diff --git a/src/app.rs b/src/app.rs
index 396cbf2..eee36c6 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -40,6 +40,11 @@ pub enum Message {
     RandomFolderTree,
     ScrollToCurrentThumbnail,
     FolderSelected(Option<PathBuf>),
+    /// Open an arbitrary path passed in from outside the app — typically the
+    /// file association case (double-clicking an image in a file manager
+    /// launches `marten /path/to/image.jpg`). Dispatches to `open_folder` or
+    /// `open_file` based on the path's filesystem type.
+    OpenPath(PathBuf),
     ImageDecoded(Result<DecodedImage, DecodeError>),
     ThumbnailsLoaded(Vec<(usize, iced::widget::image::Handle)>),
     OpenFolder,
@@ -111,6 +116,79 @@ impl Default for Viewer {
     }
 }
 
+impl Viewer {
+    /// Bootstrap entry point — called by
+    /// `iced::application(...).run_with(Viewer::init_from_env)`.
+    ///
+    /// Reads the first CLI argument (if any) to support being opened as a
+    /// file-association target. When a user double-clicks an image in their
+    /// file manager with marten set as the default app, the OS launches
+    /// `marten /path/to/image.jpg`. Without this, marten starts with an
+    /// empty viewer and the file is silently ignored — which is the bug we're
+    /// fixing.
+    ///
+    /// Behavior:
+    ///   * No argv[1]          → start with empty viewer (open-folder dialog flow).
+    ///   * argv[1] is a file   → open it (scans parent folder for next/prev nav).
+    ///   * argv[1] is a dir    → open that folder directly.
+    ///   * argv[1] doesn't exist or isn't readable → start empty + log a warning.
+    pub fn init_from_env() -> (Self, Task<Message>) {
+        let mut viewer = Self::default();
+
+        // argv[0] is the program path; the first real argument is argv[1].
+        // We deliberately only look at the first one — file managers never
+        // pass multiple files to a single-instance association target, and
+        // handling a list of files is a separate feature.
+        let task = match std::env::args_os().nth(1).map(PathBuf::from) {
+            Some(path) if path.is_dir() || path.is_file() => {
+                viewer.update(Message::OpenPath(path))
+            }
+            Some(path) => {
+                log::warn!(
+                    "Ignoring CLI argument (not a regular file or directory): {}",
+                    path.display()
+                );
+                Task::none()
+            }
+            None => Task::none(),
+        };
+
+        (viewer, task)
+    }
+}
+
+/// Given a clicked file path and the result of scanning its parent folder,
+/// decide which image list to display and at which index the clicked file
+/// lives.
+///
+/// * If the file is present in the scan → return the scan as-is with its
+///   index. This is the common case (file is supported and not anti-listed).
+/// * If the file is *not* in the scan (anti-listed codec, hidden file the
+///   user explicitly clicked, or the parent scan returned empty) → push the
+///   file into the list at its sorted position so the user still sees it and
+///   gets a proper decode error through the normal `load_current` pipeline
+///   rather than a silent "No supported images" message.
+///
+/// Returns `(images, target_index)`. If both inputs are empty, returns
+/// `(empty, 0)` and the caller surfaces "No supported images".
+fn resolve_open_file_target(
+    file_path: &std::path::Path,
+    scanned: Vec<PathBuf>,
+) -> (Vec<PathBuf>, usize) {
+    if let Some(idx) = scanned.iter().position(|p| p == file_path) {
+        return (scanned, idx);
+    }
+
+    let mut images = scanned;
+    images.push(file_path.to_path_buf());
+    images.sort();
+    let idx = images
+        .iter()
+        .position(|p| p == file_path)
+        .expect("just pushed it");
+    (images, idx)
+}
+
 /// What the image area viewport size is (window minus chrome).
 fn image_viewport(window: Size, fullscreen: bool, sidebar_width: f32) -> Size {
     if fullscreen {
@@ -144,19 +222,7 @@ impl Viewer {
                 )
             }
 
-            Message::FolderSelected(Some(path)) => {
-                let images = crate::nav::scan_folder(&path, &self.codec);
-                self.sidebar.refresh(&path, &self.codec);
-                if images.is_empty() {
-                    self.status_bar.filename =
-                        "No supported images in that folder".into();
-                    self.status_bar.total = 0;
-                    return Task::none();
-                }
-                self.navigator.set_images(images.clone());
-                self.thumbnail_bar.set_paths(images, 0);
-                self.load_current()
-            }
+            Message::FolderSelected(Some(path)) => self.open_folder(path),
 
             Message::FolderSelected(None) => {
                 if self.navigator.is_empty() {
@@ -165,6 +231,25 @@ impl Viewer {
                 Task::none()
             }
 
+            Message::OpenPath(path) => {
+                // Dispatch based on what's actually on disk. A non-existent
+                // path (e.g., broken symlink or stale .desktop entry) surfaces
+                // a clean status-bar message instead of silently doing nothing.
+                if path.is_dir() {
+                    self.open_folder(path)
+                } else if path.is_file() {
+                    self.open_file(path)
+                } else {
+                    log::warn!(
+                        "Ignoring OpenPath argument — not a file or directory: {}",
+                        path.display()
+                    );
+                    self.status_bar.filename =
+                        format!("Path not found: {}", path.display());
+                    Task::none()
+                }
+            }
+
             Message::Navigate(delta) => {
                 if self.navigator.is_empty() {
                     return Task::none();
@@ -779,6 +864,71 @@ impl Viewer {
         }
     }
 
+    // ── Path-open helpers ─────────────────────────────────────────────
+    //
+    // The two flavors of "open from outside the app":
+    //   * `open_folder` — used by the in-app folder picker AND by `OpenPath`
+    //     when the OS hands us a directory. Scans the folder, loads the first
+    //     image.
+    //   * `open_file`    — used by `OpenPath` when the OS hands us a single
+    //     image file (the file-association case). Scans the *parent* folder
+    //     so the user still gets next/prev navigation, then jumps to the
+    //     clicked file's index.
+    //
+    // Both feed into the same `load_current` pipeline so thumbnailing,
+    // sidebar, status bar, etc. all stay consistent.
+
+    /// Open a folder of images — shared by the in-app "Open Folder" dialog
+    /// (`Message::FolderSelected`) and by `Message::OpenPath` when the path
+    /// is a directory.
+    fn open_folder(&mut self, path: PathBuf) -> Task<Message> {
+        let images = crate::nav::scan_folder(&path, &self.codec);
+        self.sidebar.refresh(&path, &self.codec);
+        if images.is_empty() {
+            self.status_bar.filename = "No supported images in that folder".into();
+            self.status_bar.total = 0;
+            return Task::none();
+        }
+        self.navigator.set_images(images.clone());
+        self.thumbnail_bar.set_paths(images, 0);
+        self.load_current()
+    }
+
+    /// Open a single image file by path — the file-association case. Scans
+    /// the file's parent folder so next/prev navigation still works, then
+    /// jumps to the clicked file's index in that scan.
+    ///
+    /// If the file isn't in the scan results (e.g., it's on the codec
+    /// anti-list — `scan_folder` filters those out — or the parent folder
+    /// is unreadable), we still push it into the navigator so the user sees
+    /// *something* and gets the proper decode error via the error modal
+    /// rather than a silent "no supported images" message.
+    fn open_file(&mut self, file_path: PathBuf) -> Task<Message> {
+        let Some(parent) = file_path.parent() else {
+            self.status_bar.filename =
+                "Cannot determine parent folder".into();
+            self.status_bar.total = 0;
+            return Task::none();
+        };
+
+        let scanned = crate::nav::scan_folder(parent, &self.codec);
+        self.sidebar.refresh(parent, &self.codec);
+
+        let (images, target_index) = resolve_open_file_target(&file_path, scanned);
+
+        if images.is_empty() {
+            self.status_bar.filename = "No supported images in that folder".into();
+            self.status_bar.total = 0;
+            return Task::none();
+        }
+
+        self.navigator.set_images(images.clone());
+        // `set_images` resets current to 0; jump to the clicked file.
+        self.navigator.jump_to(target_index);
+        self.thumbnail_bar.set_paths(images, target_index);
+        self.load_current()
+    }
+
     fn load_current(&mut self) -> Task<Message> {
         let path = match self.navigator.current_path() {
             Some(p) => p.to_path_buf(),
@@ -1335,3 +1485,97 @@ mod img {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn p(s: &str) -> PathBuf {
+        PathBuf::from(s)
+    }
+
+    #[test]
+    fn resolve_target_when_file_in_scan_returns_scan_unchanged() {
+        // Common case: user double-clicks a supported, non-anti-listed image.
+        // The scan already contains it — we should hand back the same list
+        // (no clone, no sort) and the correct index.
+        let file = p("/photos/album/b.png");
+        let scanned = vec![p("/photos/album/a.png"), p("/photos/album/b.png")];
+
+        let (images, idx) = resolve_open_file_target(&file, scanned.clone());
+
+        assert_eq!(images, scanned);
+        assert_eq!(idx, 1);
+    }
+
+    #[test]
+    fn resolve_target_when_file_not_in_scan_inserts_at_sorted_position() {
+        // Anti-listed case: scan_folder filtered the clicked file out. We
+        // still want to display it, so it's inserted at its sorted position
+        // and `load_current` will try to decode it (surfacing a proper
+        // decode error if the codec really can't handle it).
+        let file = p("/photos/album/b.png");
+        let scanned = vec![p("/photos/album/a.png"), p("/photos/album/c.png")];
+
+        let (images, idx) = resolve_open_file_target(&file, scanned);
+
+        assert_eq!(
+            images,
+            vec![
+                p("/photos/album/a.png"),
+                p("/photos/album/b.png"),
+                p("/photos/album/c.png"),
+            ]
+        );
+        assert_eq!(idx, 1);
+    }
+
+    #[test]
+    fn resolve_target_with_empty_scan_yields_just_the_file() {
+        // Parent folder is empty or unreadable — fall back to showing the
+        // clicked file alone so the user gets a decode attempt rather than
+        // a silent "No supported images".
+        let file = p("/photos/album/lonely.png");
+        let scanned: Vec<PathBuf> = vec![];
+
+        let (images, idx) = resolve_open_file_target(&file, scanned);
+
+        assert_eq!(images, vec![p("/photos/album/lonely.png")]);
+        assert_eq!(idx, 0);
+    }
+
+    #[test]
+    fn resolve_target_finds_file_with_arbitrary_filename() {
+        // Make sure the lookup is by exact path equality, not by basename —
+        // a file with the same name in a different folder must NOT match.
+        let file = p("/photos/vacation/sunset.png");
+        let scanned = vec![p("/photos/other/sunset.png"), p("/photos/vacation/sunrise.png")];
+
+        let (images, idx) = resolve_open_file_target(&file, scanned.clone());
+
+        // File wasn't in the scan → inserted at sorted position.
+        assert_eq!(images.len(), 3);
+        assert_eq!(idx, images.iter().position(|p| p == &file).unwrap());
+    }
+
+    /// Sanity check that `init_from_env` returns a viewer with empty state
+    /// when no CLI argument is present. We can't easily test the "with arg"
+    /// case here because it depends on `FormatRegistry` and the filesystem,
+    /// but the no-arg path is the safe baseline.
+    #[test]
+    fn init_from_env_no_args_returns_empty_viewer() {
+        // The default-constructed Viewer (which `init_from_env` falls back to
+        // when there's no usable argv[1]) must start with an empty navigator.
+        let viewer = Viewer::default();
+        assert!(viewer.navigator.is_empty());
+    }
+
+    // Compile-time check that `Message::OpenPath` exists and accepts a PathBuf.
+    // Guards against accidental removal of the variant during refactoring.
+    #[test]
+    fn open_path_message_variant_exists() {
+        let msg = Message::OpenPath(PathBuf::from("/some/file.png"));
+        // Pattern-match to prove the variant is reachable.
+        assert!(matches!(msg, Message::OpenPath(_)));
+    }
+}
diff --git a/src/codec/image_crate.rs b/src/codec/image_crate.rs
index aa225bb..75cb24c 100644
--- a/src/codec/image_crate.rs
+++ b/src/codec/image_crate.rs
@@ -63,6 +63,26 @@ impl Codec for ImageCrateCodec {
         let height = img.height();
         let pixels = img.into_raw();
 
+        // Apply EXIF orientation if present. Phone cameras (iOS, Android)
+        // typically save the raw sensor data in landscape orientation and
+        // write an EXIF orientation tag (most commonly 6 for "rotate 90° CW"
+        // on portrait shots) so the viewer can present the image upright.
+        // Without this step, every portrait phone photo appears sideways.
+        //
+        // We parse the orientation from the original `bytes` (not the
+        // decoded pixels) because the `image` crate's decoder does not
+        // preserve EXIF metadata in its output. `kamadak-exif` reads the
+        // EXIF segment directly from the file container.
+        //
+        // If parsing fails or there's no orientation tag, `None` is
+        // returned and we fall through to orientation = 1 (identity).
+        let (pixels, width, height) = match super::parse_exif_orientation(bytes) {
+            Some(orientation) if orientation != 1 => {
+                super::apply_exif_orientation(&pixels, width, height, orientation)
+            }
+            _ => (pixels, width, height),
+        };
+
         Ok(DecodedImage {
             width,
             height,
diff --git a/src/codec/mod.rs b/src/codec/mod.rs
index 0a54bc2..c10af40 100644
--- a/src/codec/mod.rs
+++ b/src/codec/mod.rs
@@ -272,6 +272,123 @@ fn rotate_rgba_180(src: &[u8], w: u32, h: u32) -> (Vec<u8>, u32, u32) {
     (dst, w, h)
 }
 
+fn mirror_rgba_horizontal(src: &[u8], w: u32, h: u32) -> (Vec<u8>, u32, u32) {
+    // Mirror left↔right (flip X axis).
+    // Same dimensions as input.
+    let mut dst = vec![0u8; src.len()];
+    for y in 0..h {
+        for x in 0..w {
+            let src_idx = ((y * w + x) * 4) as usize;
+            let nx = w - 1 - x;
+            let dst_idx = ((y * w + nx) * 4) as usize;
+            dst[dst_idx..dst_idx + 4].copy_from_slice(&src[src_idx..src_idx + 4]);
+        }
+    }
+    (dst, w, h)
+}
+
+fn mirror_rgba_vertical(src: &[u8], w: u32, h: u32) -> (Vec<u8>, u32, u32) {
+    // Mirror top↔bottom (flip Y axis).
+    // Same dimensions as input.
+    let mut dst = vec![0u8; src.len()];
+    for y in 0..h {
+        for x in 0..w {
+            let src_idx = ((y * w + x) * 4) as usize;
+            let ny = h - 1 - y;
+            let dst_idx = ((ny * w + x) * 4) as usize;
+            dst[dst_idx..dst_idx + 4].copy_from_slice(&src[src_idx..src_idx + 4]);
+        }
+    }
+    (dst, w, h)
+}
+
+// ── EXIF orientation ───────────────────────────────────────────────────
+
+/// Apply an EXIF orientation tag to an RGBA pixel buffer.
+///
+/// EXIF orientation values are 1–8 per the EXIF specification:
+///
+/// ```text
+/// 1 = 0°                (no transform — "Normal")
+/// 2 = mirror horizontal (flip X)
+/// 3 = 180°
+/// 4 = mirror vertical   (flip Y)
+/// 5 = transpose         (mirror X + 90° CW)
+/// 6 = 90° CW            ← most common for phone portrait shots
+/// 7 = anti-transpose    (mirror X + 90° CCW)
+/// 8 = 90° CCW           ← common for upside-down phone portraits
+/// ```
+///
+/// Phone cameras (iOS, Android) typically save the raw sensor data in
+/// landscape orientation and write orientation=6 (or 8) so the viewer
+/// rotates it to portrait for display. Without applying this tag, every
+/// portrait photo appears sideways.
+///
+/// Returns `(new_pixels, new_width, new_height)`. For the rotation cases
+/// (3, 6, 7, 8) the new dimensions may swap width↔height. For the pure
+/// mirror cases (2, 4) and for 1 / unknown values, the dimensions are
+/// preserved.
+///
+/// Unknown values (0, or > 8) are treated as 1 (identity) — the EXIF spec
+/// reserves 0 to mean "unknown", and values > 8 are not valid.
+pub fn apply_exif_orientation(
+    pixels: &[u8],
+    width: u32,
+    height: u32,
+    orientation: u16,
+) -> (Vec<u8>, u32, u32) {
+    match orientation {
+        1 => (pixels.to_vec(), width, height),
+        2 => mirror_rgba_horizontal(pixels, width, height),
+        3 => rotate_rgba_180(pixels, width, height),
+        4 => mirror_rgba_vertical(pixels, width, height),
+        5 => {
+            // Transpose: reflection across the main diagonal
+            // (top-left to bottom-right). Equivalent to: mirror Y then
+            // rotate 90° CW. The main diagonal is preserved — pixels on
+            // the diagonal stay in place.
+            let (m, w, h) = mirror_rgba_vertical(pixels, width, height);
+            rotate_rgba_cw(&m, w, h)
+        }
+        6 => rotate_rgba_cw(pixels, width, height),
+        7 => {
+            // Transverse: reflection across the anti-diagonal
+            // (top-right to bottom-left). Equivalent to: mirror X then
+            // rotate 90° CW. The anti-diagonal is preserved.
+            let (m, w, h) = mirror_rgba_horizontal(pixels, width, height);
+            rotate_rgba_cw(&m, w, h)
+        }
+        8 => rotate_rgba_ccw(pixels, width, height),
+        // 0 (unknown) and any out-of-range value: identity.
+        _ => (pixels.to_vec(), width, height),
+    }
+}
+
+/// Read the EXIF orientation tag from in-memory image bytes.
+///
+/// Uses `kamadak-exif` to parse the EXIF segment (works for JPEG, TIFF,
+/// WebP, HEIF — anything that carries an EXIF container). Returns `None`
+/// when the file has no EXIF segment or no orientation tag — callers
+/// should treat that as orientation=1 (no transform).
+///
+/// Reuses the same library the EXIF properties panel uses, but reads from
+/// a `Cursor<&[u8]>` rather than re-opening the file from disk — we
+/// already have the bytes in memory at decode time, so going back to the
+/// filesystem would be wasteful.
+pub fn parse_exif_orientation(bytes: &[u8]) -> Option<u16> {
+    let cursor = std::io::Cursor::new(bytes);
+    let mut bufreader = std::io::BufReader::new(cursor);
+    let exif_reader = exif::Reader::new();
+    let exif = exif_reader.read_from_container(&mut bufreader).ok()?;
+
+    exif.get_field(exif::Tag::Orientation, exif::In::PRIMARY)
+        .and_then(|field| match field.value {
+            exif::Value::Short(ref shorts) => shorts.first().copied(),
+            _ => None,
+        })
+}
+
+
 #[cfg(test)]
 mod rotation_tests {
     use super::*;
@@ -337,3 +454,186 @@ mod rotation_tests {
         assert_eq!(out[0..4], px[(3 * 2 - 1) * 4..(3 * 2) * 4]);
     }
 }
+
+#[cfg(test)]
+mod exif_orientation_tests {
+    use super::*;
+
+    /// Build a 3×2 RGBA image with a recognizable pattern.
+    /// Each pixel is (x, y, 0, 255) so we can identify which source position
+    /// ended up at which destination position after the transform.
+    fn make_pixels(w: u32, h: u32) -> Vec<u8> {
+        let mut v = Vec::with_capacity((w * h * 4) as usize);
+        for y in 0..h {
+            for x in 0..w {
+                v.push(x as u8);
+                v.push(y as u8);
+                v.push(0);
+                v.push(255);
+            }
+        }
+        v
+    }
+
+    /// Look up the source position that ended up at destination (x, y).
+    /// Each pixel is (R, G, B, A) = (src_x, src_y, 0, 255), so we just read
+    /// the first two bytes of the pixel at (x, y).
+    fn src_pos_at(out: &[u8], dst_w: u32, x: u32, y: u32) -> (u8, u8) {
+        let idx = ((y * dst_w + x) * 4) as usize;
+        (out[idx], out[idx + 1])
+    }
+
+    #[test]
+    fn orientation_1_is_identity() {
+        let px = make_pixels(3, 2);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 2, 1);
+        assert_eq!((w, h), (3, 2));
+        assert_eq!(out, px);
+    }
+
+    #[test]
+    fn orientation_0_is_treated_as_identity() {
+        // EXIF reserves 0 for "unknown" — must not panic, must be identity.
+        let px = make_pixels(3, 2);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 2, 0);
+        assert_eq!((w, h), (3, 2));
+        assert_eq!(out, px);
+    }
+
+    #[test]
+    fn orientation_3_is_180_rotation() {
+        // 180°: top-left of source should land at bottom-right of dst.
+        let px = make_pixels(3, 2);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 2, 3);
+        assert_eq!((w, h), (3, 2));
+        // Dst bottom-right pixel (2, 1) should be src top-left (0, 0).
+        assert_eq!(src_pos_at(&out, w, 2, 1), (0, 0));
+        // Dst top-left pixel (0, 0) should be src bottom-right (2, 1).
+        assert_eq!(src_pos_at(&out, w, 0, 0), (2, 1));
+    }
+
+    #[test]
+    fn orientation_6_is_90_cw_and_swaps_dimensions() {
+        // Orientation 6 is the most common case for phone portrait shots.
+        // Source is landscape (3×2), displayed image should be portrait (2×3).
+        let px = make_pixels(3, 2);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 2, 6);
+        assert_eq!((w, h), (2, 3));
+        // For 90° CW: src top-left (0, 0) → dst top-right (w-1=1, 0).
+        assert_eq!(src_pos_at(&out, w, 1, 0), (0, 0));
+        // src top-right (2, 0) → dst bottom-right (1, 2).
+        assert_eq!(src_pos_at(&out, w, 1, 2), (2, 0));
+    }
+
+    #[test]
+    fn orientation_8_is_90_ccw_and_swaps_dimensions() {
+        let px = make_pixels(3, 2);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 2, 8);
+        assert_eq!((w, h), (2, 3));
+        // For 90° CCW: src top-left (0, 0) → dst bottom-left (0, h-1=2).
+        assert_eq!(src_pos_at(&out, w, 0, 2), (0, 0));
+    }
+
+    #[test]
+    fn orientation_5_preserves_main_diagonal() {
+        // Orientation 5 (transpose) reflects across the main diagonal —
+        // pixels on the main diagonal stay in place. For a square image,
+        // every pixel (x, x) should land at (x, x).
+        // Use a 3×3 image so the main diagonal is (0,0)→(1,1)→(2,2).
+        let px = make_pixels(3, 3);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 3, 5);
+        assert_eq!((w, h), (3, 3));
+        // Transpose: dst(x, y) = src(y, x). So dst(1, 2) should be src(2, 1).
+        assert_eq!(src_pos_at(&out, w, 1, 2), (2, 1));
+        // And dst(2, 1) should be src(1, 2).
+        assert_eq!(src_pos_at(&out, w, 2, 1), (1, 2));
+        // Main diagonal preserved: dst(1, 1) = src(1, 1).
+        assert_eq!(src_pos_at(&out, w, 1, 1), (1, 1));
+    }
+
+    #[test]
+    fn orientation_7_preserves_anti_diagonal() {
+        // Orientation 7 (transverse) reflects across the anti-diagonal —
+        // pixels on the anti-diagonal stay in place.
+        // For a 3×3 image, anti-diagonal is (2,0)→(1,1)→(0,2).
+        let px = make_pixels(3, 3);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 3, 7);
+        assert_eq!((w, h), (3, 3));
+        // Anti-diagonal preserved: dst(2, 0) = src(2, 0), dst(0, 2) = src(0, 2).
+        assert_eq!(src_pos_at(&out, w, 2, 0), (2, 0));
+        assert_eq!(src_pos_at(&out, w, 0, 2), (0, 2));
+        // Transverse: dst(x, y) = src(h-1-y, w-1-x). So dst(0, 0) = src(2, 2).
+        assert_eq!(src_pos_at(&out, w, 0, 0), (2, 2));
+    }
+
+    #[test]
+    fn orientation_2_mirrors_horizontally() {
+        let px = make_pixels(3, 2);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 2, 2);
+        assert_eq!((w, h), (3, 2));
+        // Mirror X: dst(x, y) = src(w-1-x, y). dst(0, 0) = src(2, 0).
+        assert_eq!(src_pos_at(&out, w, 0, 0), (2, 0));
+        assert_eq!(src_pos_at(&out, w, 2, 0), (0, 0));
+    }
+
+    #[test]
+    fn orientation_4_mirrors_vertically() {
+        let px = make_pixels(3, 2);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 2, 4);
+        assert_eq!((w, h), (3, 2));
+        // Mirror Y: dst(x, y) = src(x, h-1-y). dst(0, 0) = src(0, 1).
+        assert_eq!(src_pos_at(&out, w, 0, 0), (0, 1));
+        assert_eq!(src_pos_at(&out, w, 0, 1), (0, 0));
+    }
+
+    #[test]
+    fn all_orientations_round_trip_through_inverse() {
+        // For every orientation, applying the same orientation twice should
+        // NOT necessarily be identity (mirror X twice is identity, but
+        // rotate 90° CW twice is 180°). Instead, verify that applying
+        // orientation N to a 1×1 image is always identity (single pixel
+        // has no spatial orientation).
+        let one_px = vec![7, 8, 9, 255];
+        for orient in 0..=10u16 {
+            let (out, w, h) = apply_exif_orientation(&one_px, 1, 1, orient);
+            assert_eq!((w, h), (1, 1), "orientation {} on 1×1 swapped dims", orient);
+            assert_eq!(out, one_px, "orientation {} on 1×1 changed pixels", orient);
+        }
+    }
+
+    #[test]
+    fn orientation_above_8_is_identity() {
+        // EXIF spec only defines 1–8; values > 8 are out of range.
+        let px = make_pixels(3, 2);
+        let (out, w, h) = apply_exif_orientation(&px, 3, 2, 9);
+        assert_eq!((w, h), (3, 2));
+        assert_eq!(out, px);
+    }
+
+    #[test]
+    fn parse_exif_orientation_returns_none_for_non_exif_bytes() {
+        // A raw PNG without any EXIF chunk — should return None, not panic.
+        // Use a minimal PNG header + IHDR (8 + 25 bytes).
+        let png_bytes: &[u8] = &[
+            0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A, // PNG signature
+            0x00, 0x00, 0x00, 0x0D, // IHDR length
+            0x49, 0x48, 0x44, 0x52, // "IHDR"
+            0x00, 0x00, 0x00, 0x01, // width = 1
+            0x00, 0x00, 0x00, 0x01, // height = 1
+            0x08, 0x06, 0x00, 0x00, 0x00, // bit depth 8, color type 6 (RGBA)
+            0x1f, 0x15, 0xc4, 0x89, // CRC
+        ];
+        assert!(parse_exif_orientation(png_bytes).is_none());
+    }
+
+    #[test]
+    fn parse_exif_orientation_returns_none_for_empty_bytes() {
+        assert!(parse_exif_orientation(&[]).is_none());
+    }
+
+    #[test]
+    fn parse_exif_orientation_returns_none_for_random_bytes() {
+        let random: &[u8] = &[0xff, 0xfe, 0xfd, 0xfc, 0x00, 0x01, 0x02, 0x03];
+        assert!(parse_exif_orientation(random).is_none());
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 2aa463d..ef66f23 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -23,5 +23,5 @@ fn main() -> iced::Result {
         .theme(|_| iced::Theme::Dark)
         .subscription(app::Viewer::subscription)
         .window_size(iced::Size::new(1200.0, 800.0))
-        .run()
+        .run_with(app::Viewer::init_from_env)
 }