How can I convert a JPEG image to a PNG one with transparent background?

Question

Hi,

I have a JPEG format image, with a white background and a black circle.

How can I transform this image to a PNG format that the white background will be transparent and the black remains there?

I'm a programmer too, and if there are some ideas in C# code I will be very happy. Also I'm looking for a converter, tool, program anything.

Thank you.

Jeff

Answer 1

You could use the ImageMagick tool like this example.

You will need to set the -background option to transparent, set the the -alpha option to set and use the -transparent option to set the colour you want to be interpretted as transparent. See also the convert tool reference.

Answer 2

Here is working, but slow solution. You can speed it up by using Bitmap.LockBits().

using (Image img = Image.FromFile(filename))
using (Bitmap bmp = new Bitmap(img))
{
    for (int x = 0; x < img.Width; x++)
    {
        for (int y = 0; y < img.Height; y++)
        {
            Color c = bmp.GetPixel(x, y);
            if (c.R == 255 && c.G == 255 && c.B == 255)
                bmp.SetPixel(x, y, Color.FromArgb(0));
        }
    }
    bmp.Save("out.png", ImageFormat.Png);
}