Legacy ImageMagick Discussions Archive

Hello everyone! I'm sort of new to ImageMagick as whole and I would like to ask for your help.

I need to apply certain filters to the images (photos) as if it was in instagram or in a photoshop alike app. Filters come with different settings such as sepia, hue, gradients and compose modes. Where I get a problem is a multiply type of composition.

My filter is a half-transparent gradient such as this one (square):

Taking this example image (also square and same size):

Applying this command

Code: Select all

magick convert Image.png Overlay.png -compose Multiply -composite Result1.png

will result in a half-transparent image as the alpha channel is also multiplyed throughout the image:

Of course, this is not what I am looking for so the solution is to specify the -channel parameter allowing to 'skip' the alpha

Code: Select all

magick convert Image.png Overlay.png -channel rgb -compose Multiply -composite Result2.png

Result as expected and correct:

But the MagickImage class in .NET does not have a Composite method overload that accepts Channels enum as a parameter.
So this line

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magickImage.Composite(overlay, CompositeOperator.Multiply);

by default multiplies alpha as well resulting in the first example.

So I would really appreciate if you could help me with this one.

P. S.

Multiplying as is and then removing transparency may look like a decent workaround at first, but there is a catch.
If an original image has its own transparency (even a small part of it does) like this

Then after composition you can't tell what part was transparent in the beginning and what got its alpha by multiplying a filter.
So if removed transparency at that point it will also lose the one it was supposed to have:

Thanks for finding this. Could you open an issue at "https://github.com/dlemstra/Magick.NET/issues" so I don't forget adding support for this?

Just created the issue myself and made some changes to Magick.NET. Your feature request will be available in the next release.

Thank you.
In any case, if someone is wondering, I came up with a workaround on this one by saving the alpha of the image and then applying it later:

Code: Select all

MagickImage alpha = (MagickImage)magickImage.Separate(Channels.Alpha).First();
magickImage.Composite(overlay, CompositeOperator.Multiply);
magickImage.Composite(alpha, CompositeOperator.CopyAlpha);

Legacy ImageMagick Discussions Archive

Compose with -channel (command line to c#)

Compose with -channel (command line to c#)

Re: Compose with -channel (command line to c#)

Re: Compose with -channel (command line to c#)

Re: Compose with -channel (command line to c#)