Show a live preview of the candidate being rendered

sd-cli writes a latent projection of the current candidate every step
(--preview proj, no extra VRAM). While it runs, a magick display window
shows that preview scaled up to 640 px and redraws on each change; it
closes when sd-cli exits. display -update drops -resize on reload, so the
poll loop writes the enlarged copy itself. Under Hyprland a runtime
window rule (hyprctl eval) floats the window so the tiling layout leaves
its size alone; the config is not touched.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2 files changed, 38 insertions, 4 deletions

diff --git a/README.md b/README.md
index 3e57d6d..a29613f 100644
--- a/README.md
+++ b/README.md
@@ -11,7 +11,8 @@ image stage. Pages land in the book-reader library as `NNN.png`.
 - llama-server in router mode on `localhost:8181` serving `Gemma4-12B-qat-mtp`
 - `sd-cli` with Vulkan, models under `/data/LLM-models/SD`
 - ImageMagick 7 (`magick`), `$EDITOR`, `notify-send` (libnotify), and kitty for
-  inline previews (elsewhere candidates open with `xdg-open`)
+  inline previews (elsewhere candidates open with `xdg-open`); a live preview of
+  the candidate being rendered opens in `magick display`, floated under Hyprland
 - `~/.config/book-reader.conf` with a `LIBRARY="..."` line
 
 ## Usage
diff --git a/fanfictioner b/fanfictioner
index 89a3fec..063f349 100755
--- a/fanfictioner
+++ b/fanfictioner
@@ -376,8 +376,31 @@ def progress(start, log):
     print(f"\r{int(time.time() - start):4d}s {last.strip()}"[:width].ljust(width), end="", flush=True)
 
 
-def run_sd(args, log):
-    """Run sd-cli with output in log; return an error string or None."""
+def scale_preview(src, dst):
+    """sd-cli's preview is latent-sized (1/8 of the image): enlarge it, swapping dst in atomically."""
+    tmp = dst.with_name("tmp_" + dst.name)
+    if subprocess.run(["magick", str(src), "-resize", "640x640", str(tmp)], capture_output=True).returncode == 0:
+        os.replace(tmp, dst)
+
+
+def open_preview(f):
+    """Window showing f, redrawn by magick display whenever f changes (display -update drops
+    -resize on reload, hence scale_preview). Floated under Hyprland through a runtime rule,
+    so the tiling layout neither resizes it nor reflows."""
+    try:
+        if os.environ.get("HYPRLAND_INSTANCE_SIGNATURE"):
+            subprocess.run(["hyprctl", "eval", 'hl.window_rule({name = "fanfictioner-preview", '
+                            'match = { title = "^fanfictioner preview$" }, float = true})'], capture_output=True)
+        return subprocess.Popen(["magick", "display", "-update", "1", "-title", "fanfictioner preview", str(f)],
+                                stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+    except OSError:
+        return None
+
+
+def run_sd(args, log, preview=None):
+    """Run sd-cli with output in log; return an error string or None.
+    With preview, a live view of that file is open while sd-cli runs."""
+    viewer, seen = None, None
     with open(log, "w") as f:
         f.write(shlex.join(args) + "\n")
         f.flush()
@@ -390,6 +413,11 @@ def run_sd(args, log):
             while proc.poll() is None:
                 time.sleep(2)
                 progress(start, log)
+                if preview and preview.exists() and preview.stat().st_mtime != seen:
+                    seen = preview.stat().st_mtime
+                    view = preview.with_name("view_" + preview.name)
+                    scale_preview(preview, view)
+                    viewer = viewer or (view.exists() and open_preview(view)) or None
                 if proc.poll() is None and \
                         mem_available_kb(Path("/proc/meminfo").read_text()) < MIN_RAM_KB:
                     proc.kill()
@@ -401,6 +429,8 @@ def run_sd(args, log):
             raise
         finally:
             print()
+            if viewer:
+                viewer.terminate()
     if proc.returncode or "[ERROR" in Path(log).read_text(errors="replace"):
         return f"sd-cli failed (exit {proc.returncode})"
     return None
@@ -441,7 +471,10 @@ def generate(d, profile, prompt, size, refs=()):
     seed = random.randint(0, 2**31 - 4)
     log = d / f"{seed}.log"
     print(f"sd-cli {profile}, seed {seed} ...", flush=True)
-    err = run_sd(sd_args(profile, prompt, d / f"{seed}_%d.png", size, seed, refs), log)
+    preview = d / "preview.png"  # rewritten every step, one candidate after the other
+    preview.unlink(missing_ok=True)
+    err = run_sd(sd_args(profile, prompt, d / f"{seed}_%d.png", size, seed, refs)
+                 + ["--preview", "proj", "--preview-path", str(preview)], log, preview)
     files = [d / f"{seed}_{i}.png" for i in (1, 2, 3)]
     if not err and not all(f.exists() for f in files):
         err = "sd-cli wrote no output"