35 Play-Loop Infrastructure
This tour shows the key components in p_local.h: macros for game physics, data structures for collision detection, and functions for movement and item handling.
File: linuxdoom-1.10/p_local.h
The p_local.h header file includes r_local.h.
Next we’ll look at four macros that define the game’s spatial dimensions and physics: MAPBLOCKSIZE, PLAYERRADIUS, GRAVITY, and USERANGE.
File: linuxdoom-1.10/p_local.h
MAPBLOCKSIZE defines the width and height of each grid cell used for collision detection.
File: linuxdoom-1.10/p_local.h
PLAYERRADIUS defines the collision boundary size for player movement.
File: linuxdoom-1.10/p_local.h
GRAVITY defines the downward acceleration applied to objects in the game world.
File: linuxdoom-1.10/p_local.h
USERANGE defines the maximum reach for player’s use actions in world units.
File: linuxdoom-1.10/p_local.h
thinkercap maintains the list of active mobile objects (mobjs) in the game.
File: linuxdoom-1.10/p_local.h
These variables track the timing and storage of items that will respawn in the game.
File: linuxdoom-1.10/p_local.h
rejectmatrix stores data for quickly determining if two areas can see each other. blockmap divides the map into a grid for collision detection.
File: linuxdoom-1.10/p_local.h
divline_t holds a line partition: a point (x,y) and direction vector (dx,dy).
File: linuxdoom-1.10/p_local.h
intercept_t represents a ray trace hit with position and target info.
File: linuxdoom-1.10/p_maputl.c
In P_PointOnDivlineSide,
dx(line 186) is set to x minus the divline’s origin x, anddy(line 187) to y minus its origin y. A bitwise XOR ofline->dy,line->dx,dx, anddy(line 190) then inspects the sign bit to quickly determine which side of the line the point falls on without costly multiplications.
File: linuxdoom-1.10/p_maputl.c
P_TraverseIntercepts uses the frac field of intercept_t to track the nearest intersection point along a trace line. When scan->frac is smaller than the current dist, it updates both dist and the current intersection point.
Next we’ll look at movement and interaction functions: P_CheckPosition, P_TryMove, and P_UseLines. These functions handle player movement and object interaction.
File: linuxdoom-1.10/p_local.h
P_CheckPosition tests if an object can fit at a position, while P_TryMove attempts to actually move the object there, handling any collisions that occur.
File: linuxdoom-1.10/p_local.h
P_UseLines checks if a player is close enough to interact with special lines like doors and switches.
File: linuxdoom-1.10/p_local.h
P_RespawnSpecials handles respawning special items during deathmatch games.
That concludes the core play-loop tour through p_local.h: we’ve seen key macros, externs, data types, and function prototypes driving Doom’s movement, collision, usage, and respawn logic.