How to read a byte array with MagickFormat.Gray?

[kmgallahan]

I'm trying to create a MagickImage from a byte array that is of the format MagickFormat.Gray.

I always get this error:

ImageMagick.MagickOptionErrorException: must specify image size `' @ error/gray.c/ReadGRAYImage/147

It's unclear to me how, what, or where the size is supposed to come from.

I've tried many variations of this pseudo-code without luck:

        using var image = new MagickImage(@"C:\Dev\Tests\SampleImages\01HBXXRH4514PNVVVYKYZ0M6KN.tif");
        using var singleChannelImage = image.Clone();
        singleChannelImage.ColorType = ColorType.Grayscale;

        var byteArray = singleChannelImage.ToByteArray(MagickFormat.Gray);

        var newImage = new MagickImage(byteArray, MagickFormat.Gray);

        // or variations of singleChannelImage.Read(byteArray, ....

If I exclude MagickFormat.Gray on the create/read operation, then I get this:

ImageMagick.MagickMissingDelegateErrorException: no decode delegate for this image format `' @ error/blob.c/BlobToImage/460

Best I've found so far is:

        singleChannelImage.ReadPixels(
            byteArray, 
            new PixelReadSettings(imageSize.Width, imageSize.Height, StorageType.Quantum, "R"));

            // Unsure how "correct/valid" that mapping string is

---

Additional context: just trying to read an image, make it grayscale (single channel image), pass it elsewhere for processing as a byte array, get back the results, and read it in.

I started with a Span<byte>s but that wasn't working either, so tried out the array.

[dlemstra]

You should use the ReadPixels method for this. Did that not work for you?

[kmgallahan]

Yes, I did eventually came to use ReadPixels with the parameters shown above. However, I struggled a bit due to:

1. The Magick.NET MagickImage ctors take any MagicFormat without validation/complaining
2. ImageMagick throws the error shown indicating that size needs to (and can) be specified
3. Here is where I spent time trying to figure out how to pass in the size, only to eventually realize it probably just isn't an option

I do realize these things:

• Checking for invalid MagicFormat input would add a performance hit for 99.999% of usage (maybe trivial though)
• Passing the current image size to ImageMagick would require special case selection
• Adding ctors that take size params bloats the API
• Documenting valid format selections in the xml could help, but can easily be overlooked

So, I guess... 🤷‍♂️

[dlemstra]

1. The Magick.NET MagickImage ctors take any MagicFormat without validation/complaining

You were getting exceptions when you did not provide the correct byte array for the Gray format? This is valid in the constructor but not for your specific situation. But maybe I could add some extra documentation to the byte[] overloads that if you want to read pixels you should use the ReadPixels method instead.

2. ImageMagick throws the error shown indicating that size needs to (and can) be specified

This exception was thrown because the Gray format requires you to specify the dimensions of the image.

3. Here is where I spent time trying to figure out how to pass in the size, only to eventually realize it probably just isn't an option

This is a fair point but that is probably because the message from ImageMagick is thrown as an exception. But I don't want to change them because they are thrown at different spots and that would probably require some regex magic.

[kmgallahan]

You were getting exceptions when you did not provide the correct byte array for the Gray format?

I produced the byte array in Gray format from Magick.NET, used elsewhere, and then needed to ingest back into Magick.NET.

• ReadonlySpan<byte> ctor failed with size error, couldn't find size param
• MagickImage.Read ReadonlySpan<byte> same error, couldn't find size param and current size wasn't used
• Same results trying to use byte array versions
• Posted this question
• Realized ReadPixels could be used and edited the question

I imagine trying to use any of the 'raw sample' formats in the byte array or span ctors or Read methods would produce the same result (getting a size-related error message):

• BAYER
• CMYK
• CMYKA
• GRAY
• GRAYA
• RAW
• RGB
• RGB565
• RGBA
• YCbCr
• YCbCrA

The most helpful would be to check the format first and throw an error indicating that these formats require the use of ReadPixels, rather than passing anything to ImageMagick.

Not sure if any other input validation is done in Magick.NET though, so this might be something you're trying to avoid.