Comparing PNG images using ErrorMetric.Absolute option

[bokhanhanna]

Hello,
I'm trying to compare two png-screenshots using Magick.NET-Q16-AnyCPU library (v.12.1.0).
My C# code looks like this:

var expectedImagePath = @"C:\Users\Comp\Desktop\expected-image.png";
var actualImagePath = @"C:\Users\Comp\Desktop\actual-image.png";
 
using (var expectedImage = new MagickImage(expectedImagePath))
using (var actualImage = new MagickImage(actualImagePath))
{
    var comparisonResult = expectedImage.Compare(actualImage, ErrorMetric.Absolute);
    Assert.That(comparisonResult == 0);
}

Expected image:

Actual image:

The images are visually different, but the result shows they are equal (comparisonResult is "0").

I also tried using another ErrorMetric option:
var comparisonResult = expectedImage.Compare(actualImage, new ErrorMetric());
This works as expected and shows that the images are actually different (comparisonResult here is "1").

I'm just wondering why ErrorMetric.Absolute doesn’t work in this situation?

[dlemstra]

You will get the correct result when you do actualImage.Compare(expectedImage instead. This is happening because the actual image only has two channels and the expected image has 4 channels. This might be a bug that I will need to investigate in more detail. My advise would be to set the ColorType of both images to TrueColorAlpha before comparing them. Then you are comparing the correct channels. Or you could use ErrorMetric.RootMeanSquared instead.

[bokhanhanna]

Thanks, this helped!