Update app.py

app.py

@@ -155,12 +155,29 @@ def _compress_image(raw_bytes: bytes, quality: int, fmt: str) -> bytes:
             method=4,  # 0-6, higher = slower + better compression
         )
     elif fmt == "avif":
-        if not HEIF_AVAILABLE:
-            raise ValueError("AVIF output requires pillow-heif (not installed)")
+        # NOTE: Pillow 12.x has NATIVE AVIF support via AvifImagePlugin.
+        # It does NOT use pillow-heif's 'chroma' parameter.
+        # The correct parameter is 'subsampling' (default '4:2:0').
+        # We MUST use '4:4:4' to prevent alpha edge bleeding on transparent images.
+        
+        if img.mode in ("RGBA", "LA", "PA") or "transparency" in img.info:
+            img = img.convert("RGBA")
+            import numpy as np
+            data = np.array(img)
+            alpha = data[:, :, 3]
+
+            # Remove hidden RGB in fully transparent pixels
+            data[:, :, :3][alpha == 0] = [0, 0, 0]
+
+            img = Image.fromarray(data, "RGBA")
+        elif img.mode not in ("RGB", "L"):
+            img = img.convert("RGB")
+
         img.save(
             out_buffer,
             format="AVIF",
             quality=quality,
+            subsampling="4:4:4",
         )
     elif fmt == "heif":
         if not HEIF_AVAILABLE: