18 Rendering: Core Data Structures
This tour covers the core data structures that power DOOM’s software renderer: the map geometry types in the BSP tree, the deferred wall segment type for sprite clipping, and the key view state variables used during rendering.
The next steps examine the map geometry types in r_defs.h, which define how DOOM represents its 3D world structure.
sector_t
File: linuxdoom-1.10/r_defs.h (lines 101–135)
Represents a 3D space in the game world, defining its geometry and properties.
side_t
File: linuxdoom-1.10/r_defs.h (lines 144–161)
Defines the texturing and positioning for one side of a linedef.
line_t
File: linuxdoom-1.10/r_defs.h (lines 179–215)
Represents a map linedef - a line segment that divides the map into sectors.
subsector_t
File: linuxdoom-1.10/r_defs.h (lines 227–233)
Defines a convex map section by its sector and line segments.
seg_t
File: linuxdoom-1.10/r_defs.h (lines 240–258)
Represents a wall segment in the BSP tree, storing geometry and references needed for rendering a portion of a wall.
node_t
File: linuxdoom-1.10/r_defs.h (lines 265–277)
A BSP node containing a partition line and references to its two child regions.
Next we’ll look at drawseg_t and the view state globals that control how walls and sprites are rendered to the screen.
BSP traversal logic
File: linuxdoom-1.10/r_bsp.c (lines 567–570)
In R_RenderBSPNode, bsp is set to the address of the BSP node by indexing the global nodes array with bspnum. Then R_PointOnSide tests the viewer’s viewx/viewy against this node’s partition to choose which child subtree to traverse.
subsector lookup
File: linuxdoom-1.10/r_bsp.c (lines 509–512)
Accesses a subsector by indexing the global subsectors array, then uses its sector and numlines fields to determine which sector to render and how many segments to process.
segment processing
File: linuxdoom-1.10/r_bsp.c (lines 268–272)
R_AddLine processes a segment by storing it in curline and converting its endpoints into view angles using R_PointToAngle.
drawseg_t
File: linuxdoom-1.10/r_defs.h (lines 322–347)
Holds wall segment data during rendering for deferred drawing operations.
R_StoreWallRange
File: linuxdoom-1.10/r_segs.c (lines 413–415)
Stores the wall’s screen coordinates and the source segment in the drawseg_t structure.
R_RenderMaskedSegRange
File: linuxdoom-1.10/r_segs.c (lines 117–120)
Retrieves line segment data from the drawseg: the curline, front and back sectors, and midtexture from the side definition.
View state globals
File: linuxdoom-1.10/r_state.h (lines 103–120)
viewx and viewy store the viewer’s position. rw_distance holds the calculated wall distance and rw_normalangle stores the normal angle to each segment during rendering.
And that’s it for DOOM’s core rendering structures!