How can I make an image transparent?

[markrendle]

I've got an image created from a document with a white background, in PNG format. I guess there must be some way to replace the white with transparent, but I don't know where to start.

[JimBobSquarePants]

There's a few ways:

1. If your image is not antialiased you could loop through the pixels and replace each instance of Color.White with Color.Transparent.
2. Do the above but with some sort of threshold to capture variance.
3. Use the ImageSharp.Drawing package and RecolorBrush which takes source and target colors. You then pass that to the Fill method.

I think 3 is probably the easiest way to go, bear in mind though that ImageSharp.Drawing is a beta and subject to change.

using var image = Image.Load(...);

float threshold = 0..1F;
Color sourceColor = Color.White;
Color targetColor = Color.Transparent;
var brush = new RecolorBrush(sourceColor, targetColor, threshold);

image.Mutate(x => x.Fill(brush));

image.Save(...);

[Euynac]

Thanks! but RecolorBrush couldn't work when set its target color as Transparent. I tried to make target color like Red or other color and changed threshold it worked fine. Why?

[JimBobSquarePants]

You need to set PixelAlphaCompositionMode.Clear for the AlphaCompositionMode in your GraphicsOptions

See SixLabors/ImageSharp.Drawing#147

[Euynac]

Thanks you! But if I set PixelAlphaCompositionMode.Clear for the AlphaCompositionMode , my out put will become totally transparent no matter how I change the threshold.
Then I found SixLabors/ImageSharp.Drawing#26, and I set it as PixelAlphaCompositionMode.Src and it works for me. (btw, I forgot to set the PngColorType to RgbWithAlpha in the png encoder at the begining)

[JimBobSquarePants]

I'd like to see If I can make that a default for RecolorBrush. It's tricky though.

[mamdos]

what if my actual image has white pixels and i don't want to lose it , i just want to replace the background white color to transparent not the actual content itself !