secondlife/viewer-private#230: Amend terrain loading test plan

1 files changed, 28 insertions, 3 deletions

diff --git a/doc/testplans/terrain_loading.md b/doc/testplans/terrain_loading.md
index b84daf6f73..d8be930b33 100644
--- a/doc/testplans/terrain_loading.md
+++ b/doc/testplans/terrain_loading.md
@@ -1,4 +1,29 @@
-- Texture terrain should load
-- PBR terrain should load if enabled
+# Terrain Loading
+
+## Behavior overview
+
+- Texture terrain should load if applied
+- PBR terrain should load if applied, even if the feature flag is disabled
 - Related subsystem: A change to the PBR terrain loading system may affect the texture terrain loading system and vice-versa
-- Related subsystem: Minimaps should load if terrain loads (may take longer)
+- Related subsystem: Minimap should load if terrain loads
+    - They may not finish loading at the same time
+
+## Implementation details
+
+This section is provided mainly for clarification of how the terrain loading system works.
+
+The simulator sends 4 terrain composition UUIDs to the viewer for the region. The viewer does not know ahead-of-time if the terrain composition uses textures or materials. Therefore, to expedite terrain loading, the viewer makes up to 8 "top-level" asset requests simultaneously:
+
+- Up to 4 texture asset requests, one for each UUID
+- Up to 4 material asset requests, one for each UUID
+
+It is therefore expected that half of these asset lookups will fail.
+
+The viewer inspects the load success of these top-level assets to make the binary decision of whether to render all 4 texture assets or all 4 material assets. This determines the choice of composition for terrain both in-world and on the minimap.
+
+The minimap also attempts to wait for textures to partially load before it can render a tile for a given region:
+
+- When rendering texture terrain, the minimap attempts to wait for top-level texture assets to partially load
+- When rendering PBR material terrain, the minimap attempts to wait for any base color/emissive textures in the materials to partially load, if they are present
+
+We don't make guarantees that the minimap tile will render for the region if any aforementioned required textures/materials fail to sufficiently load. However, the minimap may make a best-effort attempt to render the region by ignoring or replacing data.