Transparency + Colors lost after Resize of a Transparent Image

[Maxxxel]

PngEncoder encoder = new PngEncoder();
encoder.ColorType = PngColorType.RgbWithAlpha;
encoder.TransparentColorMode = PngTransparentColorMode.Preserve;
encoder.BitDepth = PngBitDepth.Bit8;
encoder.CompressionLevel = PngCompressionLevel.BestSpeed;

using (Image<SixLabors.ImageSharp.PixelFormats.Rgba32> image = Image.Load<SixLabors.ImageSharp.PixelFormats.Rgba32>(stream))
{
  ResizeOptions resizeOptions = new ResizeOptions
  {
    Mode = ResizeMode.Max,
    PremultiplyAlpha = true
  };
  image.Mutate(x => x.Resize(Math.Min(image.Width, 1024), Math.Min(image.Height, 1024))); 
  image.SaveAsPngAsync(Exporter.Folder + "\\" + textureNames[3] + ".png", encoder);
}

Like the Titel says, when I run this on an Image with Alpha = 0, the resized image becomes a totally blank one where all values RGBA are zero.

Image i try that for:

[JimBobSquarePants]

I don't fully understand what you are saying here. Are you saying that if you resize an image where the alpha component of the image is 0, then the result has 0 for red, green, blue components?

If so, that makes perfect sense since you are pre-multiplying the other components by the alpha value. You should set ResizeOptions.PremultiplyAlpha to false to avoid this.

[JimBobSquarePants]

Sorry, that wasn't obvious.

I cannot replicate this. You know the property defaults to true yeah? You need to explicitly set it to false.

Here's the resized output. Scaled to 50%

[Maxxxel]

I'm at work now, I reply in 1.5hrs with my tests. Thanks.

[Maxxxel]

@JimBobSquarePants

With the underneath code my image turns all ARGB into zero values. Where is the mistake???

using SixLabors.ImageSharp.Formats.Png;
using SixLabors.ImageSharp.Processing;
using SixLabors.ImageSharp;

PngEncoder encoder = new PngEncoder();
encoder.ColorType = PngColorType.RgbWithAlpha;
encoder.TransparentColorMode = PngTransparentColorMode.Preserve;
encoder.BitDepth = PngBitDepth.Bit8;
encoder.CompressionLevel = PngCompressionLevel.BestSpeed;

using (Image<SixLabors.ImageSharp.PixelFormats.Rgba32> image = Image.Load<SixLabors.ImageSharp.PixelFormats.Rgba32>("test.png"))
{
    ResizeOptions resizeOptions = new ResizeOptions
    {
        Mode = ResizeMode.Max,
        PremultiplyAlpha = false
    };
    image.Mutate(x => x.Resize(Math.Min(image.Width, 1024), Math.Min(image.Height, 1024)));
    image.SaveAsPngAsync("result.png", encoder);
}

[Maxxxel]

OMG!

Sorry for wasting your time... Just noticed i dont even use the resize Options....

using SixLabors.ImageSharp.Formats.Png;
using SixLabors.ImageSharp.Processing;
using SixLabors.ImageSharp;

PngEncoder encoder = new PngEncoder
{
    ColorType = PngColorType.RgbWithAlpha,
    TransparentColorMode = PngTransparentColorMode.Preserve,
    BitDepth = PngBitDepth.Bit8,
    CompressionLevel = PngCompressionLevel.BestSpeed
};

ResizeOptions resizeOptions = new ResizeOptions
{
    Mode = ResizeMode.Max,
    PremultiplyAlpha = false,
    Size = new Size(1024, 1024)
};

using (Image<SixLabors.ImageSharp.PixelFormats.Rgba32> image = Image.Load<SixLabors.ImageSharp.PixelFormats.Rgba32>("test.png"))
{
    image.Mutate(x => x.Resize(resizeOptions));
    image.SaveAsPng("result.png", encoder);
}

Everything works as expected, very nice Lib!!!!!!!!!!!

[JimBobSquarePants]

Don't worry about it. I missed it too! 😄