Bake untranslatable worlds into an environment map

[ishaanshah]

A world that convert_world cannot translate is currently dropped with a warning, which leaves the scene without its main light. This affects common setups: a Sky Texture, a Mix or Add Shader surface (often used to separate the camera backdrop from the lighting), procedural chains, and Environment Textures that do not feed the Background directly (e.g. through a Gamma or Hue/Saturation node). On a set of about 1000 production scenes, this dropped the world in roughly 40% of them, mostly Sky Textures.

This PR bakes such a world into an envmap instead, and includes a couple of small fixes to the exporter.

Bake untranslatable worlds

When convert_world raises, export_world now renders the world once with Cycles through an equirectangular camera and exports the result as an envmap:

• The bake runs on a copy of the world; the user's data is not modified.
• The bake camera lays the map out like an environment-texture lookup, so the emitter gets the same to_world as a translated environment texture, and scale = 1 since the map already contains the strength.
• The map is written with export_image, like any other texture.
• The world's Cycles ray visibility is applied to the emitter, as for a translated world. (A world hidden from the camera is still baked, since the bake renders through a camera.)

Light Path. A world that switches on a Light Path output is baked with the values an indirect diffuse ray reports (Is Camera Ray = 0, Is Diffuse Ray = 1, depth 1, ...). The bake traces camera rays, so otherwise a world that shows the camera a backdrop and lights the scene with an HDRI would bake the backdrop. Node groups containing a Light Path node are copied before they are changed. This is a heuristic: one environment map cannot give a different answer per ray type, so for such worlds the camera backdrop or glossy reflections can differ from Cycles.

Options. bake_world (default on) and bake_world_resolution (map height, width is twice that; default 1024) on the export operator, in relaxed mode only. The bake is lossy: the map has a fixed resolution, so features smaller than a texel (such as a sun disk) are spread out, and a Light Path world keeps only one branch. Strict mode therefore still skips the world with a warning. The render engine never bakes: the bake runs a render operator, which is not available from the render job thread.

Export operator: honour the scene's exporting mode

The XML export operator now sets export_ctx.strict from scene.mitsuba.export_mode, as the render engine already does. Until now it always exported in strict mode, whatever the scene's setting.

Honouring relaxed mode exposed a bug in its colour fallback. When a colour input is driven by something the converter cannot handle, eval_color logs a warning and falls back to the socket's default colour. That fallback returned the bare _to_color(...) tuple, whereas every other branch returns a colour plugin (export_ctx.spectrum(...), or an unbounded srgb for values outside [0, 1]); Mitsuba rejects the tuple when the scene is loaded. The path was unreachable from the operator before, since strict mode replaces such a material with the error BSDF instead. The constant-colour conversion now lives in one helper, _color_constant, which both the resolved-constant branch and the fallback use, so the fallback also gets the out-of-range handling.

Examples

Tests

• tests/unit/test_world_export.py: Sky Texture and Mix Shader worlds are baked into an envmap whose file exists and loads in relaxed mode, while strict mode skips the world; an indirectly connected environment texture bakes back to its source image; the option turns baking off; translatable worlds are not baked; ray visibility is preserved; a Light Path world bakes its lighting branch.
• tests/roundtrip/test_world_roundtrip.py: a world whose colour encodes the view direction is baked, then rendered in Mitsuba and in Cycles from six directions; they agree to within 0.001, which pins the orientation of the map.
• tests/unit/test_export_warnings.py: the operator honours the exporting mode, and the relaxed colour fallback produces a valid spectrum.