Materials, lighting and rendering in Blender

You work in the user's Blender through the BlenderLab connection. The viewport capture and the final render are different things: the capture shows whatever shading mode the viewport is in, the render uses the render engine, the lights and the world. Most "it looked fine but the render is wrong" problems come from that.

Your Blender connection

You work through BlenderLab, the official Blender Lab server; its tools are blenderlab_…:

• What is in the scene: get_objects_summary; one object in detail: get_object_detail_summary (by name). The file, saved or not, and its path: get_blendfile_summary_path_info.
• Run Python: execute_blender_code. Nothing survives between calls (carry your helpers in each script); what the script prints comes back, and a dict assigned to result comes back as data. Its guard refuses a few operators (quitting Blender, factory resets).
• See the 3D viewport: get_screenshot_of_area_as_image with area_ui_type: "VIEW_3D" (the whole window: get_screenshot_of_window_as_image); point it at an object: jump_to_view3d_object_by_name.
• Check the Python API: get_python_api_docs (an identifier such as bpy.types.BevelModifier), search_api_docs; the manual: search_manual_docs.
• Quick render: render_thumbnail_to_path, render_viewport_to_path; the image goes to Blender's temp folder and the result gives the real path.
• Export: code (bpy.ops.export_scene.gltf, bpy.ops.export_scene.fbx).
• Asset libraries and 3D generators: none. Build the object, or ask the user for a file to import.

If these tools are not available, stop and say so: the user has to install the BlenderLab connection (Tools → Connections), give it to this agent, and have Blender open with the add-on's server started. Do not describe work as done when you could not do it.

Order of work

1. Look: the scene tool; which objects, which already have materials, is there a camera, a light, a world? Read bpy.app.version_string and the current render engine.
2. Materials, one per distinct surface, named for what they are (Oak_Wood, Brushed_Steel), reused across objects.
3. Light and world before judging any material: a material cannot be evaluated in the dark.
4. Camera: framed on the subject, sensible focal length.
5. Test render small (25-50 % resolution, few samples), look at it, fix.
6. Final render to a file, and report the path, engine, resolution and time.

Code for each step is in references/shading-and-render.md. Open it before writing the first script.

Materials: what goes wrong

• Color set on the wrong place. material.diffuse_color and object.color are viewport display colors only: they do not render. The color that renders is the Base Color input of the Principled BSDF node. Set both if you want the solid-mode viewport to match.
• Input names changed in Blender 4.0. Transmission became Transmission Weight, Emission became Emission Color, Specular became Specular IOR Level, Clearcoat became Coat Weight, Subsurface became Subsurface Weight. Look an input up by name and check it exists before setting it; use the API tool of your connection when unsure.
• Material made but never assigned. Append it to obj.data.materials; for different materials on parts of one mesh, assign polygon.material_index.
• Find the node by type, not by name. The node is called "Principled BSDF" only in an English interface. Use next(n for n in nodes if n.type == 'BSDF_PRINCIPLED').

Metallic is 0 or 1, almost never in between. Pure black (0, 0, 0) and pure white (1, 1, 1) base colors look wrong under light: stay within about 0.03-0.9.

Lighting and world

• No world and no light means a black render, and glass or chrome rendered against an empty world comes out black even with lights, because there is nothing to reflect or refract. Give the world a color and strength, or an HDRI.
• A dependable start: a large area light as key (45° to one side and above), a weaker fill on the other side, a rim light behind, and a world at low strength (0.2-0.5).
• Area light power is in watts and needs to be large: a 1 m area light 2 m from a small object wants roughly 100-500 W. A sun's strength is different: 2-5.
• A ground plane under the object gives contact shadows; without it the object floats.
• An HDRI gives realistic light and reflections in one step. The Blender (MCP for Blender) connection can download one from Poly Haven: it is third-party content and an approval in HydraOps, so ask first. On BlenderLab, ask the user for an HDRI file.

Engines

The engine identifier for EEVEE differs by version (BLENDER_EEVEE_NEXT in 4.2-4.5, BLENDER_EEVEE before and after): the reference shows how to set it without guessing. Do not change the user's render device or preferences.

Camera

• Create one if there is none and make it the scene camera; do not move a camera the user already placed unless asked.
• Aim it with a Track To constraint at the subject or at an empty; 50-85 mm for objects, 24-35 mm for rooms and environments.
• Check the framing with a small test render, not with the viewport screenshot (the viewport is not the camera unless you switch to camera view).

Rendering

• Render to a file with an absolute path in a folder the user can find (next to the .blend if it is saved, otherwise ask or use the user's temp folder) and report the full path. Never overwrite an existing file without saying so. Do it with code (the reference has it): BlenderLab's own render tools are for quick checks and write into Blender's temp folder whatever path you give them; report the path their result names.
• Test small first. A full-resolution Cycles render can take minutes and blocks Blender while it runs; tell the user before starting a long one.
• Look at the result before declaring success: a viewport capture does not show a render. If you cannot view the rendered file yourself, say that plainly and describe what you set up rather than what the image "looks like".
• Color management: the default view transform (AgX or Filmic) desaturates strong colors for realism; for flat graphic colors that must match exactly, use Standard.

Troubleshooting

Before you say it is done

• Every visible object has an assigned material whose Base Color is set on the node.
• There is a light or a lit world, and a camera that frames the subject.
• A render was written to a file, and you gave the path, the engine and the resolution.
• You changed only what was asked: no deleted objects, no changed preferences.