PlaneCount calculation inside ObuColorConfig

src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuColorConfig.cs

@@ -5,18 +5,28 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 
 internal class ObuColorConfig
 {
+    private bool isMonochrome;
+
     public bool IsColorDescriptionPresent { get; set; }
 
     /// <summary>
-    /// Gets or sets the number of color channels in this image.
+    /// Gets the number of color channels in this image. Can have the value 1 or 3.
     /// </summary>
-    public int PlaneCount { get; set; }
+    public int PlaneCount { get; private set; }
 
     /// <summary>
     /// Gets or sets a value indicating whether the image has a single greyscale plane, will have
     /// <see cref="Av1Constants.MaxPlanes"/> color planes otherwise.
     /// </summary>
-    public bool IsMonochrome { get; set; }
+    public bool IsMonochrome
+    {
+        get => this.isMonochrome;
+        set
+        {
+            this.PlaneCount = value ? 1 : Av1Constants.MaxPlanes;
+            this.isMonochrome = value;
+        }
+    }
 
     public ObuColorPrimaries ColorPrimaries { get; set; }
 

src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuReader.cs

@@ -465,7 +465,6 @@ internal class ObuReader
             colorConfig.IsMonochrome = reader.ReadBoolean();
         }
 
-        colorConfig.PlaneCount = colorConfig.IsMonochrome ? 1 : Av1Constants.MaxPlanes;
         colorConfig.IsColorDescriptionPresent = reader.ReadBoolean();
         colorConfig.ColorPrimaries = ObuColorPrimaries.Unspecified;
         colorConfig.TransferCharacteristics = ObuTransferCharacteristics.Unspecified;

src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuWriter.cs

@@ -112,7 +112,6 @@ internal class ObuWriter
             writer.WriteBoolean(colorConfig.IsMonochrome);
         }
 
-        colorConfig.PlaneCount = colorConfig.IsMonochrome ? 1 : Av1Constants.MaxPlanes;
         writer.WriteBoolean(false); // colorConfig.IsColorDescriptionPresent
         if (colorConfig.IsMonochrome)
         {