How to "glow" something in post that is not the brightest thing in the render

Hi guys,
can someone help me please. How can I get the blue part glowing with the glare node?
The blue part (emission shader) is driven by an alpha texture.
Any help is very much appreciated :slight_smile:

Be kind. English is not my mother language.


Enable and use the emission pass in the compositor.

Thanks. That’s pretty easy. But it only works if you have only one emissive material. If there ist a second one it will be be “glowed” (is this a real word?) as well as the first material.

It may not be ideal but you could model the blue part and give it an emission shader. Then, assign it an object pass index to isolate it in the compositor and give it the glare node.

Is there a way to get material pass only on emission material and not on Principled in the Mix ?

You can use the Object or Material ID to get a clean mask of one or multiple Objects. If you want only the emissive parts of the mask you’ll have to multiply the ID Mask with the Emission pass in the Compositor. This way you can keep your model in one piece.

Thanks Cubezz, that sounds like a good idea. I will give that a try.
Actually I found a way: I put the emissive parts to another layer too, rendered this layer without the evironment an lighting (it’s quiet like “masking the emission pass”) and then did the compositing on it. After that I multiplied the glow-layer with the original renderlayer. This worked quiet well because I rendered the glow-layer with one sample only :wink:

Now, there’s one thing to be aware of when you are using effects like this: “object-IDs don’t know about effects.” Particularly effects, like blur or glow, that affect surrounding pixels. Object-IDs also know nothing of things like (anti-)aliasing. They are literally “nothing more and nothing less” than a pixel-by-pixel identification of which object was closest to the camera at that location.

Therefore, shoot a test-render to a MultiLayer OpenEXR file with all the goodies turned-on, including Object-ID. Then, say in a separate Blend-file, use that file as the input to a node in the compositor – verify that all of the expected data-channels are present – then use various Viewers to examine one or more of these data-channels. Set up simple experimental compositing setups. (All the data has been captured into the file ahead of time, so it won’t take any “time.”) Test your assumptions of what the data will look like and how it can best be manipulated “in post.” Fifteen minutes of hands-on futzing-around will answer more questions than any forum-post ever could. :slight_smile:

Note: I specify “MultiLayer OpenEXR” because this file-format is specifically designed to capture multiple layers of high-resolution digital data, exactly as the renderer produced it. It was engineered – by Industrial Light & Magic with a little help from the Blender Foundation – for this purpose. “The data, the whole data, and nothing but the data.”

I have a couple of cents to the discussion. Sometimes when you fiddle with results of ‘ID Mask’ node as say alpha for some blending, it’s a good idea to use ‘Dilate/erode’ node to fix very rough masks.

The biggest point to be made is that you have to do all of the effects-processing “in post,” not in the renders.

An id-mask can’t be “blurred.” Each pixel has one, and only one, object-ID associated with it. It isn’t anti-aliased. Therefore, the pixel-value data that you associate with the object-ID must be a pixel that’s getting its color information only from that one object, not from surrounding areas as would be the case with in-render blur. It’s simply “the nature of the data that you have at your disposal.” (It doesn’t act like an optical mask …) If you’re careful to observe this, there are many filters that you can use in post. But don’t even try to match in-post what’s already been done in-camera because it’ll never look right.

Whats with the absolutes? It is wise to do things in post that are faster, more controllable or otherwise better to do in post. What is not, do in renders. What is an effect anyway? Why is lens bloom more of an effect than light bleed from GI?

It can. ID channel is not a mask. Once a specific mask is extracted from ID channel, one can blur blur it to his pleasure.