glow on transparent background with different layer

glow test.blend (572 KB)

so its basically just the glowing ball in front of the blue cube, I can’t get the cube show up properly, it’s either black or, if I switch the input for the last alpha over node before composite, the blue cube would show in front of the ball.

at least 10 characters

this is not I want, but I sorted it out, this is what I have

glow test.blend (710 KB)

I need some more helps, what I want is to put 3 different objects in each layer and render them out with the glow while keeping the background transparent, so far I’ve been struggling to get the desirable result, the main problem is when having alpha over, it makes the last object dark, but with mix node set to add, the result isn’t what I want either because it simply adds 3 layer altogether thus the overlapping parts became too bright, hope my elaboration on the problem is clear, please can someone take a look at blend file and point me out a way to do it.

The problem you’re running into is a result of how the alpha channel is being handled. When you put your blur through the mix node to give it color, you also give it an alpha of 1 for the whole image. Try connecting a ‘Separate RGBA’ node to different parts of your node graph to see how the alpha channel is being handled and you’ll start to see where the problems are coming up.

The image I attached below shows how you can achieve the results you’re looking for. Once the blur and color is done for the glow, I give the glow an alpha of 0. Glow effects should always have an alpha of 0 to mix correctly (if the underlying alpha math is correct of course).

Once this is done, I alpha mix it under the original render so that the glow doesn’t affect it overall, and so that the original alpha is retained for the image. This can then be correctly used in an alpha over operation for the other images.

Without taking these steps, you get the results you had before where any images blended ‘under’ the glow image would be black. This is because, with an alpha of 1, everything outside the glow area you see is black, or 0,0,0 in the RGB channels. So when you do an alpha over, those black pixels are rendered over the other pixels.


Hopefully this is clear enough to get you on the right track, but if not feel free to ask more questions. The link below is a good place to start to learn how to correctly think of and apply the concept of an alpha channel.