How multi-layer volumetric cloud rendering works¶

For cloud field configuration, instance control, preview, and the verification entry see the API reference.

Weather and density layers¶

The sky is made of independent low, middle, and high density layers, and a weather window controls the cloud clumps, gaps, and thickness variation. The continuous density can connect and overlap across weather grid boundaries, which avoids a discrete arrangement of one cloud per cell. The low layer keeps cumulonimbus envelopes, the middle layer spreads out, and the high layer uses stretched, anisotropic shapes; a cloud tower can also span several height layers. Scene height and coverage presets are art configuration, not meteorological measurements.

Rendering and cost¶

A cloud field is drawn with one sky-covering ray march and one compositing draw, and no fullscreen pass is created per cloud. It first intersects the ray with each layer's height interval, sorts and merges the overlapping intervals, and skips the gaps between layers outright. The sampling budget is allotted by the square root of the interval length, so a thin high cloud is not starved of budget by the lower layers and does not disappear; the densities of several cloud layers at one point are added. The total number of samples per ray is bounded by the budget, and the march ends early when transmission is very low. Weather windows and GPU textures use a cache of at most 8 entries, so travelling does not grow memory without limit.

A shared shape texture keeps the cumulonimbus tower and anvil envelopes; periodic Perlin-Worley/Worley noise generated in the background provides the body, billowing, and erosion of the continuous cloud layers. Each layer uses its own weather, noise phase, and shape scale, and thin clouds use a stretched, anisotropic shape. The cloud field samples nearby density along the sun direction to approximate self-shadowing and uses density scattering with highlight compression to keep the relief of the cloud body; a single cumulonimbus uses a cached lighting voxel atlas, usually updated every 0.25 seconds.

For the cloud field, FAST/BALANCED/HIGH use width and height resolutions of 1/3, 1/2, and 1 of the original viewport; the total budget per ray is 112/160/256 samples and the local lighting samples are 1/2/3; thin high clouds use simplified lighting. The corresponding resolutions for per-cloud quality are 1/3, 1/2, and 1, with at most 48/80/128 fine steps. A low-resolution result is upscaled with conservative depth and depth-aware interpolation. Both paths truncate at the opaque scene depth and write no depth; sky clouds are not limited by the terrain far plane, and the host GL state is restored.

Long-range views add atmospheric perspective and distance fade. There is currently no long-distance shadowing across clouds, no ground cloud shadow, no temporal reprojection, and no full weather simulation; translucent objects such as water are composited with the clouds by render stage. The extra gbuffer channels of an Iris shader pack are not verified yet, so acceptance with the shader pack disabled does not imply compatibility.

References¶

This implementation uses continuous density layers, procedural weather regions, and a small number of cumulonimbus shape envelopes; it does not copy the spherical-shell cloud layer or the frame-split reprojection of the original system. The hardware timings in the cited material do not represent the performance of this repository's implementation.