Unclear behavior observed in the Color Composite Node

[RaymondGuo007 (Raymond Guo)]

What is the color of the area that is not covered by either the source layer or the background layer?

1. I used a red PNG file with a transparent area on the right, connected to the source node, and a blue PNG file with a transparent area on the left, connected to the background node. However, the area where neither image is present appears black. I’m wondering why that happens.
   
   
   
   
   
2. I used a PNG file that is white on the left and transparent on the right, connected to the source alpha node, and another PNG file that is transparent on the left and white on the right, connected to the background alpha node. The area that neither image covers appears blue, which matches the background color. Interestingly, the back side of the plane also appears blue. Why is that?
   
   
   
   
   
   
   The back side of the plane
   
   
3. I encountered a case where the area not covered by either the source alpha or the background alpha displays the texture or color of the source layer. This behavior confuses me.
   I hope I’ve made myself clear, anyone has any idea about this? Thanks!

[oleksii.rybakov]

Transparent area of any texture map just reveal source material underneath.
I guess you just need red-white and white-blue input textures combined with multiply in color composite.

[oleksii.rybakov]

[RaymondGuo007 (Raymond Guo)]

But the color of the source material is defined by the Color Composite node, which includes transparent areas. So, where does the color of the source material you mentioned actually come from?

[oleksii.rybakov]

From nowhere, that’s why you have it black in your first image.

[oleksii.rybakov]

And if you will combine your original images with transparent areas, and then combine again - transparent areas will expose anything going under:

[calvin.henderson]

1. The black area in the first image comes from the “Blend with Color” part of the texture plugged in to the Background node (blue). This controls the color of transparent areas and is nominally black. If there’s “nothing” underneath the texture, it will show the color in that box.
   

   
   
   
   You can also see this behavior when only plugging one texture in:
   
   
   
   
   
   
   
   When you use the color composite node, the “Source” image (red) has an area with transparency. Underneath it, it has another “Background” texture (blue), that fills the area with transparency. But because nothing is under the “Background” texture, the color in “Blend with Color” is used - even when the box is unchecked. I think this could be better explained in the documentation. That’s why when it is combined again with another color as in @oleksii.rybakov’s example with the green image, the black area goes away because there is additional information to pull in.

2. I think that again this comes from there being nothing “underneath” the blue part of the image. In this case, there’s no “Blend with Color” color to fall back on, so the blue remains. If you chain another Color Composite here, the Background Alpha texture will behave like you expect:

   
   
   
   Not sure why the back side of your plane is blue, perhaps the mapping settings of your texture? Mine is the same on both sides:
   

3. Can you share an example of this?

[RaymondGuo007 (Raymond Guo)]

Thank you! I think you’ve fully answered my question. I really appreciate your patience and professionalism — it helped me a lot.