t2-mapper/src/file.vert

#ifdef USE_FOG
// Check runtime fog enabled uniform - allows toggling without shader recompilation
if (fogEnabled) {
  // Fog disabled at runtime, skip all fog calculations
} else {
  float dist = vFogDepth;

  // Discard fragments at or beyond visible distance - matches Torque's behavior
  // where objects beyond visibleDistance are not rendered at all.
  // This prevents fully-fogged geometry from showing as silhouettes against
  // the sky's fog-to-sky gradient.
  if (dist >= fogFar) {
    discard;
  }

  // Step 1: Calculate distance-based haze (quadratic falloff)
  // Since we discard at fogFar, haze never reaches 1.0 here
  float haze = 0.0;
  if (dist > fogNear) {
    float fogScale = 1.0 / (fogFar - fogNear);
    float distFactor = (dist - fogNear) * fogScale - 1.0;
    haze = 1.0 - distFactor * distFactor;
  }

  // Step 2: Calculate fog volume contributions
  // Note: Per-volume colors are NOT used in Tribes 2 ($specialFog defaults to false)
  // All fog uses the global fogColor - see Tribes2_Fog_System.md for details
  float volumeFog = 0.0;

  #ifdef USE_VOLUMETRIC_FOG
  {
    #ifdef USE_FOG_WORLD_POSITION
      float fragmentHeight = vFogWorldPosition.y;
    #else
      float fragmentHeight = cameraHeight;
    #endif

    float deltaY = fragmentHeight - cameraHeight;
    float absDeltaY = abs(deltaY);

    // Determine if we're going up (positive) or down (negative)
    if (absDeltaY > 0.01) {
      // Non-horizontal ray: ray-march through fog volumes
      for (int i = 0; i < 3; i++) {
        int offset = i * 4;
        float volVisDist = allFogVolumes[offset + 0];
        float volMinH = allFogVolumes[offset + 1];
        float volMaxH = allFogVolumes[offset + 2];
        float volPct = allFogVolumes[offset + 3];

        // Skip inactive volumes (visibleDistance = 0)
        if (volVisDist <= 0.0) continue;

        // Calculate fog factor for this volume
        // From Torque: factor = (1 / (volumeVisDist * visFactor)) * percentage
        // where visFactor is smVisibleDistanceMod (a user quality pref, default 1.0)
        // Since we don't have quality settings, we use visFactor = 1.0
        float factor = (1.0 / volVisDist) * volPct;

        // Find ray intersection with this volume's height range
        float rayMinY = min(cameraHeight, fragmentHeight);
        float rayMaxY = max(cameraHeight, fragmentHeight);

        // Check if ray intersects volume height range
        if (rayMinY < volMaxH && rayMaxY > volMinH) {
          float intersectMin = max(rayMinY, volMinH);
          float intersectMax = min(rayMaxY, volMaxH);
          float intersectHeight = intersectMax - intersectMin;

          // Calculate distance traveled through this volume using similar triangles:
          // subDist / dist = intersectHeight / absDeltaY
          float subDist = dist * (intersectHeight / absDeltaY);

          // Accumulate fog: fog += subDist * factor
          volumeFog += subDist * factor;
        }
      }
    } else {
      // Near-horizontal ray: if camera is inside a volume, apply full fog for that volume
      for (int i = 0; i < 3; i++) {
        int offset = i * 4;
        float volVisDist = allFogVolumes[offset + 0];
        float volMinH = allFogVolumes[offset + 1];
        float volMaxH = allFogVolumes[offset + 2];
        float volPct = allFogVolumes[offset + 3];

        if (volVisDist <= 0.0) continue;

        // If camera is inside this volume, apply fog for full distance
        if (cameraHeight >= volMinH && cameraHeight <= volMaxH) {
          float factor = (1.0 / volVisDist) * volPct;
          volumeFog += dist * factor;
        }
      }
    }
  }
  #endif

  // Step 3: Combine haze and volume fog
  // Torque's clamping: if (bandPct + hazePct > 1) hazePct = 1 - bandPct
  // This gives fog volumes priority over haze
  float volPct = min(volumeFog, 1.0);
  float hazePct = haze;
  if (volPct + hazePct > 1.0) {
    hazePct = 1.0 - volPct;
  }
  float fogFactor = hazePct + volPct;

  // Apply fog using global fogColor (per-volume colors not used in Tribes 2)
  gl_FragColor.rgb = mix(gl_FragColor.rgb, fogColor, fogFactor);
}
#endif