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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.

mnn
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1 Answers1

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

If you want transparency that will be read by other applications 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.

ChrisF
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    I think technically the BMP format supports alpha channels/32bits, but most implementations(in particular the MS ones) don't support such bitmaps. – CodesInChaos Jan 21 '11 at 12:41
  • @mnn - that's news to me, but in that case you'd need to include the flags to tell the save that it was 32 bpp. – ChrisF Jan 21 '11 at 12:44
  • I don't know any tools that support alpha channels in .bmp files, although Windows itself is fine with 32bpp alpha bitmaps as HBITMAPs. – David Heffernan Jan 21 '11 at 13:09
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    Hate to say, but 32-bit BMP with alpha isn't even supported by latest IrfanView. Also when loading such BMP using Bitmap.FromFile(), it is "degraded" to Format32bppRgb by GDI+. So the only solution is to use PNG. – mnn Jan 21 '11 at 17:00