Remove the black background color of a bitmap

Question

I need to remove the black background color of a bitmap in C# VS2013.

It is just like that I draw some points on a canvas. The canvas is black and I jus

Answer 1

This will do the job:

public Color MakeTransparent(Color c, int threshold)
{   // calculate the weighed brightness:
    byte val = (byte)((c.R * 0.299f + c.G * 0.587f + c.B * 0.114f));
    return val < threshold ?  Color.FromArgb(0, c.R, c.G, c.B) : c;
}

You could use it in a double loop over the pixels, but for fast results you should call it from the code in this post (second part!) which uses LockBits.

Change this

ModifyHue hueChanger = new ModifyHue(MaxChannel);

to the new function:

ModifyHue hueChanger = new ModifyHue(MakeTransparent);

and call it with a suitable threshold, maybe 10 or 20..:

c = hueChanger(c, 20); 

The function skips the call to the system's MakeTransparent function and directly sets the alpha channel of each pixel to 0.

If you want to create a uniform color instead of a transparent one it should be easy to modify (e.g. by returning Color.FromArgb(255, 0, 0, 0) for solid black)

Do note that, while the code in the linked post takes both 24 and 32 bbp formats you should definitely not save as JPG, as this will re-introduce artifacts and the result will not work well with e.g. a TransparencyKey color..

Instead do save it as PNG, as Hans suggests!

I hope you can modify the button code to a function :-)