Different result between Magick.NET and ImageMagick

[CrispyDrone]

I'm trying to remove table borders from a .png file. This works through ImageMagick with the following command:

magick <in.png> -type Grayscale -negate -define morphology:compose=darken -morphology Thinning 'Rectangle:1x80+0+0<' -negate <out.png>

When I try to translate this to C# using Magick.NET, small segments of the table borders remain. I'm not sure what I'm missing. Specifically, the -define morphology:compose=darken doesn't seem to work in code despite trying various things, and it seems that this option is the one that ensures the results are different and not ideal when using Magick.NET.

            var magick = new MagickFactory().Image.Create(
                $"{fileName}.png",
                new MagickReadSettings
                {
                    Density = new Density(600, 600, DensityUnit.PixelsPerInch),
                    // ColorType = ColorType.Grayscale,
                }
            );
            // grayscale image
            magick.Grayscale();
            // magick.Alpha(AlphaOption.Off);
            // magick.WhiteThreshold(new Percentage(75));
            magick.Negate();
            // effect of morphology is to darken
            // magick.Settings.SetDefine("morphology", "compose=darken");
            // magick.Settings.SetDefine("morphology:compose", "darken");
            magick.Settings.SetDefine(MagickFormat.Png, "morphology:compose", "darken");
            // magick.Compose = CompositeOperator.Darken;
            // magick.SetArtifact("morphology:compose", "darken");
            // identify white (?) areas where our kernel fits
            magick.Morphology(
                new MorphologySettings
                {
                    // Iterations = 3,
                    // Channels = Channels.All & ~Channels.Black,
                    Method = MorphologyMethod.Thinning,
                    Kernel = Kernel.Rectangle,
                    KernelArguments = "1x80+0+0<",
                    // KernelArguments = "Rectangle:1x80+0+0<",
                }
            );
            // undo negation
            magick.Negate();
            magick.Write($"{fileName}.magick.png");

I've experimented with: setting Compose; using SetArtifact; using SetDefine; playing around with the KernelArguments and UserKernel; using ColorType.Grayscale versus Grayscale as an operation; and various combinations of these.

Thanks!

[dlemstra]

You should use magick.SetArtifact("morphology:compose", "darken"); But without your input and output image(s) I cannot help you further.

[CrispyDrone]

Thank you. It seems that I've figured it out.

1. Without magick.SetArtifact("morphology:compose", "darken");, I end up with the remnants of the borders.
2. When I use, magick.SetArtifact("morphology:compose", "darken");, I end up with a completely white image.
3. When I use both magick.Alpha(AlphaOption.Off); and magick.SetArtifact, it works as expected.

I had to redact the images a little bit, but here's the input and the output for 1:

For 2, I get a completely white image.
For 3, the borders are essentially entirely removed:

Does the command line program automatically ignore the alpha channel somehow?

Also, the performance difference with ImageMagick is quite substantial. It can take up to 10-15 seconds with Magick.NET whereas it only takes around 3 seconds on average with ImageMagick. Do you perhaps have any pointers about why there could be such a substantial performance difference? I have not benchmarked it. Sometimes, it will actually run at the same speed as ImageMagick, but it's very rare. Is there anything obvious I should pay attention to if I want to process many images? I've seen you recommend ImageCollection for example in various issues. I can also open a different discussion (issue) for this if you prefer that.

Thanks again!

[dlemstra]

There are some differences between the command line and using the api directly. It might be possible that the command line use only the rgb channels.

Are you using the OpenMP build on the command line? That could explain the difference.

[CrispyDrone]

Hey, sorry for the delayed reply.

Yes, it seems so:

Version: ImageMagick 7.1.2-30 Q16-HDRI x64 344e905:20260823 https://imagemagick.org
Copyright: (C) 1999 ImageMagick Studio LLC
License: https://imagemagick.org/license/
Features: Channel-masks(64-bit) Cipher DPC HDRI OpenCL OpenMP(2.0)
Delegates (built-in): bzlib cairo freetype gslib heic jng jp2 jpeg jxl lcms lqr lzma openexr pangocairo png ps raqm raw rsvg tiff webp xml zip zlib
Compiler: Visual Studio 2026 (195136256)