Transparency in FreeImage

Question

I am successfully converting a 24-bit PNG image which has a transparent background to 4-bit PNG image using FreeImage C# wrapper. However, the 4-bit image's background becomes black instead of being transparent. Here is the code. Any ideas?

FIBITMAP dib = FreeImageAPI.FreeImage.LoadEx("C:\title_normal.png"); FreeImage.SetTransparent(dib, true); FreeImage.SaveEx(ref dib, "C:\title_normal.png", FREE_IMAGE_FORMAT.FIF_PNG, FREE_IMAGE_SAVE_FLAGS.PNG_Z_DEFAULT_COMPRESSION, FREE_IMAGE_COLOR_DEPTH.FICD_04_BPP, true);

Answer 1

Actually I figured out. Here is the code if someone is interested:

            dib = FreeImageAPI.FreeImage.LoadEx("C:\\title_selected.png");
            dib = FreeImage.ConvertColorDepth(dib, FREE_IMAGE_COLOR_DEPTH.FICD_04_BPP);
            byte[] Transparency = new byte[1];
            Transparency[0] = 0x00;
            FreeImage.SetTransparencyTable(dib, Transparency);
            FreeImage.Save(FREE_IMAGE_FORMAT.FIF_PNG, dib, "C:\\title_selected1.png", FREE_IMAGE_SAVE_FLAGS.DEFAULT);

Resulting image is 4-bit transparent PNG!

Answer 2

I dont think 4-bit png has channel for Albha.

check here

http://en.wikipedia.org/wiki/Portable_Network_Graphics