27  Rendering: Lookup Tables

This tour examines DOOM’s lookup tables used for efficient rendering calculations, including angle tables, color maps, and view mapping tables.

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tables.h declares lookup tables used for fast trigonometry calculations: - finesine and finecosine store pre-calculated sine/cosine values - finetangent stores pre-calculated tangent values - tantoangle converts slope values to angles

File: linuxdoom-1.10/tables.h (lines 56–65)

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We’ll look at how Doom loads its color lookup tables from the COLORMAP lump into memory.

• colormaps holds lighting and color tables loaded from WAD data
  > File: linuxdoom-1.10/r_data.c (lines 160–164)

• R_InitColormaps loads the COLORMAP lump into memory, aligned for efficient indexing
  > File: linuxdoom-1.10/r_data.c (lines 639–644)

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Binary Angle Measurement (BAM):

• FINEANGLES defines 8192 discrete angle steps
  

• FINEMASK enables fast angle wrapping using bitwise AND
  > File: linuxdoom-1.10/tables.h (lines 50–51)

• These constants define key BAM angles (e.g., ANG90, ANG180)
  > File: linuxdoom-1.10/tables.h (lines 68–71)

Explanation: BAM angles are converted to table indices by shifting right and masking. This maps the full 0–360° BAM range to table entries, with 90° aligning to quarter-points.

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Trigonometric and Angle Tables:

• finetangent: tangent lookup table
  > File: linuxdoom-1.10/tables.c (lines 67–75)

• finesine: fixed-point sine table (cosine via 90° offset)
  > File: linuxdoom-1.10/tables.c (lines 583–590)

• tantoangle: converts slope values to BAM angles
  > Header: linuxdoom-1.10/tables.h (lines 82–84)
  > Impl: linuxdoom-1.10/tables.c (lines 1870–1872)

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Dynamic Lighting and View Mapping Tables:

• R_InitLightTables uses colormaps to fill scalelight and zlight for shading
  > File: linuxdoom-1.10/r_main.c (lines 614–640)

• scalelight stores color ramps for light levels and scale
  > Decl: linuxdoom-1.10/r_main.c (lines 116–118)
  > Usage: linuxdoom-1.10/r_main.c (lines 755–759)

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Texture Mapping:

• R_InitTextureMapping sets up view-angle-to-screen conversions
  > viewangletox: angle → x-coordinate
  > File: linuxdoom-1.10/r_main.c (lines 551–577)
  > xtoviewangle: x-coordinate → angle
  > File: linuxdoom-1.10/r_main.c (lines 579–588)

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The next steps explore lookup tables used in DOOM’s rendering system. We’ll examine arctangent lookups, dynamic lighting calculations, and texture mapping tables.

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And that was it.