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I've been trying to do little special effects for some of my videos. For example I create a light emitting object that woud cast light on a black floor, later composit the black out and the finished result would be a light pattern on a transparent background. Using this method I could have a camera tracked scene with an object that would realstically light the floor around it.

So lets get on it! Firstly I rendered an image of a point lamp that casts light onto a floor.

The floor is a diffuse node with 0 roughness and white color. The lamp emits pure white light. And the world has its material removed so that it couldn't interrupt anything.

enter image description here Next I added a ColorRamp node and turned its black value into transparent. That would make blacks transparent, grays half transparent until white which will not be transparent.

Then I noticed an ugly white blown out clipping area in the middle which was not there when I rendered the image. It reminded me the problem of colors clipping and low dynamic range that we had before Filmic color management was introduced to Blender.

No matter how I moved the colorramp sliders, the white blob in the middle stayed there. I even tried using Separate RGBA node and used one of its outputs as a factor for where the whites should be, but the clipping area in the middle stayed there.

enter image description here Lastly i added a black background to the image and this made the problem little less obvious, but it didn't solve it. I suppose there is something to do with Blender color management tools, but I let someone more experienced decide.

My question is what is causing this issue and is there some kind of way to maintain this smooth light transition as there was on the render, when composited onto transparent background?

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Color Ramp, nor any thing in Blender, is pixel managed.

As a result, I believe the Color Ramp is taking the scene referred 0.0 to 1.0 range and clipping it, which is leading to the harsh cut transition from 1.0 scene referred onwards.

The complex solution is to mangle the data going to and from the Color Ramp. The more elegant and powerful solution is to pixel manage every single UI element to permit the audience to control the context of the tool to meet the need.

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  • $\begingroup$ Omg are you really this Troy Sobotka, who made filmic blender. Big admirer of yours... anyway. So there are higher values in the scene than 1? Does this mean that I should lower my lamps emission strength until all values in the scene are below 1 so that it would not clip? Later maybe composit the lights brighter again. I also noticed that when viewing the fresh render with filmics raw look, the colors also clipped. Maybe its something to do with it... Could explain it a little further, I got interested! :) $\endgroup$
    – Kaarel Parve
    Commented Jun 1, 2019 at 6:02
  • $\begingroup$ Good to have folks keen on pixels. At any rate, search here for “scene referred” and you’ll find some good information. No, your scene is great, the problem is Blender. The issue is that your light is being clipped, which is an issue with the ColorRamp. You might want to try math nodes, or even curves. Curves too are broken due to lack of management on the UI, but they won’t be clipping data. $\endgroup$
    – troy_s
    Commented Jun 1, 2019 at 15:02
  • $\begingroup$ Ok, thanks! I'll try. $\endgroup$
    – Kaarel Parve
    Commented Jun 5, 2019 at 9:19

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