Add real-time tilt-shift preview for gallery images

Gallery imports now show the effect live as you adjust parameters,
matching the camera preview experience. Uses a downscaled (1024px)
source bitmap for fast recomputation via collectLatest, which
cancels stale frames when params change mid-computation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src/main/java/no/naiv/tiltshift/camera/ImageCaptureHandler.kt

@@ -327,6 +327,15 @@ class ImageCaptureHandler(
         return rotated
     }
 
+    /**
+     * Applies tilt-shift effect to a bitmap for real-time preview.
+     * Runs on [Dispatchers.IO]. The caller owns the returned bitmap.
+     */
+    suspend fun applyTiltShiftPreview(source: Bitmap, params: BlurParameters): Bitmap =
+        withContext(Dispatchers.IO) {
+            applyTiltShiftEffect(source, params)
+        }
+
     /**
      * Applies tilt-shift blur effect to a bitmap.
      * Supports both linear and radial modes.

app/src/main/java/no/naiv/tiltshift/ui/CameraScreen.kt

@@ -106,8 +106,8 @@ fun CameraScreen(
     val showControls by viewModel.showControls.collectAsState()
     val lastSavedUri by viewModel.lastSavedUri.collectAsState()
     val lastThumbnailBitmap by viewModel.lastThumbnailBitmap.collectAsState()
-    val galleryBitmap by viewModel.galleryBitmap.collectAsState()
-    val isGalleryPreview = galleryBitmap != null
+    val galleryPreviewBitmap by viewModel.galleryPreviewBitmap.collectAsState()
+    val isGalleryPreview = galleryPreviewBitmap != null
 
     val zoomRatio by viewModel.cameraManager.zoomRatio.collectAsState()
     val minZoom by viewModel.cameraManager.minZoomRatio.collectAsState()
@@ -179,10 +179,10 @@ fun CameraScreen(
     ) {
         // Main view: gallery preview image or camera GL surface
         if (isGalleryPreview) {
-            galleryBitmap?.let { bmp ->
+            galleryPreviewBitmap?.let { bmp ->
                 Image(
                     bitmap = bmp.asImageBitmap(),
-                    contentDescription = "Gallery image preview. Adjust tilt-shift parameters then tap Apply.",
+                    contentDescription = "Gallery image preview with tilt-shift effect. Adjust parameters then tap Apply.",
                     contentScale = ContentScale.Fit,
                     modifier = Modifier
                         .fillMaxSize()

app/src/main/java/no/naiv/tiltshift/ui/CameraViewModel.kt

@@ -8,10 +8,14 @@ import android.util.Log
 import android.view.Surface
 import androidx.lifecycle.AndroidViewModel
 import androidx.lifecycle.viewModelScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.Job
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.collectLatest
 import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
 import no.naiv.tiltshift.camera.CameraManager
 import no.naiv.tiltshift.camera.ImageCaptureHandler
 import no.naiv.tiltshift.effect.BlurParameters
@@ -29,6 +33,8 @@ class CameraViewModel(application: Application) : AndroidViewModel(application)
 
     companion object {
         private const val TAG = "CameraViewModel"
+        /** Max dimension for the preview source bitmap to keep effect computation fast. */
+        private const val PREVIEW_MAX_DIMENSION = 1024
     }
 
     val cameraManager = CameraManager(application)
@@ -64,11 +70,19 @@ class CameraViewModel(application: Application) : AndroidViewModel(application)
 
     // Gallery preview state
     private val _galleryBitmap = MutableStateFlow<Bitmap?>(null)
-    val galleryBitmap: StateFlow<Bitmap?> = _galleryBitmap.asStateFlow()
 
     private val _galleryImageUri = MutableStateFlow<Uri?>(null)
     val galleryImageUri: StateFlow<Uri?> = _galleryImageUri.asStateFlow()
 
+    /** Downscaled source for fast preview recomputation. */
+    private var galleryPreviewSource: Bitmap? = null
+
+    /** Processed preview bitmap shown in the UI. */
+    private val _galleryPreviewBitmap = MutableStateFlow<Bitmap?>(null)
+    val galleryPreviewBitmap: StateFlow<Bitmap?> = _galleryPreviewBitmap.asStateFlow()
+
+    private var previewJob: Job? = null
+
     val isGalleryPreview: Boolean get() = _galleryBitmap.value != null
 
     // Device state
@@ -111,6 +125,24 @@ class CameraViewModel(application: Application) : AndroidViewModel(application)
             if (bitmap != null) {
                 _galleryBitmap.value = bitmap
                 _galleryImageUri.value = uri
+
+                // Create downscaled source for fast preview recomputation
+                galleryPreviewSource = withContext(Dispatchers.IO) {
+                    val maxDim = maxOf(bitmap.width, bitmap.height)
+                    if (maxDim > PREVIEW_MAX_DIMENSION) {
+                        val scale = PREVIEW_MAX_DIMENSION.toFloat() / maxDim
+                        Bitmap.createScaledBitmap(
+                            bitmap,
+                            (bitmap.width * scale).toInt(),
+                            (bitmap.height * scale).toInt(),
+                            true
+                        )
+                    } else {
+                        bitmap.copy(bitmap.config ?: Bitmap.Config.ARGB_8888, false)
+                    }
+                }
+
+                startPreviewLoop()
             } else {
                 haptics.error()
                 showError("Failed to load image")
@@ -118,11 +150,38 @@ class CameraViewModel(application: Application) : AndroidViewModel(application)
         }
     }
 
+    /** Reactively recomputes the tilt-shift preview when blur params change. */
+    private fun startPreviewLoop() {
+        previewJob?.cancel()
+        previewJob = viewModelScope.launch {
+            _blurParams.collectLatest { params ->
+                val source = galleryPreviewSource ?: return@collectLatest
+                try {
+                    val processed = captureHandler.applyTiltShiftPreview(source, params)
+                    val old = _galleryPreviewBitmap.value
+                    _galleryPreviewBitmap.value = processed
+                    old?.recycle()
+                } catch (e: Exception) {
+                    Log.w(TAG, "Preview computation failed", e)
+                }
+            }
+        }
+    }
+
     fun cancelGalleryPreview() {
-        val old = _galleryBitmap.value
+        previewJob?.cancel()
+        previewJob = null
+
+        val oldGallery = _galleryBitmap.value
+        val oldPreview = _galleryPreviewBitmap.value
         _galleryBitmap.value = null
         _galleryImageUri.value = null
-        old?.recycle()
+        _galleryPreviewBitmap.value = null
+
+        oldGallery?.recycle()
+        oldPreview?.recycle()
+        galleryPreviewSource?.recycle()
+        galleryPreviewSource = null
     }
 
     fun applyGalleryEffect() {
@@ -205,5 +264,7 @@ class CameraViewModel(application: Application) : AndroidViewModel(application)
         cameraManager.release()
         _lastThumbnailBitmap.value?.recycle()
         _galleryBitmap.value?.recycle()
+        _galleryPreviewBitmap.value?.recycle()
+        galleryPreviewSource?.recycle()
     }
 }