export transparent images in c#?

Question

I've edited an bitmap in c# and for every pixel i've changed it to a certain color if a condition was true else i've set the color to Color.Transparent ( the operations were done with getPixel/setPixel ) . I've exported the image in .png format but the image isn't transparent. Any ideas why or how should i do it ?

Regards, Alexandru Badescu

here is the code : -- here i load the image and convert to PixelFormat.Format24bppRgb if png

m_Bitmap = (Bitmap)Bitmap.FromFile(openFileDialog.FileName, false);

           if(openFileDialog.FilterIndex==3) //3 is png
            m_Bitmap=ConvertTo24(m_Bitmap);

-- this is for changing the pixels after a certain position in an matrix

for (int i = startX; i < endX; i++)

                for (int j = startY; j < endY; j++)
                {
                    if (indexMatrix[i][j] == matrixFillNumber)
                        m_Bitmap.SetPixel(j, i, selectedColor);
                    else
                        m_Bitmap.SetPixel(j, i, Color.Transparent);

                }

Can you please post your code on how you are creating the image, and saving it. — Øyvind Bråthen

honibis honibis · Accepted Answer · 2010-10-25T10:48:34

Its because pixelformat. Here is a sample code for you:

        Bitmap inp = new Bitmap("path of the image to edit");
        Bitmap outImg = new Bitmap(inp.Width, inp.Height, PixelFormat.Format32bppArgb);
        outImg.SetResolution(inp.HorizontalResolution, inp.VerticalResolution);
        Graphics g = Graphics.FromImage(outImg);
        g.DrawImage(inp, Point.Empty);
        g.Dispose();
        inp.Dispose();

        ////
        // Check your condition and set pixel here (outImg.GetPixel, outImg.SetPixel)
        ////

        outImg.Save("out file path", ImageFormat.Png);
        outImg.Dispose();

this is the code that requires minimum change to your current code. But i would recommend you to check out LockBits Method for a better performance: http://msdn.microsoft.com/en-us/library/5ey6h79d.aspx

export transparent images in c#?

3 Answers