Nothing to Do With Sunblock
I've alluded to them mysteriously before: UVs, the mystical force that binds all images to your surface. What exactly is a UV though?
A UV is a coordinate that correlates to a vertex on your object. It is placed from 0-1 in a square grid. U is one direction on the grid, V is the other. The purpose of the naming convention is to differentiate UVs from the XYZ coordinate plane. UVs tell your model which area of the image map correlates to which portion of your model. By telling Maya "between these three UVs are these blue pixels, and these three UVs corresponding to those three vertices" you make sure that Maya puts the blue pixels on your image on the appropriate faces.
You can download the cube and texture I used for the following screens in the course download area.
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The UV ensures that your image maps are placed properly on your model.
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Laying Out UVs
When you want to add texture to complicated or multiple objects, it is necessary to build UV maps. UV maps enable you to lay out your UVs in a way that makes sense, is easy to work with, and distributes your texture properly. Such a process can (and will) be tedious and require some trial and error. It's one of my least favorite parts of texturing, but it does have the solid reward of a job well done. Besides, working with good UVs instead of bad ones is enough motivation to take the time to map them. Maya provides pretty good tools for moving around UVs. To use them, open Windows >
Modeling Editors > UV Editor.
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UV Editor. Go to View > Frame All in the editor if you don't see the grid view I have above.
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Here we have the UV Editor. The area with the gray grid is the UV window, where you can see
your actual layout. It will be empty if you don't have any objects selected, or may show some white
lines if you do. If you downloaded the cube it will look like the above. The light gray area shows where
your actual UV space is, the dark gray area you can think of like a virtual table top to keep yourself
organized while you work on various pieces of your model.
The area highlighted in red has some icons to change the display of your UVs. This is crucial to being able to make sense of the sets of overlapping lines in a 2D space.
The area highlighted in green is the UV Toolkit. This is just like the modeling toolkit: a bunch of tools and menu options organized into sections for quick access. Note that in versions of Maya prior to an updated copy of 2017, the Toolkit will not exist and you will instead see all these options as small icons near the top like the display options.
Maya primitives come with their own UV layout. If you select one, the UVs will show up in the editor. The cube we've been using has the default set of UVs, and looks like an upside down "T". Other primitives might have grids or circles instead. Turning on shaded UVs by pressing the Shaded UVs icon in the display options area can make it easier to see what is part of the UV shell and what is just background in the editor.
Think of the box as a present. If you were to unwrap/unfold a present and wanted to keep all the
wrapping paper together, you might do it like the diagram above. Each square above represents a face
of the cube, and if you folded up along all the shared edges, you'd get a cube. When an image is applied, like the one above, a different color corresponds to each face of the cube. If the UVs are moved, the contents of each face changes:
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The UV is moved to the right, shifting the placement of the image map. UVs shown shaded.
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As you can see, laying out your UVs to match your image, or creating your image to match your UVs, is important. So let's get started with something a bit more meaty than a cube, but still pretty basic.
Mapping a Simple Object
For this next example, I'm going to lay out UVs for a soda can. I made this can by starting with a cylinder and then used Multi-Cut to insert edge loops and Extrude to add in detail. Follow along if you want, by making a simple can yourself or just choosing a primitive cylinder with some divisions from top to bottom.
Although things will look slightly different for each object, the process is the same. If I select my object, it shows up in the UV Texture Editor. Generally, on complicated models this looks like a mess of lines. Since I started with a primitive cylinder, and the object isn't too complicated, the original UVs of the cylinder have been preserved a bit, and it looks like a grid with a couple caps. Although our final layout will have similarities, this grid contains a UV point for each vertex in the can. That means there's a lot of overlapping and squished UVs here. I've clicked the UV distortion icon to help show what areas are actually overlapping. You can see some red up by the rim of the can where the extrudes dont have any UV area of their own. That's gotta change.
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Soda can viewed in the UV Editor.
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To move our UVs, we need to make sure we have the right component mode on. The UV Toolkit has icons on the upper right for selecting vertices, edges, and faces, but also for selecting UVs and UV shells (connected groups of UVs). With the appropriate UV component mode on, you can push them into place them into place with the tools you already know (W for move, R for scale, and so on). Moving UVs is the most basic way of arranging them in your UV space. As you saw from our cube example, a simple translate action can drastically change the results of the texture.
You can select your components in either the viewport or in the UV texture editor, depending on
what's more clear. It's usually easier to see what you're selecting when you select in the main view, but
selecting in the UV Editor is fine too. Note that you can only move UVs in the UV editor though, and
you can only move the other components in the normal viewport. If I wanted to adjust the UVs for the
top of my can, I might drag a box around the top of the can in the side view, but then I'd have to move
to my UV Texture Editor to make any adjustments.
One thing that's useful is the ability to convert to UVs from another component selection. If you
make a selection of one kind of component (say, faces) and want to select the UVs associated with
those faces, Ctrl+right click them, and select To UV. You can also use the standard vertex selection
shortcuts on UVs now, such as double-clicking adjacent points to select a ring, double-clicking your
whole object to select an entire connected group, and using < and > to expand or contract your
selections.
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Ctrl-Right Click makes converting selection back and forth easy.
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Cutting and Sewing UVs
Some UV sets are just fine to start with, and all you'll need to do is scale them up to fit more of the space, or rotate and adjust a few points. Others, like this one, will require a bit more work. If we apply a quick checker texture to our object (click the checker icon in the display area of the UV editor), we can see that some areas are a bit strange looking--parts are not checkered, but appear streaked or distorted. This checker option is great for identifying areas that need work. Ideally every face on your model will have an even size square checker.
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Without proper mapping, our checker is anything but.
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We'd better do this by hand. To ensure a process that works for any model we will start with an
entirely new set of UVs. There are a lot of options to choose from as starting places in the Create section of the UV Toolkit. Each of these options looks at your model from different ways and maps what it sees to a 2D plane. Each object will be a little different in terms of where you want to start. Since this object is a can, Cylindrical is a great choice, but the most flexible way to start building your own UV set is with Automatic Mapping. This looks at your object from all directions, and splits up the UVs into sections. This means you'll get a lot of pieces (also called shells), but at least no faces will be flat or distorted like the top of our can was.
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Automatic mapping creates the most shells but the least distortion, making it clear where every thing is located.
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Now the issue is we have more pieces than we need. Having too many pieces is tricky, because
pieces can create "seams"--areas where textures don't line up properly due to a break in the UVs. Ideally for this can, I'd have a top piece, a bottom piece, and one "unrolled" piece for the sides. We need to do two things here: Show the seams on our object, and then stitch the pieces together.
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Seams here are caused by the joins in the UVs not overlapping the checker texture properly. They can be avoided by minimizing unnecessary breaks in the UV's shells, and by proper placement of those
shells.
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To show the seams, I go into Window > Settings/Preferences > Preferences, and click the Polygons
option. There is a checkbox there (under Highlight) called Texture Borders. Turning this on highlights
borders, making it easy to see how your UV pieces correlate to your object. Additionally, you can turn
on borders for just one object using Display > Polygons > Texture Border Edges. Now, I can select the
pieces I need either in the view or in the Texture Editor.
Occasionally, selecting an edge on the object on a seam will appear to select two edges in the Texture Editor. This is actually because the two edges are the same edge. They can appear on different places in the UV editor, because each face can have its own UV coordinates. Edges that are on seams will be in
two different places. It is these edges we need to sew together to remove seams and form a single
object.
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Seam shown in the main view and the UV Texture Editor.
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First, I want to sew up my sides into one big sheet. I can sort of see where the sides are already, in my UV layout, but to make sure I select all the right edges, I'm going to select them in my world view. The same faces become selected in the UV editor. Now, I can connect these edges by clicking the Sew
icon in the Cut and Sew section of the UV Toolkit. In this case, the edges just snap together and become one fairly even surface. You can also use the adjacent Sew Tool which lets you click and drag around your model to sew together any nearby sewable edges if you prefer.
On the next seam though, if we do the same, all the edges become connected, and you can see a bit of crossover occurring in the UV Editor. This is because some of the sections were flipped by the
Automatic Mapping.
You can fix this by first selecting the shell and using the tools in the Transform section of the UV Toolkit to rotate the shell 180 degrees before sewing. For now we are going to ignore this weird
flipping though, because we are going to explore a couple other tools.
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Often you'll want to keep your shells at easy to work with 90 degree angles..
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Once we have sewed up the seams in the can, we are going to want to actually make a new one
ourselves. Why not just leave one seam un-joined and save ourselves the trouble of un-doing work? Well, seams are best when they are as hidden as possible. This prevents trying to line up textures in obvious areas. By making your own seams, you can place them strategically, like at the back of the can, or in little creases. To make a new seam, simply select the edges you want and click Cut in the UV Toolkit. You should see the edge become one of the fat border edges but no other movement in your UV layout. Now we have a tube with one seam where we want it, but its a bit of a mess, so let's talk
about ironing out tangles.
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The purple faces are a sure sign of UVs that need untangling.
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Unfolding and Straightening
Alright, so we have a possible tube for our UV set. But how do we get it flattened out so there's no overlap? We could move those points and lines by hand, but man, that is time consuming!
Luckily, Maya has an Unfold tool. The Unfold tool tries to unwrap your object like a package, preserving relative surface area. However, in order to do this without distortion, it needs some seams in your object. Imagine trying to peel an orange and lay it flat. Unless you cut some notches in the orange, the skin will be all wrinkled and bent. This is why we cut our tube open. To perform an unfold on your object, select the UVs you want affected and click on the Unfold button in the Unfold section of the UV Toolkit. You can also perform this on your whole model, but generally you just want to do one piece at a time to prevent Maya from messing with your other shells that you've laid out.
You'll notice it's a bit tilted now. I could rotate it into place. But let's explore Unfold a bit more. When you only select a few UVs Maya will unfold the selected ones to match the unselected ones. This means you can force it to arrange the shell in certain ways by playing with the borders of the area you
want to unfold. I can take the corners of the shell here and placing them on the grid. I can then select every UV in the shell besides the corners and unfold again. That's a little awkard though, and we already have the corners where we want them, so let's Pin them. Pinning prevents you from moving UVs and shells by accident, or with operations like Unfold. It works exactly as you'd expect, clicking Pin with a UV selected will lock it in place until you unpin it.
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Guiding the unfold with corners and pinning. |
The last unfold creates a kind of puffy pillow, since Maya is trying to minimize distortions in
the mesh, and the top and bottom of the can are smaller than the sides. This is great for distortion but we have some real world considerations here. We need to be able to work with these UV layouts to easily make our textures, and often that means we are working in image editing software that plays nicer with right angles and nice grids. (In rare cases you will be only painting textures in a 3D sculpting application and care less about these restrictions, but it pays to make good habits now).
This is an easy fix now that the UV toolkit contains a Straighten UVs tool. This takes the selected UVs and tries to force the layout into a nice square box, prioritizing position and only secondarily distortion. It's essentially the opposite of the optimize tool. Note that occasionally UV locations are too far from being a nice grid, and you'll have to drag a few points around to get the tool to understand exactly where you want the layout to be and re-straighten. You can use the Align and Snap tools to make this quick, or start with an Unfold first like we did here.
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The last unfold on the left leaves a pillow shape that is low distortion but difficult to work with in Photoshop. Straighten fixes that into a really easy to work with grid. |
The same process (Sew, Cut, Unfold, and sometimes Straighten) can be used on the rest of the pieces. For instance, the tops can just be sewn together and Unfolded. Unfold works beautifully on radially symmetric objects. The rim of the can illustrates a place for personal preference. It can be sewn up into the top and easily unfolded to create a round border. This reduces the number of seams and shells and simplifies the layout. However, if detail is needed on that rim with a texture, it is often easier to work in a strip than in a loop. Also, the rim is actually a different object from the can, so having a texture border can actually help create a sharp edge on the texture map instead of having to line up the two different colored pixels exactly (this can also avoid blurry streaking that occurs in low resolution filtered textures). I chose to unfold the rim much like the can, but keep it separate. Since the seam is at the underside of the rim and in the crease of the top, it shouldn't matter too much that we have an extra break, and it will help in the Photoshop work.
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Unfolded rim, two different ways.
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The final thing we want to do is arrange our pieces to optimally fill the space. Unused space is just wasted pixels. I select each shell and scale it up, then arrange them as best I can. The tools in Align and Snap and Arrange and Layout can both help with final shell arrangements, just make sure you are working in UV shell mode so you don't accidentally tear a shell apart. Finally, my can is ready to have
texture applied! Now just imagine doing this for a complicated object... oh wait, that's the exercise.
Excuse me while I stifle maniacal laughter.
Now that the UV is done, it's time to apply our map. It can help to take a snapshot of your UVs and bring this into Photoshop so you have a reference layer (a reverse rotoscope). There is a button to do this automatically, the small camera icon near the UV display options at the top. For this one, I took a coke label, and an image of a can top and bottom and brought them into photoshop with my snapshot to assemble. I made the strip on the rim by sampling the colors from the top image. The end result is a
suitable texture for a game prop:
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The Coke can images are arranged to match the UV map, and therefore match the can. Success! |
So far so good. But a single soda can isn't super complicated. To help you understand the strategy of attacking a larger object, let's go to a more complicated and organic model. In this case, I'll use a chef character from a former project.
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Never trust a skinny cook.
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Right now, the chef is one object (I Combined it for export). Since it already has a UV set and texture, I'm going to reset it by choosing Automatic Mapping. In general, this is a good place to start. It splits things up into mostly undistorted pieces, which are usually identifiable. Unfortunately, on complicated objects it also produces a hideous mess. Let's talk about organization.
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Automatic mapping produces a bunch of shells. Some shapes are recognizable, some aren't, but at least everything has a starting place now. |
The easiest way to do this is to select a set of faces on the model you wish to work on. You can also select a shell with the Select Shell option in the UV Editor toolkit. Then choose Toggle Isolate Select Mode by pressint Ctrl+1, or clicking the Isolate Select icon in the display area of the UV Editor. This in turn hides everything but what you have selected. You can then add to the set any UVs you want by selecting faces or UVs in the viewport (since they won't be visible in the UV Editor of course) and choosing View > Isolate Select > Add Selected. I often find it easier to toggle the whole mode off, add to my selection, and turn it back on again.
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Isolate Select lets you work on just one part of a mesh, like this hat, without worrying about the rest.
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This option is great for things like characters, which are often just one piece. However, for mechanical models, there is a simpler way of hiding unwanted shells: separate them. The Mesh > Combine and Mesh > Separate tools (in the menu bar of the main Maya window, not the UV Texture Editor) will preserve your UVs, and since Maya only shows the UVs for the current object, you can make little UV groups by temporarily joining and unjoining objects.
I'm going to do this on my chef now, since it has a lot of discrete parts. As you can see, it makes things easier.
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After a Separate action, the various parts of the model are easy to select in isolation, and only those UVs show up in the editor. In this case, UVs for the pants and left foot.
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Now I need to pick a place to start. I generally choose my most prominent features first, then work down to the details. In this case, I think the prominent features are the main body, the face, and the apron. The feet, hat, and mustache will be easier to fit in later. Since it's the biggest area, let's start with the body.
The body shape is simpler if we break it down into a few areas: The torso, the arms, the apron, and the tails. The apron and tails are more flat, like planes, so we will start a little differently on those, and do them second. The torso and arms are much like a cylinder, so we can approach them a little bit like our coke can. However, unlike the coke can we have a significantly fatter middle, and we also have the pleasure of having this character wear clothes. Clothes are great because they naturally have seams, so we can put our UV seams where the seams on the clothes would be and no one will be able to tell. With that in mind, I think I want the front of the torso to be one piece.
There are two ways to go about this. We will start with one and do the second way on the apron
and tails. The first way is to approach like the coke can. Sew together all the seams in the torso, and
then place the cuts where you need them for good unfolding. So first I select all the UVs in the jacket
and choose Sew UVs. This makes them one messy shell.
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Sewing everything together makes a mess, but we can untangle it in the viewport and clean it up in the editor.
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Then I select sets of edges that run where the seams are: Around the shoulders and under the arm pits, across the shoulders up to the neck, and down each side. Normally, I would do these one edge loop at a time, but the image below shows them all highlighted for illustration sake. Clicking Cut UVs will separate the shell at this point. If I don't choose edges that connect to each other, I won't end up with discrete shells so I can unfold.
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Choosing where to place a seam is important. Look for natural breaks or hidden areas.
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You can confirm your cuts make the shells you desire by turning on UV Shell selection mode in the
UV toolkit and clicking in the big mess that are the current UVs. Highlighted areas should crop up as
you had planned, and you can drag those shells off to the side to their own area for the time being.
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The highlighted shell shows up on the correct faces in the viewport, so I know I have the right thing to work with. Tangle to the left is the rear of the apron, both arms below. |
The next step is just to unfold each section and adjust orientation and position. If your mesh is
completely symmetrical, you can use symmetry and/or the Symmetrize functions in the UV Toolkit to
ensure UV layouts are identical on each side, but for a character that is rarely the case. Fortunately, if
your topology is mostly the same, and unfold action will generally lay out shells in such similar
fashions as to be acceptable, such as in the arms below. No more tangle!
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Unfolded jacket parts look clear and identifiable. Tangles busted!
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The face is a very important area, so moving on to that next the approach is the same: Sew up
all the unwanted seams, and make new ones. Since the face is very scrutinized, I kept the seams
entirely to the back of the head, using one down the middle but not entirely to the front brow, and one
around the back of the head. You can see the seam in the rear is clearly visible. This was a choice
between a more distorted face mesh and harder to see seam (tucked in the fold of his neck maybe) vs a
less distorted face but clear seam. I chose the visible seam since the textures on this part of the
character are fairly simple and shouldn't be hard to get to match well.
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Location of face cuts on left, results of unfold on right.
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You'll notice this looks pretty good...OK, it looks creepy. All unfolded faces look creepy. But also there is something strange with the mouth, notice how wide open it is. That's because this character has a "mouth bag", an area of geomety that is inside his mouth so that there's something there when his mouth is open. Generally any area like this that won't be seen often doesn't need the same kind of detail that the face does, so let's cut it off. Plus it will make our face unfold better.
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Any significant interior spaces should be removed from the exterior side to aid in both shell layout and texture space.
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Moving along a bit faster, let's look at a new piece: the apron. The apron we said we were going to do a little differently. Although we started with Automatic Mapping, that's not always the best choice. Sometimes a shape is better suited to a different sort of layout, and we can use different mapping techniques for different objects, or even different faces on the same object.
Since the apron is pretty flat, I'm going to use a planar projection. This is one of several modes that work with flat projections. Best Plane and Normal based attempt to find the best fit, while Camera Base let's you pick the angle by using the direction you are looking at the model through. Planar just picks one of the three axis, but frequently that's all you want as a starting point. Because we have selected all the faces in the apron front, the resulting projection has no seams and the added benefit of being less of a tangle. This can make working with it a little simpler.
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Projection modes can be used to start closer to your desired final outcome for a UV shell.
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Now that the apron is sort of flat, we still want to cut the back side off of it since it won't be seen nearly
as much as the front side. A quick select of the edge loops, an unfold, and done!
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Separating the front and back of the apron, then unfolding. Note the back of the apron has less map area currently, as it is less visible.
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In some cases, a planar projection is sufficient, or a planar projection followed by a simple unfold. The buttons for instance. Because the buttons are curved, automatic mapping splits them up into several pieces, which isn't really necessary for simple objects. In this case a simple projection unifies the button, and an unfold removes any distortion caused by being slightly off on the projection angle.
This actually gives us a great opportunity to explore another strategy for making efficient UV layouts: transferring UVs from one object to another. Frequently models will have items that are identical. It makes sense to duplicate features around the model, and if you or your modeler was exceptionally savvy they might have made UVs for that object before they did all the duplicating, but sadly this isn't often the case. Luckily, if objects are the same, we can use Transfer Attributes to save ourselves from laying out a dozen buttons.
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These buttons are the same shape. Top is the button that was just unfolded, bottom a button that still has automatically generated UVs.
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To transfer attributes between meshes, select the source mesh, then the target mesh, and choose Mesh > Transfer attributes. Note that this is an active node in the history of the object, so changes to the source model will be reflected in the destination. This means if you wish to move the UVs after the transfer, you need to delete the history on both meshes.
You'll want to make sure two options are set: Only UV sets is transferring, and the sample space is set
to Topology. You can transfer a lot of different things and sample in different ways, but for objects with
the same geometry, Topology is the most reliable choice. Once the options are set, you can apply and
you should see a messy set of UVs disappear. The target button now has the UVs of the source button
and they are overlapping, which brings us to a conundrum: What to do about overlapping UV areas.
Generally speaking, overlapping UVs is not ideal. A lot of options work best when everything on your model has a 1:1 correlation with some unique space on the UV map. If this is true, your model will be able to be painted in a 3D painting application, have lighting or other environmental stuff baked on to it's textures, and have all bump and normal maps work appropriately. Because of this, when laying out shells, it's best to let each shell have it's own area and not overlap at all. There is even a setting on the Move UV Shell tool to prevent this, and most of the layout tools will layout all the shells without overlapping even one pixel.
That said, sometimes you don't care about baking textures or flipped normal maps. Sometimes you have a hundred tiny objects and would just rather they each get 100 times more texture area than fill up your map with a million dots or all require their own draw call for their own texture. In these cases, stacking up your shells is the best solution. Shells that overlap will use the same texture data, since they are in the same place on your map. So clearly this can only be used for objects that are identical. If you have transferred attributes between similar objects they will already be stacked up. If not, using the Orient Shells and Stack Shells, or combo option of Stack and Orient will suck a bunch of shells to share the same mid point and rotation values. You can then select them with a marquee select
and move all dozen of them around wherever you want.
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Stacked items will have a bolder appearance. Unstacked items are best oriented and aligned the same anyways to make map creation easier.
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Now that we have gone through the biggest parts of the chef, I won't bore you with repeat
pictures of every little step, let's just do a quick run down of how each of the other parts were
approached.
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Legs: Sewn together, then cut down the middle, and along the inside edge of the pants. Unfolded one leg, then used Symmetrize to match the other side
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Feet: Cut along the border of the sole, and split along the back. Tops and bottoms unfolded, and UVs flipped to other side with transfer attributes. Deleted history so each foot can have its own space.
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Mustache and Eyebrows: Cut the backs off each part, unfolded each piece. Mirrored to the other
side.
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Hands: Cut along the midline between each fingers, like tracing out your hand on to a piece of paper. Unfolded into back of hand and palm areas. Note that the hands always end up kind of
distorted and weird just due to the complexity of the shapes involved. In the case of this chef, I was painting textures in a 3D application and unconcerned about the specific shapes, but some level of straightening on fingers after unfolding can be useful for texture painting in a 2D setting.
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Once all the parts are done, you'll have a mess of shells stuffed here and there around your UV space. Packing the shells up into the 0-1 space in your UV map is a bit of an art. For a character like this it's best to have areas that will see a lot of visibility have more texture detail, like the face, where as areas like the shoes should be proportionately slightly smaller.
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Shells stashed all over the UV workspace ready for final arrangements.
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A good place to start with the Gather Shells or Layout tools. Gather Shells just pulls all the outer shells into the 0-1 quadrant, which is great if you've kept stuff at specific scales and just want to pull them
around yourself to layout. In a situation like above though, sometimes the best option is to use the Layout button. Layout will scale, translate, and rotate your shells. Layout can also approximate the proportions based on actual model surface area, so generally creates fairly good starting places for maps.
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All the shells efficiently using space, but not arranged well for human use.
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The layouts automatically generated are generally very good at using all the availible map area, but can sometimes be difficult to work with when editing textures. Often shells are very close together, which means it's easy to accidentally overlap a texture element, and shells that you might want adjacent
(both shoes, for instance are frequently stuffed on far corners of the map. The best solution if you are
going to be editing this texture by hand at all is to start from a Layout (or a blank page if you prefer)
and then re-arrange the shells so that things are organized in a sensible way.
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The final layout has all the skin tones at the top, jacket in the bottom left, and proportions suitable for how visible and important each element is.
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Before I leave you to your Exercise, let's just quickly recap and go over some of the strategies you
should be thinking about as you lay out UVs.
Space
UV space is precious--there is exactly one grid for you to arrange your stuff on. Important areas may demand more of this space. Human faces, for instance, may take up a good ¼ of your character's UV map, while areas that don't get seen at all may get scaled down to just a few pixels. Weigh the size of everything relative to its importance, and arrange your UVs from there. Think of items as having a certain percentage: When one item grows, others must shrink. An object given too little UV space can appear pixelated or blurry in the final product.
Resolution
The available UV space can be augmented by the resolution of your image. An image that is 512
pixels square has ¼ the space of an image that is 1024 pixels square. This means your objects on your
UV map could take up ¼ the space and still have the same resolution. Bigger maps, however, require more memory, and are a luxury.
Packing
Since space is important, how the objects are arranged is important as well. Rotate triangular shells
to form squares. Arrange rectangular objects close together so they pack flat. If you have a large ring or
circle, put smaller shells into the center so the space is not wasted. The Layout tool option in the UV Toolkit attempts to do this for you, but the best results come from a human touch, so tweak away.
Overlap
Overlapping is generally to be avoided. Any areas that overlap will have the same texture, which is
not generally advisable. However, if a part of your object has the same features as other parts, and you
want the same texture on it, putting all the UVs for those areas on the same part of the map can get you
more pixels on your object than are on your image. That's some good efficiency. If you need to, you
can also lay out symmetrical objects "folded" in half and overlapping. Keep in mind that one will truly
be a mirror of the other, however, and features like bump mapping don't work well with this (they form a seam, because the bump presses into one side, and out on the other).
Additionally, if you wish to keep things visually discrete, a UV placed at .5 is exactly the same as a
vertex at 1.5. So if you had, say, two bicycle wheels, but wanted to see them both, you could place one
in the usual box, and move the other exactly 1 unit to the right. The texture just repeats for any value higher than 1, so it will display the same.
Note: UV space really only exists in the upper right quadrant. The other quadrants are really just illusory. They exist only so that you can put things in the same UV space, but not in the same visual space. Any map can be displayed to repeat across all quadrants, but you can't have, say, a different map in each quadrant.
Duplication
If you've laid out UVs for an object and it was a pain, you'll be happy to know that duplicating an
object preserves its UVs. So if your model has dozens of rivets, layout your UVs on the first, then duplicate that one to produce your rest. Additionally, you can transfer UVs from one object to another using the Mesh > Transfer Attributes function.
Shells
Keep your shells logical. It is obnoxious to paint a texture onto a face if that face is in multiple pieces. People usually divide up their models so that it is easy to layout the details in Photoshop. This means that things are at right angles, broad surfaces are contiguous, and seams occur at corners and crevices. Arrange objects with as little distortion as possible, like the soda can above.
Applying Multiple Maps
One last thing to note: You can apply multiple materials to your model, as well as have multiple objects share one material.
To apply multiple materials to your model, select the faces you want the first material applied to, and
apply materials to the selection. Select the rest of the faces, and apply a second material (or third, fourth, and so on). You can now lay out UVs in two separate sets (for example, face, pants, shoes, and belt in one material; jacket, hands, backpack, and pager in the other). The advantage here is you can use double the texture area (two 512 maps are 524k pixels) without moving up to quadruple (a 1024 map is 1048k pixels). Another consideration is when two materials have vastly different properties. A character made of metal and stone might use two materials with different shading properties, and to save on calculation use simpler maps for the simpler non-reflective material.
To apply one material to multiple objects, simply assign the material to the objects. Then, lay out
UVs for each one individually. Selecting all the objects will display them in the Texture Editor, and
allow you to see their relative position. This approach lets you maintain each object separately while you work. Separate and Combine can be used to lump up different meshes while you work as well.
Note that if you are exporting to a game engine, you will sometimes not be able to apply more than one texture to any object. In this case, splitting up your model with the Mesh > Separate function and then applying separate materials to each half might work better.
Learn about the role of UV maps in model texturing.
