add flip image functionality

app/src/main/java/eu/kanade/presentation/reader/settings/ColorFilterPage.kt

@@ -117,6 +117,10 @@ internal fun ColumnScope.ColorFilterPage(screenModel: ReaderSettingsScreenModel)
         label = stringResource(MR.strings.pref_inverted_colors),
         pref = screenModel.preferences.invertedColors(),
     )
+    CheckboxItem(
+        label = stringResource(MR.strings.pref_flip_horizontally),
+        pref = screenModel.preferences.flipHorizontally(),
+    )
 }
 
 private fun getColorValue(currentColor: Int, color: Int, mask: Long, bitShift: Int): Int {

app/src/main/java/eu/kanade/tachiyomi/ui/reader/setting/ReaderPreferences.kt

@@ -109,6 +109,8 @@ class ReaderPreferences(
 
     fun invertedColors() = preferenceStore.getBoolean("pref_inverted_colors", false)
 
+    fun flipHorizontally() = preferenceStore.getBoolean("flip_horizontally", false)
+
     // endregion
 
     // region Controls

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/pager/PagerConfig.kt

@@ -48,6 +48,9 @@ class PagerConfig(
     var landscapeZoom = false
         private set
 
+    var flipHorizontally = false
+        private set
+
     init {
         readerPreferences.readerTheme()
             .register(
@@ -106,6 +109,9 @@ class PagerConfig(
                 { dualPageRotateToFitInvert = it },
                 { imagePropertyChangedListener?.invoke() },
             )
+
+        readerPreferences.flipHorizontally()
+            .register({ flipHorizontally = it }, { imagePropertyChangedListener?.invoke() })
     }
 
     private fun zoomTypeFromPreference(value: Int) {

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/pager/PagerPageHolder.kt

@@ -174,6 +174,10 @@ class PagerPageHolder(
     }
 
     private fun process(page: ReaderPage, imageSource: BufferedSource): BufferedSource {
+        if (viewer.config.flipHorizontally) {
+            return ImageUtil.flipImage(imageSource, true, false)
+        }
+
         if (viewer.config.dualPageRotateToFit) {
             return rotateDualPage(imageSource)
         }

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/webtoon/WebtoonConfig.kt

@@ -44,6 +44,9 @@ class WebtoonConfig(
 
     val theme = readerPreferences.readerTheme().get()
 
+    var flipHorizontally = false
+        private set
+
     init {
         readerPreferences.cropBordersWebtoon()
             .register({ imageCropBorders = it }, { imagePropertyChangedListener?.invoke() })
@@ -96,6 +99,9 @@ class WebtoonConfig(
             .distinctUntilChanged()
             .onEach { themeChangedListener?.invoke() }
             .launchIn(scope)
+
+        readerPreferences.flipHorizontally()
+            .register({ flipHorizontally = it }, { imagePropertyChangedListener?.invoke() })
     }
 
     override var navigator: ViewerNavigation = defaultNavigation()

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/webtoon/WebtoonPageHolder.kt

@@ -213,6 +213,10 @@ class WebtoonPageHolder(
     }
 
     private fun process(imageSource: BufferedSource): BufferedSource {
+        if (viewer.config.flipHorizontally) {
+            return ImageUtil.flipImage(imageSource, true, false)
+        }
+
         if (viewer.config.dualPageRotateToFit) {
             return rotateDualPage(imageSource)
         }

core/common/src/main/kotlin/tachiyomi/core/common/util/system/ImageUtil.kt

@@ -163,11 +163,26 @@ object ImageUtil {
         return output
     }
 
+    fun flipImage(imageSource: BufferedSource, flipX: Boolean, flipY: Boolean): BufferedSource {
+        val imageBitmap = BitmapFactory.decodeStream(imageSource.inputStream())
+        val flipped = flipBitMap(imageBitmap, flipX, flipY)
+
+        val output = Buffer()
+        flipped.compress(Bitmap.CompressFormat.JPEG, 100, output.outputStream())
+
+        return output
+    }
+
     private fun rotateBitMap(bitmap: Bitmap, degrees: Float): Bitmap {
         val matrix = Matrix().apply { postRotate(degrees) }
         return Bitmap.createBitmap(bitmap, 0, 0, bitmap.width, bitmap.height, matrix, true)
     }
 
+    private fun flipBitMap(bitmap: Bitmap, flipX: Boolean, flipY: Boolean): Bitmap {
+        val matrix = Matrix().apply { postScale(if (flipX) -1f else 1f, if (flipY) -1f else 1f) }
+        return Bitmap.createBitmap(bitmap, 0, 0, bitmap.width, bitmap.height, matrix, true)
+    }
+
     /**
      * Split the image into left and right parts, then merge them into a new image.
      */

i18n/src/commonMain/resources/MR/base/strings.xml

@@ -375,6 +375,7 @@
     <string name="pref_custom_brightness">Custom brightness</string>
     <string name="pref_grayscale">Grayscale</string>
     <string name="pref_inverted_colors">Inverted</string>
+    <string name="pref_flip_horizontally">Flip horizontally</string>
     <string name="pref_custom_color_filter">Custom color filter</string>
     <string name="pref_color_filter_mode">Color filter blend mode</string>
     <string name="filter_mode_overlay">Overlay</string>