Fix front camera rotation mirroring in shader

Add uIsFrontCamera uniform to shader and adjust coordinate
transformations for front camera's mirrored texture coordinates:
- Position transform: (1-posY, 1-posX) instead of (posY, 1-posX)
- Angle transform: -angle - 90° instead of +angle + 90°

Applied to linearFocusDistance, radialFocusDistance, and blur
direction calculations in sampleBlurred.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/src/main/java/no/naiv/tiltshift/effect/TiltShiftRenderer.kt

@@ -126,7 +126,7 @@ class TiltShiftRenderer(
         GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT)
 
         // Use shader and set parameters
-        shader.use(cameraTextureId, blurParameters, surfaceWidth, surfaceHeight)
+        shader.use(cameraTextureId, blurParameters, surfaceWidth, surfaceHeight, isFrontCamera)
 
         // Set vertex positions
         GLES20.glEnableVertexAttribArray(shader.aPositionLocation)

app/src/main/java/no/naiv/tiltshift/effect/TiltShiftShader.kt

@@ -24,6 +24,7 @@ class TiltShiftShader(private val context: Context) {
     // Uniform locations
     private var uTextureLocation: Int = 0
     private var uModeLocation: Int = 0
+    private var uIsFrontCameraLocation: Int = 0
     private var uAngleLocation: Int = 0
     private var uPositionXLocation: Int = 0
     private var uPositionYLocation: Int = 0
@@ -65,6 +66,7 @@ class TiltShiftShader(private val context: Context) {
         // Get uniform locations
         uTextureLocation = GLES20.glGetUniformLocation(programId, "uTexture")
         uModeLocation = GLES20.glGetUniformLocation(programId, "uMode")
+        uIsFrontCameraLocation = GLES20.glGetUniformLocation(programId, "uIsFrontCamera")
         uAngleLocation = GLES20.glGetUniformLocation(programId, "uAngle")
         uPositionXLocation = GLES20.glGetUniformLocation(programId, "uPositionX")
         uPositionYLocation = GLES20.glGetUniformLocation(programId, "uPositionY")
@@ -82,7 +84,7 @@ class TiltShiftShader(private val context: Context) {
     /**
      * Uses the shader program and sets uniforms.
      */
-    fun use(textureId: Int, params: BlurParameters, width: Int, height: Int) {
+    fun use(textureId: Int, params: BlurParameters, width: Int, height: Int, isFrontCamera: Boolean = false) {
         GLES20.glUseProgram(programId)
 
         // Bind camera texture
@@ -92,6 +94,7 @@ class TiltShiftShader(private val context: Context) {
 
         // Set effect parameters
         GLES20.glUniform1i(uModeLocation, if (params.mode == BlurMode.RADIAL) 1 else 0)
+        GLES20.glUniform1i(uIsFrontCameraLocation, if (isFrontCamera) 1 else 0)
         GLES20.glUniform1f(uAngleLocation, params.angle)
         GLES20.glUniform1f(uPositionXLocation, params.positionX)
         GLES20.glUniform1f(uPositionYLocation, params.positionY)

app/src/main/res/raw/tiltshift_fragment.glsl

@@ -10,6 +10,7 @@ uniform samplerExternalOES uTexture;
 
 // Effect parameters
 uniform int uMode;              // 0 = linear, 1 = radial
+uniform int uIsFrontCamera;     // 0 = back camera, 1 = front camera
 uniform float uAngle;           // Rotation angle in radians
 uniform float uPositionX;       // Horizontal center of focus (0-1)
 uniform float uPositionY;       // Vertical center of focus (0-1)
@@ -24,9 +25,15 @@ varying vec2 vTexCoord;
 // Calculate signed distance from the focus region for LINEAR mode
 float linearFocusDistance(vec2 uv) {
     // Center point of the focus region
-    // Transform from screen coordinates to texture coordinates (90° rotation)
-    // Screen (x,y) -> Texture (y, 1-x)
-    vec2 center = vec2(uPositionY, 1.0 - uPositionX);
+    // Transform from screen coordinates to texture coordinates
+    // Back camera: Screen (x,y) -> Texture (y, 1-x)
+    // Front camera: Screen (x,y) -> Texture (1-y, 1-x) (additional X flip for mirror)
+    vec2 center;
+    if (uIsFrontCamera == 1) {
+        center = vec2(1.0 - uPositionY, 1.0 - uPositionX);
+    } else {
+        center = vec2(uPositionY, 1.0 - uPositionX);
+    }
     vec2 offset = uv - center;
 
     // Correct for screen aspect ratio to make coordinate space square
@@ -35,9 +42,15 @@ float linearFocusDistance(vec2 uv) {
     float screenAspect = uResolution.x / uResolution.y;
     offset.y *= screenAspect;
 
-    // Adjust angle by +90 degrees to compensate for the coordinate transformation
-    // The position transform is a 90° CW rotation, so angles transform as θ + 90°
-    float adjustedAngle = uAngle + 1.5707963;
+    // Adjust angle to compensate for the coordinate transformation
+    // Back camera: +90° for the 90° CW rotation
+    // Front camera: -90° (negated due to X flip mirror effect)
+    float adjustedAngle;
+    if (uIsFrontCamera == 1) {
+        adjustedAngle = -uAngle - 1.5707963;
+    } else {
+        adjustedAngle = uAngle + 1.5707963;
+    }
     float cosA = cos(adjustedAngle);
     float sinA = sin(adjustedAngle);
 
@@ -50,9 +63,13 @@ float linearFocusDistance(vec2 uv) {
 // Calculate signed distance from the focus region for RADIAL mode
 float radialFocusDistance(vec2 uv) {
     // Center point of the focus region
-    // Transform from screen coordinates to texture coordinates (90° rotation)
-    // Screen (x,y) -> Texture (y, 1-x)
-    vec2 center = vec2(uPositionY, 1.0 - uPositionX);
+    // Transform from screen coordinates to texture coordinates
+    vec2 center;
+    if (uIsFrontCamera == 1) {
+        center = vec2(1.0 - uPositionY, 1.0 - uPositionX);
+    } else {
+        center = vec2(uPositionY, 1.0 - uPositionX);
+    }
     vec2 offset = uv - center;
 
     // Correct for screen aspect ratio to make coordinate space square
@@ -61,9 +78,13 @@ float radialFocusDistance(vec2 uv) {
     float screenAspect = uResolution.x / uResolution.y;
     offset.y *= screenAspect;
 
-    // Apply rotation
-    // Adjust angle by +90 degrees to compensate for the coordinate transformation
-    float adjustedAngle = uAngle + 1.5707963;
+    // Apply rotation with angle adjustment for coordinate transformation
+    float adjustedAngle;
+    if (uIsFrontCamera == 1) {
+        adjustedAngle = -uAngle - 1.5707963;
+    } else {
+        adjustedAngle = uAngle + 1.5707963;
+    }
     float cosA = cos(adjustedAngle);
     float sinA = sin(adjustedAngle);
     vec2 rotated = vec2(
@@ -120,8 +141,13 @@ vec4 sampleBlurred(vec2 uv, float blur) {
     vec2 blurDir;
     if (uMode == 1) {
         // Radial: blur away from center
-        // Transform from screen coordinates to texture coordinates (90° rotation)
-        vec2 center = vec2(uPositionY, 1.0 - uPositionX);
+        // Transform from screen coordinates to texture coordinates
+        vec2 center;
+        if (uIsFrontCamera == 1) {
+            center = vec2(1.0 - uPositionY, 1.0 - uPositionX);
+        } else {
+            center = vec2(uPositionY, 1.0 - uPositionX);
+        }
         vec2 toCenter = uv - center;
         float len = length(toCenter);
         if (len > 0.001) {
@@ -132,7 +158,12 @@ vec4 sampleBlurred(vec2 uv, float blur) {
     } else {
         // Linear: blur perpendicular to focus line
         // Adjust angle for coordinate transformation
-        float blurAngle = uAngle + 1.5707963;
+        float blurAngle;
+        if (uIsFrontCamera == 1) {
+            blurAngle = -uAngle - 1.5707963;
+        } else {
+            blurAngle = uAngle + 1.5707963;
+        }
         blurDir = vec2(cos(blurAngle), sin(blurAngle));
     }