Commit af7bf1ce for libheif

commit af7bf1ce0bf3af1edd24fa30828ea48542da3394
Author: Dirk Farin <dirk.farin@gmail.com>
Date:   Sun Sep 20 21:52:23 2026 +0200

    Explain why a filter array may not have a separate alpha plane yet

diff --git a/libheif/image/pixelimage.cc b/libheif/image/pixelimage.cc
index 8d294459..443082c0 100644
--- a/libheif/image/pixelimage.cc
+++ b/libheif/image/pixelimage.cc
@@ -979,6 +979,10 @@ Error HeifPixelImage::check_plane_layout() const
         return layout_error("Chroma format is not valid for a filter-array image");
       }
       colour_planes = {heif_channel_filter_array};
+      // A filter array with an alpha plane is representable in 'unci', but nothing produces or
+      // consumes it yet: the uncompressed decoder only recognizes the filter-array component on
+      // its own, Op_bayer_bilinear_to_RGB24_32 carries no alpha, and Op_drop_alpha_plane does not
+      // copy a filter-array plane. Once those handle it, this may be allowed.
       separate_alpha_allowed = false;
       break;