Jesper Rahlff

there are never stupid questions. Better to ask and spend 5 minutes to solve the problem than spending 2 hours trying to do it yourself. Sometimes one can't see the forest because of all the trees

your technique is correct. your seed in your rand() function is not. $NCY is equal to the total number of copies, hence the same looking glass as it will always stay the same number. What you want to use is $CY which is the number of the current copy.

you could do a simple particle simulation and then rasterize them to get the volume. then control density based on age of your points.

all you need to do to create a vector that goes from each point to your target point is the following: vector p0 = point(1,"P",0); vector myvector = p0 - v@P; to calculate the degrees between the two vectors you can use this formula angle = arccosine((vector1*vector2)/(length of vector1*length of vector2)) and this will yield the angle in radians which you then can convert to degrees. you can read more about that here: https://www.wikihow.com/Find-the-Angle-Between-Two-Vectors in vex that looks like this in a point wrangle: //normalize normal vector to make sure the vector length is 1 v@N = normalize(v@N); // get the length of myvector float length = length(myvector); // get the dot product between the two vectors float dotproduct = dot(myvector,v@N); //get the angle between in radians. Note that we do not have to multiply length of my vector with the length of v@N because we know that v@N = 1; f@angle = acos(dotproduct/length); //convert from radians to degrees. f@angle = degrees(f@angle); Lastly you can isolate whichever points you want based on a threshold of f@angle

hm yeah I see the problem. That is a good question. I am at work right now, but I will try to dive deeper into it when I am off.

I will have to look at a scene in order to be able to trouble shoot properly. Right now I am just firing from the (dot)hip

Thats awesome. Yeah it makes retiming mostly pain free for sure. so I am not sure this is the reasons why, but if you go to advanced < bindings on the up res node, at the bottom you can specify extra fields that it needs to take into consideration from the low res volumes. Try to input the name of your color volume there.

can you share the file when you get home?

that is correct. however if you dive inside the retime node and look at how it is build, you will see there is no advection going on. its a sequence blend, trying to interpolate v@P over time. This calculation is fairly easy to do on points, but not on voxels.

you can cache subframes into whatever format you want. coming back to the original question, where is the smoke getting the color from?

the way you can retime volumes is to use a Gas Up Res node to basically create subframes from your original simulation and then do a velocity advect on your source volume to gain the exact desired speed. The theory goes that if you know you want to slow down your simulation to 10% you need 10 subframes. meaning frame numbers like 1.0, 1.1, 1.2, 1.3 etc.. By default the retime node will try to create the interpolation on its own, but if you cache out subframes from your original sim, you can feed the retime node those frames and it can do the retime correctly. This way you avoid going through another simulation step. The help file if you search Gas Up Res will at the bottom of the page give you an example file of how to retime the volume.

what happens if you cache out your sim at full speed with substeps and retime it after the simulation? You should be able to use the new Retime node from Houdini 17 to do this quite easily.

yeah thats the idea. So in the wrangle you would do: v@initialP = v@P; v@P=v@uv; do your operations with group and smooth. then you can do in a new wrangle after the smooth: v@P = v@initialP; I am not sure if it will revert the uvs back to the distorted look, but if so you should be able to transfer the smoothed uvs to the mesh with the distorted uvs replacing those.

try setting vP = v@uv, and then group your points by unshared edges. use a uvsmooth and input your group as constrained points, maintaining your edges in place. after smoothing, set your Position back to original position.

if you go to Edit > preferences > hip file options, you can check if your units are set to 1 meter and 1 kg. (this is standard for houdini). Then IF you modelled to real world scale you will get accurate values in meters as described above by LaidlawFX