CODEWITHSUNDEEP
c#bitmapdrawingtransparency
mnn
mnn

Reputation: 2010

Saving Bitmap as 32bpp doesn't keep transparency

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.

Upvotes: 1

Views: 5502

Answers (1)

ChrisF
ChrisF

Reputation: 137148

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.

Upvotes: 5

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