Hi all,

Another day, another feature! The last couple of days have kept me busy creating a "physics powered" grab arm for picking up the crates. When the player gets the crates to the exit, the claw comes to get it. This is as creepy looking as it sounds. I decided to build the claw from primitive static mesh types, and bind the meshes together with physics constraints so that they acted in a hinged way, using forces and impulses to manipulate the arms.

The reasoning behind this is so that when an object is within the claws, the physics library (in UE4's case, PhysX) can automatically clamp the four arms onto the box, or whatever else is there, and lift it. Once the item is lifted it can let go at any point and let it drop (which it will do if you give it the wrong crate). The other approach to this would involve a skeletal mesh, rigging, and inverse kinematics. That set of options is a non-starter for me because I am extremely lousy at Blender, not even able to create something with simple shapes, never mind rig or animate a model, and secondly, in my opinion, inverse kinematics in unreal engine 4 is a bit awkward to implement and last i tried it, a bit unstable causing random crashes.

It took me a couple of hours to create a model i was happy with, using static meshes and some material work, and then another hour or so to fine tune the physics of the hinges on the arms, giving me a rather disturbing looking grab claw:

The claw can be controlled by two simple physics radial forces, one to draw the arms together into the centre, restricted by a maximum angle, and one to push the arms wide apart ready to pick something up (this is required so that they don't clip through the thing theyre trying to grab, and cause them to go completely nuts).

These actions are shown in the video below:

Finally, the proof is in the seeing, here is the claw in action, with some really cool new explosion effects!

Comments and feedback as always are more than welcome!