Collision Data and Cels: Tying Frame Data, Animations, and Hitboxes Together

In Night Sky Engine, the concept of a Cel is what ties a character's frame data, animations, and hitboxes together. As such, it's the most vital data for a character. I briefly touched upon it in Getting Your Bearings from the Quick Start Guide, but here, I'll explain in more detail how Cels work and how to make new Cels.

Frame Data

A Cel can be thought of as a sort of "keyframe". They're named as such to invoke the idea of an animation cel. When combined with a State Blueprint, the Cel determines how long the character's current hitboxes and pose lasts for.

The reason such a system is used, and not something like a frame data timeline (such as using Unreal Engine's Anim Notifies), is that it makes adjusting the animation to match the frame data incredibly easy. For example, if you have a punch animatoin, and put a Cel at frame 0 (punch start) and another cel at frame 10 (punch fully extended), but then decide that the punch should actually have 4 frames of startup instead, you don't have to edit the animation, reimport it, modify the Cels, and the State. Instead, you can just set the duration of the Set Cel and Blend Cel node in your State blueprint to 4.

Animation

This section assumes that you have an Anim Sequence in Unreal Engine for your character. If not, here's a link to official documentation for animation importing.

In the Collision Data asset of your character, each Cel has associated animation data.

This screenshot has quite a bit going on, so let's break it down:

• Anim: An array of animations for your Player Object. It's structured in an Array format to allow different animations on different Components.
  • Part Name: This is the name of the component that will be assigned the animation. For example, if you have a Skeletal Mesh Component named "Body", it will use the given animation.
  • Anim Sequence: For Skeletal Mesh Components, this is the animation that will be used for the given part.
  • Flipbook: For Paper Flipbook Components, this is the flipbook that will be used for the given part.
• Anim Frame: The frame of animation that will be used for this Cel. This depends on your animation's frame rate.
• Anim Blend In: For Skeletal Mesh Components, this controls the "blend in" time. This is used for when the animation asset is different from the currently playing asset.
• Anim Blend Out: For Skeletal Mesh Components, this controls the "blend out" time. This is used for when the animation asset of the next Cel is different from this Cel's.

Hitboxes

Finally, every Cel can have an arbitrary number of boxes. The Collision Data editor comes with a built-in preview for the boxes, relative to the animating character.

To set the character to preview collision for, use the dropdown below the Cel Browser and select the blueprint for your character.

Every Collision Box comes with some properties. Here's what they all do:

• Type: There are five types of boxes.
  • Pushbox. This is currently unused, as push collision is handled elsewhere. However, this is planned to be an option as a separate push collision system.
  • Hurtbox.
  • Hitbox.
  • Offset. This can be used as a "root motion" system for sprites.
  • Custom. This box type can be used in conjunction with the Custom Type property for custom logic.
• Custom Type: For boxes with the Custom type, this specifies what kind of custom box it is. You can set any tag you want, but it should preferably use the naming convention Collision.<Type>.
• X Position: The box X axis position relative to the character's origin in Night Sky units, divided by 1000. The position is used as the box center.Updating this will update the X Position (Unreal) property, and vice versa.
• Y Position: The box Y axis position relative to the character's origin in Night Sky units, divided by 1000. The position is used as the box center. Updating this will update the Y Position (Unreal) property, and vice versa.
• X Size: The box width in Night Sky units, divided by 1000. Updating this will update the X Size (Unreal) property, and vice versa.
• Y Size: The box height in Night Sky units, divided by 1000. Updating this will update the Y Size (Unreal) property, and vice versa.
• X Position (Unreal): The box X axis position relative to the character's origin in centimeters. The position is used as the box center. Updating this will update the regular X Position property, and vice versa.
• Y Position (Unreal): The box Y axis position relative to the character's origin in centimeters. The position is used as the box center. Updating this will update the regular Y Position property, and vice versa.
• X Size: The box width in centimeters. Updating this will update the regular X Size property, and vice versa.
• Y Size: The box height in centimeters. Updating this will update the regular Y Size property, and vice versa.

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