Sky Renderer

Outdoor sky configuration and the Glide renderer’s BSI sky plane and sun disc.

Overview

In the shipped Glide build, the sky uses world_skyfx[N] and a matching .COL file. The texture is projected onto a horizontal quad before scene geometry. A separate billboard draws the sun. The GXA file named by world_sky[N] is not loaded by this renderer’s sky setup.

Lifecycle

Background Fill

The shipped Glide build selects world_fogrgb[N] as the RGB color used to clear the next back buffer after presenting a normal game frame. An omitted value defaults to (0, 0, 0). The finite sky plane draws over this fill; uncovered regions retain the fog color, including when the sky is disabled.

For example, world 0 uses (200, 100, 100), world 1 uses (200, 190, 250), world 6 uses (57, 49, 49), world 27 uses (0, 0, 10), and world 28 uses (200, 100, 100).

The shipped Glide world-settings reader does not read world_background[N].

Sky fog

The outdoor frame enables Glide table fog for the sky draw and restores the scene fog table afterwards. The sky draw does not disable fog. Its 64-entry table contains zero in entries 0 through 32, increases by eight from 8 through 240 in entries 33 through 62, and ends at 255 in entry 63.

The sky raster supplies reciprocal camera-space depth to Glide. The Glide fog lookup describes table interpolation and RGB blending.

GXA assets

world_sky[N] names a GXA asset in system/. The Glide renderer does not load this asset; its sky comes from the BSI asset instead.

Glide sky plane

The shipped Glide build uses the following geometry and texture mapping. Coordinates use engine units, with positive Y pointing down.

• The plane is centered horizontally on the camera. Its four relative X/Z corners are (-65000, 65000), (65000, 65000), (65000, -65000), and (-65000, -65000).
• Its absolute Y is world_skylevel[N]. Zero or omitted selects -3500.
• world_skyscale[N] is the half-span of texture coordinates in texels. Zero or omitted selects 2000. This is not the sun disc’s default scale.
• For camera position (cx, cy, cz), the plane’s relative height is world_skylevel[N] - cy. Initial texture-coordinate centers are (0.009 * cx, -0.009 * cz).
• For relative point (x, z) on the plane and scroll offset (su, sv), the texture coordinates in texels are (scale * x / 65000 + 0.009 * cx + su, -scale * z / 65000 - 0.009 * cz + sv). Divide by image width and height for normalized coordinates and repeat the texture. The sky draw uses near/far distances of 1 and 65000.
• The BSI file is a standalone chunk stream, not a TEXBSI archive record. Its matching .COL file supplies the colors independently of the world’s terrain/model palette. Sky sampling is opaque, including palette index 0.

Glide renderer initialization selects bilinear minification and magnification on texture unit 0. The sky texture binding and draw path do not change the filter mode. The initial boat sky uses bilinear minification and magnification, an eight-bit paletted texture, wrapping in both directions, and the base mip level. Palette lookup precedes color filtering.

For the software Voodoo 1 renderer, bilinear filtering operates on encoded eight-bit channels, without an sRGB-to-linear conversion. Texel centers are at half-integer coordinates. Each fractional coordinate is rounded down to a multiple of 1/16. Each row is interpolated first, rounding its channel result down to an integer; those two results are then interpolated vertically and rounded down again. For channel endpoints a, b and fractional weight w, each interpolation is a + floor((b-a)*w). For example, the midpoint between encoded channel values 0 and 255 is 127.

The installed files SKY888.BSI, SKY899.BSI, SKYNIT.BSI, and SKYNEC.BSI each contain one 256 by 256 frame and have matching COL files. The seven outdoor world entries are 0, 1, 6, 14, 27, 28, and 30.

Scrolling and rotation

Raw world_skyspeed[N] is an unsigned byte converted to texels per BIOS timer tick by dividing by 16. The timer runs at 1193182 / 65536 ticks per second. The initial scrolling direction is selected randomly from 2048 angle steps. Scroll offsets start at zero, advance with the shared engine sine/cosine table, and wrap at plus/minus 256 texels. A frame processes at most 36 elapsed ticks. The plane tilts around a camera-relative axis as described below.

The fxskyspeed console command interprets its value as a signed integer. It sets the current sky speed to that full value divided by 16 and stores the low eight bits in the current world’s speed setting. The current speed can therefore differ from the byte-derived speed used when the world is loaded again. Each successful command consumes one random value and selects a new direction from its low 11 bits, including when the requested speed is zero. It retains the accumulated texture offsets and the last processed timer tick.

Plane tilt

The sky is a plane that can be crossed vertically and is visible from either side. It is not an enclosing cube or sphere.

The initial tilt is 24 of the engine’s 2048 angle units, approximately 4.21875 degrees. It is a fixed angle, not an animation rate. The tilt setter masks its input to 11 bits.

Let h be the normalized horizontal camera-forward direction in engine coordinates. For an unrotated plane point with horizontal position p, let q = dot(p, h) and a = 24 * 6.2831852 / 2048. Its tilted horizontal position is p + (cos(a) - 1) * q * h, and its relative vertical position is sin(a) * q + world_skylevel - camera_y. Texture coordinates stay attached to the unrotated corners. The camera-relative tilt is applied before adding the height, not around the world origin.

Initial scroll direction

The sky consumes one value from the engine’s shared random stream and keeps its low 11 bits as the angle. The stream updates its unsigned 32-bit state with state = state * 1103515245 + 12345, wrapping at 32 bits; the returned value is (state >> 16) & 32767. Therefore the sky direction is (updated_state >> 16) & 2047. Reproducing a particular session requires the state immediately before sky initialization or the resulting direction, not just a world identifier. Other engine users also consume this stream.

Global Engine Toggles

The [xngine] section of SYSTEM.INI provides master controls that apply to all worlds:

Sun disc

A separate billboard renders the sun as a textured sprite in the sky, independent of the BSI sky plane:

The billboard center, relative to the camera, is the negative of (truncate(16000*sin(angle)), truncate(16000*cos(angle)), truncate(16000*sin(skew))), using world_sunangle and world_sunskew in the 2048-step engine angle table. It does not use world_sun as its position. The sprite faces the camera and remains the same angular size during camera translation. Its near/far range is 1/65535 engine units.

Each billboard half-size is the corresponding BSI image dimension multiplied by 0.5 * (tex_scale / 256) * (world_sunscale / 100). A zero or omitted sun scale defaults to 12800. The installed SUN001.BSI is a single 256 by 256 frame with texture scale 256, so the installed sun scale 4000 gives half-size 5120 engine units. Missing sun image names suppress the disc.

The sun texture is bound as eight-bit alpha: its indexed pixel bytes supply opacity, independently of its palette. The sun draw explicitly selects bilinear minification and magnification on texture unit 0. Constant encoded RGB comes from world_sunimgrgb, defaulting to (255,255,200). The draw uses source-alpha / one-minus-source-alpha blending and disables fog for the sun, restoring scene fog afterwards. Scene geometry drawn later can cover it.

In the software Voodoo 1 renderer, the sun’s alpha channel uses the same four-bit fractional bilinear interpolation described for sky colors. Texel centers are at half-integer coordinates; each fractional coordinate is rounded down to a multiple of 1/16. Horizontal alpha interpolation truncates to an eight-bit integer before vertical interpolation, which truncates again. The resulting integer alpha feeds framebuffer blending. Sun texture coordinates wrap in both directions.

Sun framebuffer blending

Sky texture filtering and fog produce colors before framebuffer encoding. The RGB565 conversion and dither arithmetic in this section describe the software Voodoo 1 framebuffer backend; they are not additional stages in the BSI texture or sky fog calculation.

The sun draw enables four-by-four framebuffer dithering and alpha dither subtraction. In the software Voodoo 1 renderer, blending reads the existing RGB565 destination by shifting the stored red and blue fields left by three bits and the green field left by two bits. It does not replicate the high bits into the vacated low bits.

Before blending, the destination channels undergo dither subtraction using the pixel’s four-by-four dither value d. Red and blue become floor((2 * channel + 15 - d) / 2); green becomes floor((4 * channel + 15 - d) / 4).

For an encoded source channel s, corrected destination channel t, and filtered eight-bit sun alpha a, the blended channel is floor(s * (a + 1) / 256) + floor(t * (256 - a) / 256). The source and destination terms truncate separately before addition. The result is clamped to an eight-bit channel and then passes through the normal framebuffer quantization.

Console Commands

The developer console (F12) exposes runtime sky adjustment:

The show world console command displays current sky parameters in the on-screen debug overlay, including the sky texture name, scale, level, and speed.

World Sky Assignments

Seven outdoor world entries define sky parameters, including world 14. All others are indoor/dungeon locations with no sky.

Worlds 1, 27, and 28 share the same ISLAND.WLD terrain and PVO node maps. Different palettes, BSI sky textures, and lighting parameters implement the time-of-day variants as separate world entries rather than dynamic sky transitions.

External References

• WORLD.INI documentation — per-world sky, sun disc, and background fill keys
• SYSTEM.INI documentation — global sky engine toggles in the [xngine] section
• UESP: Redguard Console — show world command displays sky parameters at runtime