Customizable Client-side Directional Damage Indicator (CCDDI) adds a simple yet extremely helpful damage indicator around your crosshair, alerting the player to nearby damage sources.
Depends on ModMenu and YetAnotherConfigLib (YACL) for the extreme customization of the indicator, including its position and appearance in the heads-up-display (you can even choose your own textures for the indicator!)
Vanilla Minecraft already has a damage indicator (shaking the camera in the direction of the damage), but it is more disorienting than useful.
I am considering porting to Neo/Legacy Forge, so please refrain from asking!
