Force backend by WGPU_BACKEND_TYPE env var (#170)

* Force Vulkan only on Windows

* Use WGPU_BACKEND_TYPE env for forcing backend type

* Use backend name instead of int

* Remove unused import

Author: berendkleinhaneveld

wgpu/backends/rs.py

@@ -212,12 +212,24 @@ def request_adapter(self, *, canvas, power_preference=None):
         else:
             surface_id = get_surface_id_from_canvas(canvas)
 
-        # Force Vulkan on Windows, to avoid DX12 which seems to ignore
-        # the NVidia control panel settings. I guess Vulkan is more
-        # mature than Metal too, so let's just force that for now.
+        # Try to read the WGPU_BACKEND_TYPE environment variable to see
+        # if a backend should be forced. When you run into trouble with
+        # the automatic selection of wgpu, you can use this variable
+        # to force a specific backend. For instance, on Windows you
+        # might want to force Vulkan, to avoid DX12 which seems to ignore
+        # the NVidia control panel settings.
         # See https://github.com/gfx-rs/wgpu/issues/1416
-        # force_backend = lib.WGPUBackendType_Vulkan
-        force_backend = enum_str2int["BackendType"]["Vulkan"]
+        force_backend = enum_str2int["BackendType"]["Null"]
+        if "WGPU_BACKEND_TYPE" in os.environ:
+            try:
+                backend = os.environ["WGPU_BACKEND_TYPE"]
+                force_backend = enum_str2int["BackendType"][backend]
+                logger.warn(f"Forcing backend: {backend} ({force_backend})")
+            except KeyError:
+                logger.warn(
+                    f"Invalid value for WGPU_BACKEND_TYPE: '{backend}'.\n"
+                    f"Valid values are: {list(enum_str2int['BackendType'].keys())}"
+                )
 
         # H: chain: WGPUChainedStruct, backend: WGPUBackendType
         extras = new_struct_p(