VB.NET - Problem with transparent background PNG and ImageSharp quantizer-CodePudding

I am trying to make a WinForms app in VB.NET (Framework 4.7.2) that optimizes an electronic signature using ImageSharp, and I am running into a problem. When I try to use the OctreeQuantizer to decrease the number of colors in the PNG, I get an inverted / negative image if the source has a transparent background. This is a problem since the signature will need to be transparent when it is applied to a document.

Application Window - Top is original signature

Original Save Dialog from Paint.NET

As you can see, I saved the original image as an 8bpp PNG with transparency using Paint.NET

Here's my quantize function:

    Public Shared Function QuantizeImageGrayscalePNG(ByVal vstrSource As String, ByVal vintMaxColors As Integer) As Bitmap
        Dim poqQuant As New OctreeQuantizer()
        Dim pencPNG As New PngEncoder() With {.BitDepth = PngBitDepth.Bit8, .ColorType = PngColorType.GrayscaleWithAlpha}

        vintMaxColors = Math.Min(256, vintMaxColors)
        If vstrSource IsNot Nothing AndAlso IO.File.Exists(vstrSource) AndAlso vintMaxColors > 0 Then
            Using piNew As Image = Image.Load(vstrSource),
                  pstOutput As New IO.MemoryStream()

                poqQuant.Options.MaxColors = vintMaxColors

                piNew.Mutate(Sub(x) x.Quantize(poqQuant))
                piNew.Save(pstOutput, pencPNG)

                Return Bitmap.FromStream(pstOutput)
            End Using
        Else
            Return Nothing
        End If
    End Function

If I use an image with no transparency as the source, the issue does not occur. I also tried commenting out the Mutate function, and it "works" but of course does not change the number of colors or image size.

CodePudding user response：

Answer from JimBobSquarePants on GitHub:

Octree only supports a single transparent color . For Png you need to use the Wu quantizer which is the default quantizer.

However, if you are trying to reduce the colors in a png you should be saving it in indexed format by setting the PngColorType in the encoder along with a custom Wu quantizer with your max color count.

Not sure that is a full answer, but my issue was closed so that's all I'm getting.