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Scale signed

what's up you guys it's Peter here and today I want to cover another one of these useful tools for your visual effects toolbox it is going to be the SDF or the signed distance field so we're gonna cover a little bit on how it works and then also some examples on you know how to use them into Houdini and make them do and perform kind of tasks that are common to do for visual effects so let's dive inside and have a closer look in Houdini how it all works so first let me shrink this down a little bit and then we can dive into okay so I have a little example over here that shows you the signed distance field and on the one hand we have the SDF in value we also have the gradient lengths over here and I've made a little point a little sphere that is going to animate through it and as it enters into the active voxels it is going to start sampling the SDF to the sign business field and basically display a variety of information that we can then use for our Sims or for snapping or for any kind of operations we want to do so let's explain what how it works and maybe it's easier to understand backwards how to you know understand it so signed distance field the first thing to understand is that it is a field that means we are dealing with voxels and volumetric data the second part of it is what is actually being stored in those voxels and that is going to be the distance now the distance what distance the distance to the surface so I have over here this isoh contour represents the slice of the pig's head and you can kind of sort of see that we are measuring the distance right in red the SDF value we are measuring the distance to that surface now what happens when we are outside right of that the zone of that active region is that this value goes to wherever the closest one is so this even though you can kind of sort of see right this is animating moving this distance should be updating but it isn't so why not right well because we're dealing with a V DB and a fee DB has got an active region it can have what is called an exterior and also an interior band we'll talk about that in a moment but the point being is as soon as my point goes inside of this active region the outside band currently then we can kind of see and will correctly start sampling the values now the second part of it we'll talk about that later is the gradient now we'll talk about what a gradient is and you can kind of sort of see this big blue vector right is a gradient now let's keep going we already covered field it's a volume we already covered distance that means it is the distance to that surface lastly we have a signed right so why would it be signed ever heard of a signed distance and negative distance well no not really but by convention it is agreed upon that any value that is on the inside of the geometry it has a negative value so it's got a negative sign in front of it so I've kind of visualized that here with all the little plus signs on the outside the negative signs on the inside and if you look closely you can zoom in a little bit right I've made over here a region right where the ISO contour is zero by the way ISO contour what does that mean right so ISO means the same and contour means a region so a region where all the values are the same in my case here this ISO contoured is a region where all the valleys are the same right where we are on the surface and remember as the f the signed distance field and the distance to the surface well if we are on the surface and what is the distance to the surface it will be zero so therefore we start with a positive distance on the outside we're going to be zero when we're on the surface and we will have a negative distance more like a representation that we're inside of the surface and that our distance value over here will go to negative so has my value over here goes to the inside we can see I am negative zero point two six four units from that surface right okay so what I've built here is a little system showcases the difference between the SDF of value and the gradient length notice that the gradient length here is not necessarily normalized it's got zero point nine two four and it's literally just a direction vector now this is the direction of exit from the current point to the closest point on the surface that's quite useful to have to have access to when you're inside of your you know systems so the other bit then is well what if I combine the gradient lengths together with that SDF value that's going to be that little red vector over here that is pointing in the same direction as the gradient vector but that has an SD F value so the lengths over here of this little bit is 0.25 so if we kind of see that you know as we kind of get closer to this surface we can kind of see that the lengths over here is gonna get closer and closer to zero now we can't quite hit zero because we are very close to that surface already but as we kind of steps through it you know it's kind of nicely sampling until eventually boom it is outside of my volume again let's talk a little bit about the volume next we now know that ok we're dealing with a field we have distance that were sampling and this is science of sine means for inside or outside now we have the end the entire volume over here of our pig's head as well so these are the bounds on the volume and then these represent the active voxels so that's the inside and outside banding currently this is the outside banding and the inside banding you can't really see it because currently it is turned off let's dive inside of our SDF explanation and step through this so I'm gonna dive inside of this SDF explanation and we have over here the pig's head so we start off with the pig's head and it's a polygonal surfaces you as you can kind of see we're going to convert that into a volume so if we look here V to be from polygons then we can kind of see this is the entire signed distance field represented as sort of a as the F as a voxel so if you ever see this in the viewport this is not polygonal geometry if we hover over this you can see points primitives 1 vertices 1 v:d bees 1 and this is the resolution over here of your VDB notice also as we get closer you can almost see the individual sort of you know visualization planes for representing that this is indeed a VDB object it's not a polygon object the last thing that is also quite useful to see is where that active region is so if we turn on over here the hulls of your geometry and I'll show you the active region of the active and foxholes now currently on the VDP we can see exterior bands of three interior band of three and we can turn on the fill interior too first that's showcase what happens if we increase the exterior band so if we go from three to say ten is going to expand over here this region this this area where the SDF is going to get computed gets calculated is going to be expanded next we can also look at the interior now in order to showcase the interior I'm going to go to wireframe so we can see the interior data over here on the inside of our voxels of our SDF if we turn on fill interior you can sort of see that that insight built is now completely full so if we turn it off again there's my inside banding that's based off of three voxels we can turn this into years of world space as well and we might later on make use of that when we're doing displacement of our volumes and this might indicate our displacement amount or our displacement bounce so let's turn this back to voxels and let's go to this other one that I've already preset pre-configured so if we go back to all the way at the bottom and my visualization over here can visualize this start off the wireframe and I'm going to go back to my camera over here so let's showcase what happens if we turn off fill interior right so currently fill interiors turn on so here if we turn it off you can clearly see the interior information is just gone and again my as DF value is not correctly being sampled because there is no data over here right so it'll try to get grab whatever data is closest so this is true for both the outside as well as the inside now oftentimes you know the outside volume could be massive right there could be you know a lot of empty space over here the inside of a volume or the inside of an object might be a bit more contained a bit more reasonable also this could be good for liquid particle sampling that kind of information it could be good for little creatures crawling inside of another object or on top of an object so there's there's a lot of use cases for this so let me turn fill interior back on and now my values are immediately computed again okay so let's keep going and let's see as we kind of step through this we can now sort of visualize how can we actually sample these values so I have a sphere I mentioned this before I have a point now this is a primitive sphere I like using primitive spheres because at the inside at the center over here of that primitive sphere there is indeed a single point that sits there so that's mainly for visualization purposes and that I can easily move it around in the viewport but I don't care about the primitive sphere I care about that center point so I could have just as well done this with an ADD operation and a static point now here with the transform I could kind of say okay I've got this animating right but does that access so that it moves over 100 frames from negative 2 to 2 I've also scaled it down so you can kind of just sort of see this little sphere move through my scene this is going to move in relative to where my where my things that is so we can kind of see whoops so we can kind of see as it steps through it is going to reference you know all these values as we kind of go through how do we do this well we can use an attribute vault and we're going to sample the SDF now notice in the first input I have my point in my second input I have my VDB right so this over here is going to be my SDF my surface that I'm going to go in a sample so let's dive inside and have a closer look now all of this we can basically ignore just for a moment these are basically my rounding of my strings so that in my phone stops is not going to continue giving me all of this data so you can kind of ignore that but here this is where we where we get to the good stuff that is the volume sample from file so we have two nodes here that are going to be quite useful one is volume sample from file and the other one will be volume gradient from file so volume sample file is going to give me over here using as inputs that point that's my points position is going to sample in the volume and where is that volume coming from well we can either save file name so it could be a BG on disk and a BG on disk might be useful if you are using mana or Karma for rendering and then you can basically here also specify whilst we're in salt that the file name is going to come from of input two which is going to be our SDF all right so my point then is going to spit that information out into an attribute I'm calling that at which magnitude now magnitude it can be named anything you can name this you know as the F value if you want but I like calling it magnitude because that allows me to sort of you know visualize in the viewport or I can reuse it later on as well it's not a reserved attribute magnitude is not a reserved attribute by Houdini is just my naming so the volume gradient then notice the outputs the volume sample file is sort of this you know this I suppose a bit darker green that means it's a float it's a single value whereas volume gradient is going to be a bright green one that means it's a vector so even though for example exports a single float which is the value of that distance right that side distance the volume gradient is going to compute a vector that is pointing from a low value to high value so remember when we go back up let's just visualize over here template this we can kind of keep an eye on it as we can step through it right and kind of sort of see it so that means that over here this point right it's going to create a vector that is going from a low value or negative value to high value a positive value so let's let's go in and see this so we can visualize these values here in the spreadsheet we can sort of showcase I've got the gradient visualized and we can also showcase the magnitude now these are the same values that I already had right so we have that negative 0.98 it was zero point nine eight and then my actual gradient over here is going to be pointing in a certain direction now there's updates right so this is nicely sampled and if we were to feed in a hundred points it will very quickly sample that volume without having to recompute all of those those distance values because that's ultimately the point of why you would want to use the SDS you want to sample the distance rather than compute the distance in a later example I'm going to show you the difference between say using array operation with a minimum distance and a volume sample with an SD after this is already pre computed so let's get back in here so here I'm going to get rid of my my point basically or rather my sphere and we're left with just one single point which has our gradient vector and my magnitude attribute so next we can copy and I'm going to copy a line this line is aligned with the z axis it's of unit length and I'm going to create a normal on this point that is pointing exactly over here turn on the normal and we get a little closer and kind of see there's my point and there it has a normal and that normal is pointing in the direction of the SDF right so basically over here that normal is pointing in the direction of the gradient of the SDF I should say now a gradient doesn't need to be just on an SDF a gradient can also work on a temperature field and let's say you know on the inside it's hot and on the outside this is cold it's got a point from the outside to the inside so temperature fields have could be gradients any kind of scalar value whereby you have a difference so let's see we bring in the gradient over here now notice when we said that gradient vector over here has a length of 1.0 but as we can keep going forward right we can kind of see at some point maybe here for instance gradient length is 0.36 so that means this gradient value is not yet normalized so it's a good idea to normalize it right and then shove it into the normal so that way we're dealing with a normalized gradient vector on the other hand I can also display over here the length and feed that into the pea scale so this is going to be the length of that gradient vector and shove it into P scale so that when I later on copy overhere my little line segment on it it is going to be off length I mean in this case almost normalized right 0.99 but and that is not a guarantee so it's definitely a good idea that you know you basically hear 0.36 that you normalize your vector okay and that basically after a poly wire and a coloring you know we can basically sort of see it's going to show up as this tiny little line segment right that it's basically representing that gradient vector on the other hand I'm also going to do this is basically more useful I'm going to make this little red vector which is pointing in the same direction as the gradient but it is going to be using the SDF M values so notice the gradient lengths 0.98 the SDF value zero point or negative zero point two now we're going to need to take into account that negative and positive value because notice that over here right on the outside if we look a little closer you can kind of see that the gradient vector always points from a lower value to a higher value so that means from a positive value to an even more positive value or on the inside if we're over here on the inside from a negative value to a positive value now notice what is happening with my other vector my little red vector over here my red vector is going to be pointing from the outside to this inside and then over here from the inside to the outside so it's basically pointing to the surface so what I'm going to do is I'm going to make use of the gradient vector we're gonna normalize it and then I'm going to negate it so that instead of pointing outwards it's gonna point inwards now this is going to be used in collaboration with that magnitude right that magnitude can go positive and can go negative so we're almost doing a double negation when we're going to be on the inside whereby even though we're going to negate our gradient vector so it's pointing to the inside because the value over here is negative it is going to flip it around again so it's like a double negation so let's dive inside of our gradient right and see this in action so here's my point I've moved this one over slightly so that I'm not sitting right on top each other and I'm going to say map gradient too and and also update our P scale so the gradient over here same deal we're going to normalize our gradient and here's that negation that I was talking about so has our vector is on the outside right it's basically going to showcase the normal to the inside let's just visualize that normal there's that normal remedy this kind of a little hard to see but it's basically sitting right over there so that normal is pointing to the inside now the the other part of it is going to be that magnitude that I'm going to feed into P scale now remember what I was saying magnitude is not a reserved attribute P scale is the reserved attribute that is going to get used by the copy operation okay so let's keep going and let's have a look so here my copy now I'm going to copy that little line segment onto the Geo poly wire red color it so we've got now both of our line segments nice so I've got the one that the gradient on the outside and the little inverse gradient or the distance direction and the distance to the SDF displayed over here in red okay let's see what else do we have here well these three phone stops represent what I have in the viewport they basically show me the signed distance feel so that's the signed distance field they show me the SD - value so that's literally just going to use a point string a reference the magnitude string that I've already computed and then here same thing for the other gradient length as well then I have my pig's head right so my pig's head outline so notice when we start with this pig's head I don't really want it to display everything in full 3d space even though the SDF works in 3d space I want to do this brace this as sort of a slice in 2d right so it's a little bit easier to understand because it works in 3d it works in 2d is the same deal so let's make this representation of this contour and the way I'm doing that is I'm grabbing my pig's head over here right and I'm going to match a grid to it so we made a little grid I'm gonna use match size and make sure that's scaled to fit us turn on and it's going to match this grid to the size this is really useful for you know creating outlines or creating Isis or things like that then we can transform this up ever so slightly so over here scaled it by 1.1 and I've used an expression here the centroid expression together with a input path of the incoming geometry and then we have the dimensions in X NY and NZ yeah so these expressions are quite useful to have as a potential default or potentially you can say okay save them as a new preset and as you can kind of see I already have one which is called centroid expressions as a preset so it's useful just to have that would be nice if side effects added that as well but nice to have as well all right so there's my centroids then we can do a boolean operation and literally slice off the area where both my grid intersects with my pig's head and then it's going to be over here using the same operation so most of times you're doing intersections of shatter but I'm going to use the seam which is going to give me that line that is basically intersecting between the plane and my pig's head we can use that and visualize that so poly wire it and then you know change the color so that we can kind of see it in the viewport notice I've also used that later on to sort of scatter all of these little zeros on to it so that we can kind of say okay this value over here of the SDF is going to be 0 ok let's talk a little bit about the visualization of the of the SDF of the active of volumes so we said before right that we have over here this is my SDF and notice my hulls are turned on right here and they line up exactly with over here it is a little you know box now what are these blocks right well so with these blocks we can basically see with the visualize 3 node right this can showcase right over here all the active voxels now you can say solid boxes are or by default it's a wireframe right so you can kind of sort of see these are all the folks also represented by these boxes I just said solid boxes it's fine now these are individual little boxes so if you have to have two voxel eyes an object it's very easy to do now or if these sort of visualize three operation if you want to and then you can say it with the assemble node I'm going to pack this geometry now let me just go and turn each of these into its own tiny little box into its tiny packed primitive so that means if you look inside we have all these points and all of these individual boxes I'm gonna do about the operations select just the middle section over here and say okay scale the bounding box down by you know negative whatever make sure it has a minimum size offset it just in the positive direction so it's kind of slightly offset over here we're moving it a little bit and then using a group operation I could say group inside grab the points enable the bounding box enable the bounding objects now these points really represent packed primitives all right so this is really going to select all of these little cubes because we're dealing now after the assemble I turned on create pack geometry so we're dealing here with 1700 packed objects so the ones that I've got selected over here or 170 so great blast everything except for that little regions and now are left with you know our basically our voxel representation of you know these points cool next I want to sort of indicate whether they're positive or negative so we can use all of these boxes and on the one hand I'm already visualizing them so that's gonna be visualized on them over here is it unpack all of these little boxes right you can fuse them together to clean them up so there's no overlaps and then turn them into a poly wire so now this is actually tiny little poly wire challenging great next we this is part of the visualization but we basically want to say okay go and sample the SDF right so this is what I was saying instead of sampling one point I'm going to use all of these Centers of these little boxes to sample all the values so I can say hundred table sample the SDF in the second input right over here this we follow this is my same as the F again that is coming from my V DB from polygons okay sample the SDF same deal right so that will just volume to sample shove it into the magnitude for ingredients we don't actually need this but we are after the magnitude and so what can I say well we can split them now using their gradient right so or using that magnitude if my magnitude value is positive I wanna copy some plus signs onto it if my magnitude is negative copy some negative signs on it so here with this split we can also see in the first input and this input here it's going to be all the points on the outside all the positive values and I'm here on the second input it's gonna be all the negative values so let's hide everything for a second here's all my points and it really just made a plus sign and copy that to the points there's all my plus signs right and then we'll do the same thing for the negative signs over here with the thumbs up and copy all this on the inside just today you can visualize them together and then lastly that outline of my pig's head over here how are we sampled it so that we can kind of copy some zeros onto it so I made over here just a zero copy down on it so we have all of the zeros visualizing as well so if we merge all of that together you can kind of see outside zone positive inside zone negative and then on the surface it'll be zero right okay so that's so far for the visualization of all of this stuff all right so hopefully in kind of sort of start to see where this is useful in the next section I want to talk about the examples of what we can do it is the application of it right so this is good for the theory but in the next section we're going to get into the examples of it now I hope you enjoyed this kind of videos if you do please like and subscribe and see you in the next video thank you Cheers