Hi I am doing some particle stuff with tops etc and want to mix it with some videos - theres a lot of black parts in the particle stuff in the background I want to make invisible/transparent how do I make the black parts of the particle stuff invisible/transparent using chrioma key ? Tx
Hello, Perhaps it would be better to give more informations but I would avoid chroma key on particles, not very computer friendly. The first question is how you produce particules.
With geometry instances, you can change the activation channel or the alpha channel and obtain easily transparency for the desired particles. It is less easy with the Particles Sop way because you have few command on each particule color.
But I would definitely doesn’t chroma key for that. Its sufficiently difficult with slow moving bodies on well lighted green background. Isolate black tiny fast moving particles on transparent background… Good luck!
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Thanks for the reply. Heres a screenshot hopefully will give you enough info. DOnt worry about the red cross - thats just me cocking something up trying to cram everything onto a screenshot :
Double click the above to enlarge
Its difficult to understand what you want, do you want to mask some particles before or after post-production effects (feedback, edges…), or do you want to mask some of the produced image?
To post you proper project as .toe would help to analyze it.
I just want the black background on which the particles move to be transparent - so when I add a video the black background doesnt mask it . Basically the particles expand in response to audio sometimes filling the whole screen , I use feedback etc - however the bits where the particles “are nt” is black. I want to mix the particles with a video - I want the particles to be visible over the video - but the negative space/black background - to be transparent so the video can show through that bit, I do not want the video to mask the particles. In most video software chroma key is used to do this.
Yes and no, When I import moving object from 3D software (Houdini, Blender or C4D) I have an alpha layer and I doesn’t need chromakey and in most movie its the case. When I produce particles in TD I do the same, first using the render output to comp it on my other images. My humble opinion is first to obtain directly your transparency. And certainly I never try to obtain transparency after feedback Top! But i repeat, without more informations it is difficult to help you.
Thabks Jaques - I attach the toe file but it will be difficult to see whithout an audio input - I agree making the background transparent in the first place is the way to go - I guess thats what I want to know how to do. To get this to work youll need to set stereo mix as the input and play through your onboard soundcard some kind of musicnew.toe (32.7 KB)
Ok, I looked quickly to your project. The problem (one of) is that with lookup, you replace grey and alpha values with only color values without alpha. I have tried quickly to put some alpha in your color ramp and then you can re introduce some transparencies. I also changed pixel format to 16 bits float, with that you can obtain a better return to transparency after feedback. Very rough proposition, but it can be a path.
new.1.toe (49.2 KB)
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Jaques thanks a million
So in relation to this - if I then wanted to make certain colours in the movie transparent, say dark brown - so it seemed a bit as if the particles were weaving in and out of the tree image /vwahtever video Im using - how would I do that ?
I would use a glsl top where you can very precisely, for each pixel, calculate the level of the A layer depending of RGB value. You need to make a little bit of glsl programming. In a sense thats how work chromakey works 
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well thats my homework sorted out - thanks again Jaques - really helpful forum and community here