Added AssumeAlpha to the Jp2ReadDefines that allows a user to assume …

…a 2nd or 4th channel is an alpha channel (#1449).

src/Magick.NET/Formats/Jp2/Jp2ReadDefines.cs

@@ -16,6 +16,11 @@ public sealed class Jp2ReadDefines : IReadDefines
     public MagickFormat Format
         => MagickFormat.Jp2;
 
+    /// <summary>
+    /// Gets or sets the flag that assumes an alpha channel when the image has 2 or 4 channels (jp2:assume-alpha).
+    /// </summary>
+    public bool? AssumeAlpha { get; set; }
+
     /// <summary>
     /// Gets or sets the maximum number of quality layers to decode (jp2:quality-layers).
     /// </summary>
@@ -33,6 +38,9 @@ public IEnumerable<IDefine> Defines
     {
         get
         {
+            if (AssumeAlpha.HasValue)
+                yield return new MagickDefine(Format, "assume-alpha", AssumeAlpha.Value);
+
             if (QualityLayers.HasValue)
                 yield return new MagickDefine(Format, "quality-layers", QualityLayers.Value);
 

tests/Magick.NET.Tests/Formats/Jp2/Jp2ReadDefinesTests/TheAssumeAlphaProperty.cs

@@ -0,0 +1,34 @@
+// Copyright Dirk Lemstra https://github.com/dlemstra/Magick.NET.
+// Licensed under the Apache License, Version 2.0.
+
+using ImageMagick;
+using ImageMagick.Formats;
+using Xunit;
+
+namespace Magick.NET.Tests;
+
+public partial class Jp2ReadDefinesTests
+{
+    public class TheAssumeAlphaProperty
+    {
+        [Fact]
+        public void ShouldTreatMetaChannelAsAlpha()
+        {
+            var settings = new MagickReadSettings
+            {
+                Defines = new Jp2ReadDefines
+                {
+                    AssumeAlpha = true,
+                },
+            };
+
+            using var image = new MagickImage();
+            image.Read(Files.Coders.TestJP2, settings);
+
+            Assert.Equal("true", image.Settings.GetDefine(MagickFormat.Jp2, "assume-alpha"));
+            Assert.True(image.HasAlpha);
+            Assert.Equal(4, image.ChannelCount);
+            Assert.Contains(PixelChannel.Alpha, image.Channels);
+        }
+    }
+}

tests/Magick.NET.Tests/Formats/Jp2/Jp2ReadDefinesTests/TheConstructor.cs

@@ -17,6 +17,7 @@ public void ShouldNotSetAnyDefines()
             using var image = new MagickImage();
             image.Settings.SetDefines(new Jp2ReadDefines());
 
+            Assert.Null(image.Settings.GetDefine(MagickFormat.Jp2, "assume-alpha"));
             Assert.Null(image.Settings.GetDefine(MagickFormat.Jp2, "quality-layers"));
             Assert.Null(image.Settings.GetDefine(MagickFormat.Jp2, "reduce-factor"));
         }