How to add transparent padding to a jpg and save it as png with transparency?

Question

The documentation for SixLabors ImageSharp is very limited, and most google searches leads to GitHub, which is not very helpful.

How can I upload a jpg, .Mutate it with transparent padding and save it as a png with transparency?

This is the code I have so far. If the uploaded image is a png, transparent padding works, but jpgs get black padding:

private static void ResizeAndSavePhoto(Image<Rgba32> img, string path, int squareSize)
{
    Configuration.Default.ImageFormatsManager.SetEncoder(PngFormat.Instance, new PngEncoder()
    {
        ColorType = PngColorType.RgbWithAlpha
    });
    img.Mutate(x =>
        x.Resize(new ResizeOptions
        {
            Size = new Size(squareSize, squareSize),
            Mode = ResizeMode.Pad
        }).BackgroundColor(new Rgba32(255, 255, 255, 0))
        );
    img.Save(path);
    return;
}

.SaveAsPng() takes a filestream, but I have an Image<Rgba32> and a path...

Answer 1

You can explicitly save as a png via SaveAsPng, set the path extensions to .png, or pass an IImageEncoder to the Save methods.

You'll find API docs at https://docs.sixlabors.com/api/index.html

private static void ResizeAndSavePhoto(Image<Rgba32> img, string path, int squareSize)
{
    img.Mutate(x =>
        x.Resize(new ResizeOptions
        {
            Size = new Size(squareSize, squareSize),
            Mode = ResizeMode.Pad
        }).BackgroundColor(new Rgba32(255, 255, 255, 0)));

    // The following demonstrates how to force png encoding with a path.
    img.Save(Path.ChangeExtension(path, ".jpg"))

    img.Save(path, new PngEncoder());
}

Additionally, if saving to a stream.

img.SaveAsPng(path);