CODEWITHSUNDEEP
c#.netbitmap
briba
briba

Reputation: 2987

Remove image transparency

I'm working on a Machine Learning problem at college and my first job is to convert images into black and white.

The problem is that my image has transparency, and I don't know how to remove it.

What I am trying:

public static Bitmap RemoveTransparency (Bitmap src)
{            
    Bitmap   target = new Bitmap (src.Size.Width, src.Size.Height);
    Graphics g      = Graphics.FromImage (target);

    g.Clear (Color.White);            
    g.DrawImage (src, 0, 0);

    return target;
}

An example of an input image:

My captcha image

An example of output image after "RemoveTransparency" call:

Output image

Does anyone know what's going on? Seems like background an the letters have the same color... and my my background is black if I'm coloring to white?

Thank you!

Upvotes: 4

Views: 2836

Answers (1)

Timothy Shields
Timothy Shields

Reputation: 79441

You need to set the CompositingMode of your Graphics object to SourceOver before drawing the other image on top of it.

g.Clear(Color.White);
g.CompositingMode = CompositingMode.SourceOver;
g.DrawImage(src, 0, 0);

The default CompositingMode is SourceCopy, which is taking the transparent black (R=G=B=A=0) pixels in your src image and rendering them as black pixels. SourceOver will do alpha blending, which is what you're looking for.

See here for details: CompositingMode Enumeration

Upvotes: 5

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