Saving Bitmap as 32bpp doesn't keep transparen

问题:

using (var bmp = new Bitmap(image.Width, image.Height, PixelFormat.Format32bppArgb))
using (var g = Graphics.FromImage(bmp))
{
    g.Clear(Color.Transparent);
    g.DrawImage(image, 0, 0);
    bmp.Save("image.bmp", ImageFormat.Bmp);
}

The question should be clear: why saving to BMP trashes transparency to black, while saving to PNG keeps it ?

Just to clarify: image is in Format8bppIndexed format and its palette does contain transparent colors (e.g. it draws correctly onto form/picturebox)

Edit: My bad, Bitmap.Save() actually saves BMP in Format32bppRgb format, even though the bitmap format is Format32bppArgb.

回答1:

It's because by default the bmp file format doesn't support transparency while the png file format does.

If you want transparency you are going to have to use png. The compression algorithms are lossless so you're not going to get artefacts in your image. The file will take up less space on disk too.