marge 2 image with "add" operation

I want image that RGB value are added from two image.
I expected that "Screen" mode in merge node is suitable for this purpose, but it was not what i want, foreground black pixel (RGB:0, 0, 0) affect result image.
How can i achieve composite image simply added 2 input RGB value like as Photohsop layer screen mode?

Photoshop's screen blend mode is not additive.

If you want to add in Fusion using the Merge, set the Gain to 0.

thanks for your comment.

I tried set the screen mode gain to 0 on merge node, but it made result as only background image.
Is there any other node for additive composite?

No, just leave the Merge in Normal mode and turn down the Alpha Gain slider.

Another way to get a simple Add is with the ChannelBooleans node, but remember to set Alpha to Do Nothing because you generally don't want to add that.

To clarify the screen thing: It's a blend mode invented to get around the limitations of 8-bit int images, where you can't simply add things together without the risk of going above 1 (or 255 in Adobe-speak). The trouble when using screen in floating-point is that if you have sources that do go above 1, the blend works opposite the way you'd expect. Let's suppose, for example, you want to combine two images where parts of them have a value of 1.25:
1 - (1 - 1.25) * (1 - 1.25) = 0.9375

You likely wouldn't want to combine two bright sources and get a dimmer one in return, right? Given this behavior, it's better to just delete screen from your brain—it only works in a color mode you don't want to use in Fusion.

I thought "alpha gain 0" means completely transparent so there might be no effects
I could get what I want. thank you.

This is the basic Merge operation for premultiplied images: Out = FG + BG * (1 - A)
where A is the Foreground Alpha.

Alpha Gain operates as a mutiplier on A, reducing how much the Foreground's alpha knocks out the Background's pixels, resulting in this modified formula: Out = FG + BG * (1 - A * Gain)

So when Gain = 0, you wind up with: Out = FG + BG * (1 - 0) = FG + BG

What you describe is the function of the Blend control, which acts as a multiplier on the Foreground (including its alpha), resulting in a further modified formula:

Out = FG * Blend + BG * (1 - A * Gain * Blend)