPath, "eng.traineddata");
  
      // 复制tessdata到持久化目录
      if (!Directory.Exists(targetTessPath))
      {
          Directory.CreateDirectory(targetTessPath);
          string sourcePath = Path.Combine(Application.streamingAssetsPath, "tessdata/eng.traineddata");
  
          #if UNITY_EDITOR || UNITY_STANDALONE
          File.Copy(sourcePath, trainedDataPath, overwrite: true);
          #else
          UnityWebRequest request = UnityWebRequest.Get(sourcePath);
          await request.SendWebRequest();
          File.WriteAllBytes(trainedDataPath, request.downloadHandler.data);
          #endif
      }
  
      await Task.Run(() =>
      {
          try
          {
              using (var engine = new TesseractEngine(targetTessPath, "eng", EngineMode.Default))
              {
                  using (var img = Pix.LoadFromFile(imagePath))
                  {
                      using (var page = engine.Process(img, PageSegMode.SingleBlock))
                      {
                          resultText = page.GetText().Trim();
                      }
                  }
              }
          }
          catch (TargetInvocationException ex)
          {
              Debug.LogError($"OCR Inner Error: {ex.InnerException.Message}\n{ex.InnerException.StackTrace}");
          }
          catch (Exception ex)
          {
              Debug.LogError($"OCR Error: {ex.Message}\n{ex.StackTrace}");
          }
      });
  
      return resultText;
  }
  

3. 捕获深层异常信息 #

• 原异常是包装后的TargetInvocationException，需捕获内部异常才能看到真实错误原因（如库文件缺失、架构不兼容），在catch块中添加专门处理逻辑

4. 优化图像预处理 #

• 在灰度化基础上添加二值化处理，强化文本与背景对比度，提升OCR准确率：

  // 替换原灰度化代码
  float threshold = 0.6f;
  for (int i = 0; i < pixels.Length; i++)
  {
      float gray = pixels[i].grayscale;
      pixels[i] = gray >= threshold ? Color.white : Color.black;
  }
  

内容的提问来源于stack exchange，提问作者Kayle Brown