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);

score 2 · Accepted Answer · answered Jan 21 '11 at 16:12

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!

score 1 · Answer 2 · answered Jan 21 '11 at 15:53

1

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

check here

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

answered Jan 21 '11 at 15:53

Shekhar_Pro

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You need at least an 8-bit PNG to support alpha – Chris Haas Jan 21 '11 at 15:56
How is this possible? I tried to set black color to transparent in IrfanView and it worked. – Mert Sevinc Jan 21 '11 at 15:59
after setting black to transparent.. was the image still 4-bit i doubt that – Shekhar_Pro Jan 21 '11 at 16:02

Transparency in FreeImage

2 Answers2