How to convert a 4-channel image to 3-channel image in an elegant way?

The second example from RemoveAlphaChannel:

• Remove opacity by blending with a white background:

img2 = RemoveAlphaChannel[img, White]

ImageChannels /@ {img, img2}

{4, 3}

ColorSeparate /@ {img, img2} // Grid

Row[{Labeled[Framed@AlphaChannel@img, "img", Top], 
     Labeled[Framed@AlphaChannel@img2, "img2", Top]}]

@kglr 's solution is very convenient and powerful. However RemoveAlphaChannel[#, colorname]& does not work for all cases. So I spent some time searching in the web forimage channels andfound the principle of the alpha channel.

Based on that, I get the following solution which can be used for other cases(different images with different alpha channels).

reduceImageChannels[img_] :=
 Map[Most,
    Map[(#*#[[4]] + 1*(1 - #[[4]])) &,
     (Flatten[ImageData@img, 1])]] // Partition[#, First@ImageDimensions[img]] & // Image

Let's check it,

img = Import @ "https://i.stack.imgur.com/XvzDc.png"

img2 = reduceImageChannels@img

ImageChannels@img2

3

It given me the image exactly what I want!