How to Create a Retractable Wrist Blade: Part 1 - Modeling
Whether you're working on a video game, animated short film or movie a large part of the production is going to be creating all the assets. When you think about it, everything that inhabits the world, whether it's in your movie or game engine has to be created by a 3D modeler. In production, large teams of artists are tasked with asset creation, this can be creating everything from a rocking chair in the corner of a living room or the weapon the main antagonist wields. Being able to create great looking assets that not only fit within the art style for the project, but can be completed in a short amount of time is vital for any 3D modeler. In this article, we're going to walk you through the workflow for creating a weapon asset in Maya. This article will be split up into a few parts; this part will cover the modeling process. So the weapon we are going to be creating is a retractable wrist knife or 'hidden blade', you know, the kind you might find in your favorite video game. The knife is concealed until the wearer is ready to attack, perfect for those stealth takedowns. The weapon is actually going to serve two purposes, the first is that it will be good protection for the characters forearm, as well as an effective means of defense. Since the knife is going to be hidden until it's ready to be used we know there is going to be some rigging involved. Not to panic though, the rigging will consist of a few simple set driven keys that will drive the extension of the knife. When first approaching the model it's best to start with the most important pieces of the asset. Work biggest to smallest, or most important to least important. While every part of the asset is important, there are things that will need to be modeled first, as well as overall shapes and silhouettes that need to be established. For instance, you don't need to model the knife first, without the mechanism it protrudes from. Because I know this is going to be wrapped around a forearm it's helpful to bring in another mesh to use as a guide when establishing the shape of the wrist guard. If you already have the character model created, then bringing in the asset to help you set the size for the weapon is extremely helpful and will save a lot of time in the end. In this particular scenario I brought in the character asset from the Creating a Sci-Fi Hero in Maya and Silo tutorial. There is no right or wrong way when it comes to creating assets, you can use a box or edge modeling workflow. Often, you'll use a mixture of both, which I found myself doing quite often when creating the asset. There are certain pieces that make sense to create by dropping in a plane and extruding the edges, and others that will be much quicker by bringing in a cube or cylinder. It just depends on the situation. The first two pieces of the weapon I created was the shape for the area that the knife sits in, as well as half of the forearm guard. The forearm guard was creating by taking a cylinder and setting the number of subdivision axis to 18 and deleting half of the faces. Once those were selected and deleted you can go in and select all the faces in the middle, and delete those as well. This leaves you with a half circle made up of a strip of polygons. You can begin tweaking vertices on the cylinder, adding in edge loops as needed to establish the rough shape. You can see the final result with both the knife portion created as well as the top half of the forearm guard. At this stage we're not worried at all about the small details. We've not added any thickness to the forearm guard yet, that can wait until later. You really just want to create the rough shapes first and work in the details later. You can repeat the same process you did for the top portion of the forearm guard for the bottom half. Now, we could have created a cylinder and kept it complete, and used it as the entire forearm guard, but I wanted it to be obvious that these are two separate pieces of material. From the other side of the mesh you can see how the top of the guard overlaps the bottom, and it's a slightly different shape. I've also selected all the faces and extruded them out slightly, creating the thickness. The area that the knife sits in has four different shapes that extrude from both sides. These will eventually connect to the straps, but they need to be a separate piece of geometry from the main area that holds the knife. To do this you can create an edge loop right where you want the pieces to be separate. Once that is done go in and delete all the faces and bridge them back together. After that add another edge loop in the newly separated piece of geometry and create a cut between it, but delete the two middle faces and bridge the sides together. Next, we can create the straps that hold everything together. The easiest way to do this is to use the CV Curve tool and draw out a curve around the top of the forearm guard. Once you're happy with the shape of the curve, create a cube, scale it roughly into the size and shape you want and select the front face and extrude it along the curve, setting the subdivisions to around eight, or whatever works for you. This will give you the basic shape of the strap, and you can duplicate this piece to create the other three. At this point we have the basic structure of our retractable knife, all the major pieces are created, while it's really just a rough block in at this point we can begin to go back and start adding in those smaller details to flesh our weapon out more. For instance, I've created a small metal plate that fits on top of the knife mechanism. This can be created by dropping in a plane and extruding the edges to create the basic shape you want and adding some thickness to it. Next I went and added a bit more detail to the forearm guard by adding a ring of edge loops around the sides of the mesh and using those new faces to extrude out some detail around the edges of the model. You can repeat this same process for the bottom piece as well. I did the same thing for the straps, creating a ring of edges and extruding them out. You want to be conscious of how your model is going to look once it's smoothed. It's a good idea to periodically go into smooth preview by hitting "3" to check your model and ensure you have edge loops in the right place so that your model maintains its basic shape once it's smooth. In the area where the knife sits, I went back over it and began adding a few more final details. Remember, we've done a rough block in of the mesh, so now we're going over everything again and adding in those smaller details. I extruded the faces in on all four sides of the mesh, as well as created an indention in the metal plate. Up until this point, our straps have just been sitting there and not really attached to anything. While we want to make sure our model looks "cool" or appealing, it still needs to be functional. So, we need to create a type of buckle area for where the strap connects to the plate on the front. Select the faces at the front of the strap, begin extruding those out, and wrap it around the ring. There are two ways you can do this, create another CV Curve and extrude the faces along that curve, or in this case, I just did it the old-fashioned way and extruded a face, rotated slightly, extruded another, rotated slightly, etc, until I created a loop. Either way works as long as it gets the job done. You can also see that I created another ring a little further down the strap to "hold" it together. Of course, this is not a functioning buckle, but looking at it, it makes sense. While it may not hold up to very close scrutiny, most people are never going to look at it that closely, especially if you're going to use this for a game model. The last thing that we need to do until we get to the fun part of creating the knife and working rig is to create one more piece that sits inside the entire weapon, which acts as the palm guard. Since this knife is going to be extending out right near the wearer's hand, they would probably want a little protection there so they don't slice a few fingers off by accident. It's also going to be the place where the strap to activate the knife mechanism is fashioned to. This was created by first dropping in a plane and using the edge modeling method to flesh out the shape. As you can see it extends to both sides of the weapon, however, toward the back it's actually a slightly different material. The area that is extruded out on the back is actually metal, where as the palm guard is going to be more of a thick leather material. Because of this, I separated the mesh at the middle, that way you can insert the edge loops as needed to create the hardness of metal, and keep the other side softer. The knife is going to be made up of two parts, the housing for the knife to sit inside, and the knife itself. It's going to telescope out as it extends and be driven by a spring. You can create the knife by dropping in a plane and extruding the edges to create the basic shape. The style of your knife might differ, but I kept it very basic with serrated edges, similar to a combat or military knife. To create the indentions in the knife drop in a series of edge loops, select every other vertex and drag it up slightly. In order to keep its sharpness when smoothed add in edge loops on either side. The area where the spring mechanism sits was created by dropping in a simple cube, shaping it slightly and extruding the middle faces inward. You can create a spring by dropping in a helix and adjusting its radius, height, and number of coils. The spring is really an aesthetic choice, you can have everything hidden inside this area, or create the spring to show how the knife works. The last step is to create the set driven key that will drive the opening of the knife. To do this first drop in a NURBS circle and position it somewhere in front of the weapon and add in two new attributes "Sheath Open" and "Knife Open," Since these are the only two attributes that will need to be animated you can lock and hide the rest. Now you want to move everything inside of the area where the knife rests until it's ready to be extended. Place the actual knife inside of the spring mechanism, which I call the "sheath" for lack of a better word, and scale the spring down. First select the sheath, go to Animate>Set Driven Key>Set... Inside the set Driven Key options select the knife control and choose Load Driver. Highlight Sheath Open, and the Translate X inside the driven, and choose Key. Select the knife control and change the Sheath Open value to 10 and translate the sheath to its original position where its fully extended out and go back into the Set Driven Key options and choose key. Repeat this same process but this time for the knife. So select the Knife Open as the driver, and Translate X as the driven and choose Key. Change the Knife Open value to 10 and translate the knife until it's fully extended. Finally, we just need to apply the spring to the Sheath Open attribute. So open up the Set Driven Key options again, load the Knife_Control as the driver, and the spring as the driven. With the Sheath Open highlighted in the driver, and Scale X highlighted in the drive, choose key. And then change the Sheath Open value to 10, and scale the spring back out until its fully extended, and choose Key again. Now when you change the value of both the Sheath Open and Knife Open attributes to 10 the knife is fully extended, and at a value of 0 it's back inside the mechanism completely hidden by view. The very last thing to do is create the strap that wraps around the thumb and index finger. This can be done by extruding a cube along a CV Curve, just as you did for the previous straps. With that, we have a completed model. The knife retracts through the set driven keys we created, making it extremely easy to animate. In the next article we will take this same weapon model through the texturing process so be sure to check back to the Blog for more updates.