Glide fog and clipping

Scene visibility and sky blending use separate clipping ranges and fog tables.

Clipping distances

The scene uses SYSTEM.INI’s [xngine] front_plane and back_plane, in engine units. A nonzero world_back_plane[N] replaces the system far distance on world load. The installed GOG configuration uses 7 and 3800, and none of its 29 worlds declares a world override. The sky plane independently uses near/far 1 and 65000.

Fog lookup

The sky supplies reciprocal positive camera-space depth to Glide. The sky table is active during that draw; the scene table is restored afterwards. Fog color is world_fogrgb[N]. Scene densities come from FOG.INI.

The Glide table’s entry i corresponds to camera depth 2^(3 + floor(i / 4)) / (8 - (i modulo 4)) in engine units. For positive depth d, let e = floor(log2(d)). The continuous table coordinate is 4*e + 8*(1 - 2^e/d), clamped to the interval 0 through 63. Interpolation between adjacent entries is linear in reciprocal depth, rather than distance. This is a camera-depth curve, rather than radial fog.

Glide uploads each density byte with a delta byte equal to 4*(next density - current density) modulo 256. The last delta is zero. The interpolated density is the current byte plus one quarter of the delta byte times the fractional coordinate. The hardware quantizes that fraction and result, and can dither them. For monotonic adjacent differences below 64, this has the usual adjacent-value interpolation curve. The shipped scene table’s transition from entry 62 to 63 wraps its delta; this lies beyond the installed scene far plane.

The RGB blend weight is (interpolated density + 1) / 256. Blending operates on encoded RGB channel values toward the fog color, before framebuffer quantization. Density 255 replaces RGB completely. Density zero has a 1/256 blend weight. Fog does not replace alpha. This channel operation differs from mixing linear-light RGB. Fixed-point rounding, fog dither and framebuffer RGB565 precision can change individual pixels around the continuous curve.

The sky table is zero through entry 32, then rises by eight per entry through 240 at entry 62, followed by 255 at entry 63. Its density reaches 255 while approaching the finite sky boundary, so the texture blends into the same fog RGB that fills uncovered pixels.

Integer precision and framebuffer dithering

Table fog uses an unsigned 16-bit reciprocal-depth encoding. For depth greater than one, let t = floor(2^32 / depth). Values of t below 65536 select 65535. Otherwise, with e the count of leading zero bits in the 32-bit t, the encoding is min(65535, ((e << 12) | ((~t >> (19-e)) & 4095)) + 1). Depth at or below one selects zero. The table index is the encoding divided by 1024; its interpolation fraction is bits 2 through 9.

The density is the table byte plus floor(delta_byte * fraction_byte / 1024). Voodoo 1 does not apply fog-density dithering; the fog-dither switch is a Voodoo 2 feature. For incoming encoded 8-bit channel c, fog channel f and density d, the resulting channel is clamp(c + floor((f-c)*(d+1)/256), 0, 255).

Glide initializes framebuffer dithering to 4 by 4, and the shipped game’s graphics setup explicitly selects that mode. The threshold matrix, indexed by framebuffer Y then X modulo four, is:

 0  8  2 10
12  4 14  6
 3 11  1  9
15  7 13  5

For threshold b and encoded channel c, the stored red/blue five-bit value is floor((2*c - floor(c/16) + floor(c/128) + b)/16). The green six-bit value is floor((4*c - floor(c/16) + floor(c/64) + b)/16). Display conversion repeats high bits into the low bits: five-bit v becomes (v << 3) | (v >> 2), and six-bit v becomes (v << 2) | (v >> 4).

External References

• Glide table-depth conversion
• Glide table upload
• Software Voodoo pixel operations