DivideSrc produces almost-white image in Magick.NET 14.10.4+ (image resizing)

[Athari]

I'm using the following method for resizing images, which uses linear RGB and resizes alpha separately.

It works in Magick.NET 14.10.3 and below, but in Magick.NET 14.10.4 and above, DivideSrc operation produces an almost-white image. Am I doing anything wrong?

async Task ResizeImage(IMagickImage<Q> img, MagickGeometry size, FilterType colorFilter)
{
    if (size.Width == img.Width && size.Height == img.Height)
        return;

    if (!img.IsOpaque) {
        using var alpha = img.Separate(Channels.Alpha)[0];
        img.ColorSpace = ColorSpace.RGB;
        img.Alpha(AlphaOption.Off);
        img.Composite(alpha, CompositeOperator.Multiply);
        img.Resize(size, colorFilter);
        alpha.Resize(size, colorFilter);
        img.Composite(alpha, CompositeOperator.DivideSrc);
        img.ColorSpace = ColorSpace.sRGB;
        img.Alpha(AlphaOption.On);
        img.Composite(alpha, CompositeOperator.CopyAlpha);
    }
    else {
        img.ColorSpace = ColorSpace.RGB;
        img.Resize(size, colorFilter);
        img.ColorSpace = ColorSpace.sRGB;
    }
}

color:

alpha:

divide correct (14.10.3-):

divide wrong (14.10.4+):

Preceding images are identical. Subsequent images become white in opaque areas, naturally.

[dlemstra]

Your input image has no alpha channel? And I have no clue what the size and colorFilter need to be?

[Athari]

ResizeImage function resizes image img to size size using filter colorFilter. It uses linear RGB to avoid dilate/erode effect and resizes alpha separately to avoid premultiplication effect.

The sample images I provided apply to img.Composite(alpha, CompositeOperator.DivideSrc) line which has different behavior in different versions:

• color — img before the call
• alpha — alpha
• divide correct — img after the call (v14.10.3-)
• divide wrong — img after the call (v14.10.4+)

If you want to test the function in full, you can pass any image with alpha to hit the problematic code path. Color filter can be any supported value, it doesn't affect the result much. I've provided the whole function just for context, the problem is with the img.Composite(alpha, CompositeOperator.DivideSrc) line.

[dlemstra]

Can you give it another try with the latest release? There have been some changes in the Composite method and it looks fixed?