PeachImage
PeachImage.Formats.Png.PngEncoderOptions
PngEncoderOptions.TransparentColor Property
When set, and the source image has no alpha channel, encodes a tRNS chunk marking this
single RGB (or gray, for single-channel sources) value as fully transparent, instead of the
default of never emitting a color-key transparency chunk. Ignored for sources that already have
a real alpha channel (Rgba32/Rgba64), which
always encode with a true per-pixel alpha channel.
public System.Nullable<(byte R,byte G,byte B)> TransparentColor { get; init; }