tesseract fallback engine

frontend/src/recognition/tesseractEngine.ts

@@ -0,0 +1,57 @@
+import type { Stroke, RecognitionCandidate } from './types'
+import { strokesToCanvas } from './preprocess'
+import type Tesseract from 'tesseract.js'
+import type { PSM } from 'tesseract.js'
+
+let worker: Tesseract.Worker | null = null
+let loading = false
+
+export function isReady(): boolean {
+  return worker !== null
+}
+
+export async function init(): Promise<void> {
+  if (worker || loading) return
+  loading = true
+
+  try {
+    const Tesseract = await import('tesseract.js')
+    worker = await Tesseract.createWorker('eng')
+    await worker.setParameters({
+      tessedit_char_whitelist: 'ABCDEFGHIJKLMNOPQRSTUVWXYZ',
+      // SAFETY: '10' is exactly PSM.SINGLE_CHAR, so the literal lands inside the enum's value set
+      tessedit_pageseg_mode: '10' as PSM, // single character
+    })
+  } catch (e) {
+    console.warn('Tesseract init failed:', e)
+    worker = null
+  }
+  loading = false
+}
+
+export async function recognize(strokes: Stroke[]): Promise<RecognitionCandidate[]> {
+  if (!worker) return []
+
+  try {
+    const imageData = strokesToCanvas(strokes, 128)
+
+    // convert ImageData to canvas for tesseract
+    const canvas = new OffscreenCanvas(128, 128)
+    const ctx = canvas.getContext('2d')!
+    ctx.putImageData(imageData, 0, 0)
+    const blob = await canvas.convertToBlob({ type: 'image/png' })
+
+    const result = await worker.recognize(blob)
+    const text = result.data.text.trim().toUpperCase()
+
+    if (text.length === 1 && text >= 'A' && text <= 'Z') {
+      const confidence = result.data.confidence / 100
+      return [{ letter: text, confidence }]
+    }
+
+    return []
+  } catch (e) {
+    console.warn('Tesseract recognition failed:', e)
+    return []
+  }
+}