diff --git a/crates/gpu/src/lib.rs b/crates/gpu/src/lib.rs
index 5ef3011b7..312d42635 100644
--- a/crates/gpu/src/lib.rs
+++ b/crates/gpu/src/lib.rs
@@ -270,12 +270,39 @@ pub fn set_pool_limit(bytes: u64) {
 pub fn trim_pool(keep: u64) {
     #[cfg(not(target_arch = "wasm32"))]
     if let Some(g) = existing_device() {
+        if keep == 0 {
+            render::release_shared_source();
+        }
         g.trim(keep);
     }
     #[cfg(target_arch = "wasm32")]
     let _ = keep;
 }
 
+/// Device bytes of the one big source (a zoom window's original) shared by every view of the photo.
+pub fn shared_source_bytes() -> usize {
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        render::shared_source_bytes()
+    }
+    #[cfg(target_arch = "wasm32")]
+    {
+        0
+    }
+}
+
+/// Big sources uploaded to the shared copy so far (a diagnostic: one big photo is uploaded once, not once per window).
+pub fn source_uploads() -> u64 {
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        render::source_uploads()
+    }
+    #[cfg(target_arch = "wasm32")]
+    {
+        0
+    }
+}
+
 /// Device bytes held by a view's GPU stages (kept with its [`StageCache`]); 0 when it has none.
 pub fn stage_bytes(stages: &StageCache) -> usize {
     #[cfg(not(target_arch = "wasm32"))]
diff --git a/crates/gpu/src/render.rs b/crates/gpu/src/render.rs
index 46b47f1a4..8fda204f5 100644
--- a/crates/gpu/src/render.rs
+++ b/crates/gpu/src/render.rs
@@ -417,6 +417,43 @@ const COVER_ROWS: usize = 16;
 /// The largest source (bytes on the device) a view's stages keep uploaded between renders.
 const RETAIN_SOURCE_BYTES: usize = 96 << 20;
 
+/// Big sources uploaded to the shared copy so far (diagnostics and tests).
+static SOURCE_UPLOADS: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(0);
+
+/// The one big source (the original a zoom window is cut from) kept on the device for all the
+/// views of the photo: the Before and After windows and every pan share it, instead of each
+/// view's stages holding (or re-uploading) 288 MB of a 24 MP photo. It does not count as a view's
+/// stages; it is released with the buffer pool when the app idles.
+static SHARED_SOURCE: std::sync::Mutex<Option<(std::sync::Weak<Rgb32f>, Arc<Buf>)>> = std::sync::Mutex::new(None);
+
+fn shared_source(src: &Arc<Rgb32f>, upload: impl FnOnce() -> Buf) -> Arc<Buf> {
+    let mut g = SHARED_SOURCE.lock().unwrap_or_else(|e| e.into_inner());
+    if let Some((weak, buf)) = &*g
+        && weak.upgrade().is_some_and(|s| Arc::ptr_eq(&s, src))
+    {
+        return buf.clone();
+    }
+    *g = None; // the previous photo's copy goes before the next is made
+    SOURCE_UPLOADS.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+    let buf = Arc::new(upload());
+    *g = Some((Arc::downgrade(src), buf.clone()));
+    buf
+}
+
+/// Free the shared big source (see [`shared_source`]).
+pub(crate) fn release_shared_source() {
+    *SHARED_SOURCE.lock().unwrap_or_else(|e| e.into_inner()) = None;
+}
+
+/// Device bytes of the shared big source, 0 when there is none.
+pub(crate) fn shared_source_bytes() -> usize {
+    SHARED_SOURCE.lock().unwrap_or_else(|e| e.into_inner()).as_ref().map_or(0, |(_, b)| b.len * 4)
+}
+
+pub(crate) fn source_uploads() -> u64 {
+    SOURCE_UPLOADS.load(std::sync::atomic::Ordering::Relaxed)
+}
+
 /// The reference framing decision ([`Warp::covers`](lightcraft_pipeline::optics::Warp::covers)) for every
 /// output pixel, one bit per pixel, rows padded to 32-bit words. Blocks of 32 × [`COVER_ROWS`] pixels whose
 /// interval bounds place them clearly inside or outside the image are filled at once
@@ -555,11 +592,12 @@ pub fn render(
         Some(e) => e.sampled.clone(),
         None if gpu.fits(src.data.len() * 3) => {
             let upload = || gpu.upload(rgb_words(src));
-            // (a source too big to keep is uploaded for this render only: a window is cut from the
-            // photo's own pixels, and what its stages keep is its own sampled pixels)
+            // (a source too big for a view's stages to keep is the one shared copy: a window is
+            // cut from the photo's own pixels, and what its stages keep is its own sampled pixels)
             let src_buf = match stages {
                 Some(c) if src.data.len() * 12 <= RETAIN_SOURCE_BYTES => c.source(src, upload),
-                _ => Arc::new(upload()),
+                Some(_) => shared_source(src, upload),
+                None => Arc::new(upload()),
             };
             sample(&mut cx, src, src_buf, &plan)
         }
diff --git a/crates/gpu/tests/equivalence.rs b/crates/gpu/tests/equivalence.rs
index bfb72e02f..cb6e7c024 100644
--- a/crates/gpu/tests/equivalence.rs
+++ b/crates/gpu/tests/equivalence.rs
@@ -706,8 +706,12 @@ fn windows_match() {
 /// upload with the view's stages counted it against the stage budget on every tick of a drag and
 /// got the stages cleared, and the upload and resampling redone: the window's stages are its own
 /// pixels, not the photo's.
+// (these two share the process's one big source: a test lock keeps them from replacing each other's)
+static BIG_SOURCE_TESTS: std::sync::Mutex<()> = std::sync::Mutex::new(());
+
 #[test]
 fn a_window_does_not_keep_the_whole_source_on_the_device() {
+    let _lock = BIG_SOURCE_TESTS.lock().unwrap_or_else(|e| e.into_inner());
     if !gpu() {
         return;
     }
@@ -727,3 +731,31 @@ fn a_window_does_not_keep_the_whole_source_on_the_device() {
     assert_eq!((again.image.width, again.image.height), (640, 480));
     assert!(lightcraft_gpu::stage_bytes(&stages) < 100 << 20);
 }
+
+/// …but the 288 MB is uploaded once for every window of every view of the photo, not once per
+/// window: panning at 1:1, and the Before and After windows, share one device copy of it, which
+/// goes when the app idles (`trim_pool(0)`).
+#[test]
+fn windows_of_a_big_photo_share_one_uploaded_source() {
+    let _lock = BIG_SOURCE_TESTS.lock().unwrap_or_else(|e| e.into_inner());
+    if !gpu() {
+        return;
+    }
+    use lightcraft_pipeline::PixelWindow;
+    let src = scene(1, 6000, 4000);
+    let info = SourceInfo { raw: true, ..Default::default() };
+    let s = DevelopSettings::default();
+    let (after, before) = (StageCache::default(), StageCache::default());
+    let at = |x: usize| RenderRequest { window: Some(PixelWindow { x, y: 1500, w: 640, h: 480 }), ..RenderRequest::fit(6000, 4000) };
+    let start = lightcraft_gpu::source_uploads();
+    for (cache, x) in [(&after, 1000), (&after, 1256), (&after, 1512), (&before, 1512), (&before, 1768)] {
+        lightcraft_gpu::render(&src, &info, &s, &at(x), Some(cache)).expect("gpu render");
+    }
+    assert_eq!(lightcraft_gpu::source_uploads() - start, 1, "one upload for five windows of two views");
+    assert!(lightcraft_gpu::shared_source_bytes() >= 280_000_000, "{}", lightcraft_gpu::shared_source_bytes());
+    assert!(lightcraft_gpu::stage_bytes(&after) < 100 << 20, "and it is not in the views' stages");
+    lightcraft_gpu::trim_pool(0);
+    assert_eq!(lightcraft_gpu::shared_source_bytes(), 0, "an idle app gives it back");
+    lightcraft_gpu::render(&src, &info, &s, &at(1000), Some(&after)).expect("gpu render");
+    assert_eq!(lightcraft_gpu::source_uploads() - start, 2);
+}