How to paste one image to another one, by forcing the former's background as transparent?

Question

I have 2 images, imgBlackOnly and imgRedOnly as below, which already have the same ImageDimensions.

I want to paste imgBlackOnly to cover imgRedOnly, while the white part of imgBlackOnly is treated as "transparent" as what we did in the tiny software Paint of Windows. Thus my desired output is imgBlackCoverRed as below:

But one point to note is, the white part of imgBlackOnly is not totally white (since it is a screen capture from somewhere else), and therefore we need to force some "nearly white" part to be "totally white" by adding a manual threshold.

How can I do that? Many thanks!

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To avoid misunderstanding, I also want to point out that the original picture of imgBlackOnly is not just black. It has several colors, with a "nearly white" background. Thanks.

Answer 1

The definition of the function ColorReplace[img, color] is that it takes colors that are similar to (not exactly the same as) color and replaces those pixels with transparent pixels.

i1 = Import["https://i.stack.imgur.com/wOPPys.png"];
i2 = Import["https://i.stack.imgur.com/KdDgSs.png"];
i1 = ColorReplace[i1, White];
i2 = ColorReplace[i2, White];
ImageCompose[i1, i2]

You may notice that there are still some white pixels around the black figure. We can fix that by providing our own threshold for how far away a color can be from white and still be considered similar:

i2 = ColorReplace[i2, White, 0.5];
ImageCompose[i1, i2] 

Answer 2

imgBlack=Import["https://i.stack.imgur.com/KdDgSs.png"];
imgRed=Import["https://i.stack.imgur.com/wOPPys.png"];
ImageCompose[imgRed,RemoveBackground[imgBlack]]  

Answer 3

The threshold mentioned by @C.E. can also be used with RemoveBackground to remove the white edge.

Module[
 {img1, img2},
 {img1, img2} = Import /@ {
    "https://i.stack.imgur.com/KdDgSs.png",
    "https://i.stack.imgur.com/wOPPys.png"};
 ImageCompose[
  img2,
  RemoveBackground[img1, {White, 0.5}]]
 ]

Answer 4

cover=#2~ImageCompose~SetAlphaChannel[#1, ColorNegate@#1] &

cover[imgBlackOnly,imgRedOnly]