- I've examined some of the hurdles involved with scale in ZBrush in other videos and courses but the true depths of the problem have yet to be covered. In this video I'll show some of the ways in which ZBrush has difficulty calculating the size of an object and show you how to overcome them. Okay I'm starting with a fresh start of ZBrush. I'm just going to hit comma to make the lightbox go away and let's just start with a sphere 3D. I can drag it into the canvas and go into edit mode and what I want to do is make it a PolyMesh3D.
So this locks in its polygons so that it's no longer a procedural primitive. Okay so previously I looked at how the scale value in the export sub-palette acts as a scale multiplier. It multiplies the scale of the object by this number. So if we set this number to one. I'm just going to type in one here and then we import a model it should come in at the exact scale. So let's go to Import and in our exercise files let's go to the advanced scale folder and get trex 29inches and open.
I'll just hit F to zoom on the model. Let's use a couple different methods to measure this. So I'm going to come around to a side view and hold down Shift 'til it snaps and let's get the move manipulator out and let's get out of the gizmo mode so we have the old fashioned measurement tool here and I just want to measure this out. You can see it is about 29 inches long which is exactly what it should be. Let's also look in the geometry sub-palette under size. You can see it's telling us that it is indeed 29 units in the longest dimension.
Now let's try something else. Let's go down to the export and let's change this scale to 10 and now let's try to import the model again. So let's go to Import, trex 29inches and Open. Okay so what is happening here is that this number, the scale, under Export not only affects the scale of the model when you export it but when you import it as well. So a scale of 10 actually divided the size of the model by 10. So if we look under the geometry sub-palette here under Size you can see now it's 2.9 units in the longest dimension.
Now if you leave the export number set to, well 9.999 but really 10, if you leave it set to that when you export the model will get enlarged by a factor of 10 and it should end up the exact same size it was before you imported it. So it's not a big deal right? Well it does get a little bit complicated because a lot of times you want to be able to see what the size of the model is quickly by looking here in this palette and if the scale is set to something other than one then this size is going to be off and you could confuse yourself as to what the true size of this model really is.
So that's why I suggested that it's a good idea to keep the scale number set to one so every time you import the model will come in at the right size. Makes sense right? Well buckle up because if it wasn't complicated enough there are more special cases. What I've shown you is true if you're importing a model into a sub-tool that is already a PolyMesh. So if you have your ZBrush set up to start up with a sphere ready to start sculpting on for example this is how it will work. However let's go about this a different way.
Let's go back up in our tool palette and let's click on Sphere3D. So what this does is it selects a sub-tool that has not actually been turned into a true mesh 3D object yet. Its scale has not been locked in. So let's see what happens if we import a model in this mode. Let's go ahead get that same 29 inch t-rex. Let's scroll down. Let's take a look at its export scale. Okay you can see it put in a number of 14.5 basically.
Now why did it pick 14.5? Well let's go and take a look at its size under Geometry. Okay you can see it is two units in the longest dimension. Now this is a calculation that ZBrush does under the hood because ZBrush is kind of set up to work best with models that are about two units in the longest dimension. Okay, again, in a lot of ways this isn't a huge problem. ZBrush will take care of this under the hood for you. When you export it'll just multiply two times 14.5 and that will give you 29 units so when you export it should come out to the correct size.
Now all of this is terribly complicated so in order to keep things simple in the previous video I recommended importing with PolyMesh objects selected where the primitive's converted to PolyMesh3D objects. Then if you keep the scale to one when you import everything should come in at the exact size. And this is usually a good thing to do. However there are a few times when you have to get more complicated. So, for example, let's say we want to clean up a 3D scan of a large object that needs to be fabricated with a CNC milling machine.
So let's set the scale to one preparing ourselves to import a model that will come in at correct size and let's go to import and let's get the trex 1500mm file. That's a pretty big object so I'm going to hit F to frame on it. Let's just confirm that everything is correct. Okay the scale is one. Let's zoom out a little bit and I'm going to go and get my transpose line and we'll just drag this out and we'll see how long this is. So you can see our scale up there is about 1,500 units.
That's correct. Now if we look under Geometry we can see that the size it's telling us that it's 100 on each axis. Now that doesn't mean that this model is a 100 unit cube. It just means that the sliders don't go any higher than 100 and this makes it hard to know exactly what the size is. Now let's add to this another problem. Let's say that we want to export this for 3D printing or CNC milling. So we could go back into draw mode. We could go to Zplugin, 3D Print Hub and let's prepare this to export with the correct size.
So Update Size Ratios and so what this does is it tries to guess what the size of the model should be in inches and millimeters. And it's actually getting it wrong. For one thing if the model is larger than 1,270 units in the longest dimension it will try to divide by 10 to bring that number down into a range that it thinks is reasonable for 3D printing. It just assumes that I couldn't possibly want to print anything bigger than 1,270 millimeters even though there are printers that print that big and I have needed to print models bigger than that myself.
So let's hit Escape to get out of that. Now there's a couple possible solutions. One is to not export through the 3D Print Exporter and instead just simply go through Export here in the Tool palette and we can save this as an OBJ. I'm not going to click Save but yeah you could save it through here. The problem with that is if you have a lot of different sub-tools that all need to be exported into the same model it can become complicated because you either have to export them individually or you have to merge them all into one model.
And if they're really high polygon counts on all those models then it can bog down your computer and become very complicated. So the other solution that we're going to use is to just convert the unit of measurement from millimeters to inches. So I'm just going to open up my calculator here and let's set it to Converter with length. And if you don't have the Windows calculator if your calculator doesn't have a unit converter in it you can use Google.
Just type in convert whatever unit and measurement you want to whatever other unit and measurement you want and it'll convert it for you in Google. So let's set this to millimeters and I'm just going to type in 1,500 and inches it's a little bit over 59. 59.055. So let's come into our Geometry sub-palette. Let's go to the size and let's just type that number into the longest dimension here. So 59.055.
So now basically we've converted this from millimeters to inches. Now let's see what happens if we try to go to the 3D Print Exporter. We'll set this to inches and Update Size Ratios. So the interesting thing about this is that there's also a limit on how many inches ZBrush wants to expect a model to be for this 3D Print Exporter and 59 inches is bigger than they expect. So it's still dividing by 10 but what's interesting about this is that Zbrush doesn't know if this is actually in inches or millimeters.
So what we could do is pick 59.06 millimeters and then when we go to Export the file that it saves actually does not internally save what the unit of measurement is. It doesn't know whether it's millimeters or inches. So even though it says millimeters we can still assume that it's inches. So if we go to Export STL and we were to save this trex 59inches and save it.
There is actually literally no difference between that file saved with the exporter thinking that it's millimeters or thinking that it's inches. Okay so I hope this all wasn't too complicated. The issue here is that there really isn't one single workflow that will work for every situation. In order to truly work with scale in ZBrush you have to know how it works under the hood because every project is different and every approach you take with scale in ZBrush has advantages and disadvantages.
And there's always some complication waiting to trip you up but keep at it and you'll be able to get yourself through any problem.
Skill Level Intermediate
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