Alpha channel missing

[daxpandhi]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am using the latest version of Magick.NET
• I have searched open and closed issues to ensure it has not already been reported

Description

Alpha channel is ignored when saving to several formats that support it. When debugging, I can see that the MagickImage has an alpha channel with the data provided. However, the saved file does not show the A channel in Photoshop or other applications.

Maybe I'm not doing something write, but feels like if Alpha is On/Set, and data is being added it should save too. I've tried it on 7.14.x, 7.15.x and 7.16.x to confirm.

Steps to Reproduce

Here is a test program to create 3 files that should have alpha channels.

    internal class Program
    {
        private static void Main(string[] args)
        {
            MagickImage mm = new MagickImage(MagickColors.Black, 1024, 1024)
            {
                ColorType = ColorType.TrueColorAlpha,
            };

            mm.Alpha(AlphaOption.Set); // Have tried all "positive" options

            float[] data = new float[1024 * 1024 * 4];

            Random r = new Random(256);

            for (int i = 0; i < data.Length; i++)
            {
                data[i] = (float)(Quantum.Max * r.NextDouble());
            }

            using (var pix = mm.GetPixelsUnsafe())
            {
                pix.SetPixels(data);
            }

            mm.BitDepth(16);

            mm.Write("D:\\rgba.psd", MagickFormat.Psd);
            mm.Write("D:\\rgba.tif", MagickFormat.Tiff);
            mm.Write("D:\\rgba.png", MagickFormat.Png);

            mm.Dispose();
        }
    }

System Configuration

• Magick.NET version: Magick.NET-Q16-HDRI-OpenMP-x64 7.16.0.0
• Environment (Operating system, version and so on): Win 7 SP1, Win 10 (all versions) .NET Fx 4.8
• Additional information:

[dlemstra]

I am unable to reproduce your issue:

64 bit depth = 16 bit per channel * 4 . This means there is an alpha channel. And when I check the files with ImageMagick (maybe biased?) all the files have an alpha channel. Not sure why Photoshop seems to think that these images have no alpha channel.

[daxpandhi]

Very strange. I'll investigate more on my end see if I can reproduce it more extensively or if there is something wrong in my workflow that is causing the problem. Will check and report back. Thanks!

[dlemstra]

Feel free to reach out when you have more info.

[daxpandhi]

This is what the untouched output from the code I wrote above gives me, btw.

[dlemstra]

32 seems like an 8 bit image? 4 * 8 = 32 and 32 / 3 = 10.6666666667 that seems like a strange bit depth.

[daxpandhi]

I called BitDepth(16) so not sure why it would be 32. I'll test entire process again on my main computer tomorrow and report back.

[dlemstra]

BitDepth(16) means 16 bits per channel. And I know PNG can move it down to 8 when it thinks it can.