[Partner Nodes] feat(bria): add Transparent option to Remove Video Background

Bria's /remove_background endpoint accepts background_color='Transparent'
when paired with an alpha-capable container/codec; pass output_container_and_codec=
'webm_vp9' in that case (otherwise mp4_h264, unchanged). The returned WebM
flows through ComfyUI's VIDEO type as a file reference, so Save Video with the
default auto/auto settings stream-copies it byte-for-byte with the alpha plane
intact (covered by new test_save_to_auto_preserves_vp9_alpha_via_stream_copy).

BriaTransparentVideoBackground is kept as-is because it remains the supported
path for the IMAGE+MASK workflow: Get Video Components currently cannot expose
alpha for VP9 streams (PyAV's default vp9 decoder drops the side alpha layer),
so the per-frame compositing workflow still needs the dedicated node that
decodes via libvpx-vp9 itself.

comfy_api_nodes/nodes_bria.py

@@ -251,12 +251,17 @@ class BriaRemoveVideoBackground(IO.ComfyNode):
             node_id="BriaRemoveVideoBackground",
             display_name="Bria Remove Video Background",
             category="partner/video/Bria",
-            description="Remove the background from a video using Bria. ",
+            description="Remove the background from a video using Bria. Pick a color to composite "
+            "the foreground over a solid background (returns an MP4), or pick 'Transparent' to "
+            "get a WebM/VP9 video with a real alpha channel. Connect Transparent output directly "
+            "to Save Video to write the alpha-channel WebM to disk; for per-frame compositing "
+            "(images + mask sockets), use 'Bria Remove Video Background (Transparent)' instead.",
             inputs=[
                 IO.Video.Input("video"),
                 IO.Combo.Input(
                     "background_color",
                     options=[
+                        "Transparent",
                         "Black",
                         "White",
                         "Gray",
@@ -268,7 +273,11 @@ class BriaRemoveVideoBackground(IO.ComfyNode):
                         "Magenta",
                         "Orange",
                     ],
-                    tooltip="Background color for the output video.",
+                    tooltip="Background color for the output video. 'Transparent' returns a "
+                    "WebM/VP9 video with an alpha channel (best paired with Save Video); other "
+                    "options composite the foreground over the chosen solid color and return an MP4. "
+                    "For per-frame compositing (images + mask sockets) use 'Bria Remove Video "
+                    "Background (Transparent)' instead.",
                 ),
                 IO.Int.Input(
                     "seed",
@@ -301,13 +310,17 @@ class BriaRemoveVideoBackground(IO.ComfyNode):
         seed: int,
     ) -> IO.NodeOutput:
         validate_video_duration(video, max_duration=60.0)
+        # Bria returns 422 if background_color="Transparent" is paired with mp4_h264
+        # (no alpha plane). VP9-in-WebM carries the alpha; downstream VIDEO handling
+        # preserves it as long as Save Video uses auto/auto (stream-copy) settings.
+        output_container_and_codec = "webm_vp9" if background_color == "Transparent" else "mp4_h264"
         response = await sync_op(
             cls,
             ApiEndpoint(path="/proxy/bria/v2/video/edit/remove_background", method="POST"),
             data=BriaRemoveVideoBackgroundRequest(
                 video=await upload_video_to_comfyapi(cls, video),
                 background_color=background_color,
-                output_container_and_codec="mp4_h264",
+                output_container_and_codec=output_container_and_codec,
                 seed=seed,
             ),
             response_model=BriaStatusResponse,
@@ -504,8 +517,9 @@ class BriaTransparentVideoBackground(IO.ComfyNode):
             display_name="Bria Remove Video Background (Transparent)",
             category="partner/video/Bria",
             description="Remove the background from a video using Bria and return the cut-out frames "
-            "plus an alpha mask. Connect both to a compositing node, or feed them to Save WEBM to "
-            "write a transparent video.",
+            "plus an alpha mask, ready for per-frame compositing or feeding into Save WEBM. Use this "
+            "when you need IMAGE + MASK sockets; if you only need to save a transparent video file, "
+            "use 'Bria Remove Video Background' with background_color='Transparent' instead.",
             inputs=[
                 IO.Video.Input("video"),
                 IO.Int.Input(

tests-unit/comfy_api_test/video_types_test.py

@@ -5,10 +5,13 @@ import os
 import av
 import io
 from fractions import Fraction
+import numpy as np
+
 from comfy_api.input_impl.video_types import VideoFromFile, VideoFromComponents
-from comfy_api.util.video_types import VideoComponents
+from comfy_api.util.video_types import VideoComponents, VideoContainer, VideoCodec
 from comfy_api.input.basic_types import AudioInput
 from av.error import InvalidDataError
+from av.codec import CodecContext
 
 EPSILON = 0.0001
 
@@ -237,3 +240,63 @@ def test_duration_consistency(video_components):
     manual_duration = float(components.images.shape[0] / components.frame_rate)
 
     assert duration == pytest.approx(manual_duration)
+
+
+def _make_webm_vp9_with_alpha(width=32, height=32, frames=5, fps=10) -> bytes:
+    buf = io.BytesIO()
+    with av.open(buf, mode="w", format="webm") as container:
+        stream = container.add_stream("libvpx-vp9", rate=fps)
+        stream.width = width
+        stream.height = height
+        stream.pix_fmt = "yuva420p"
+        for i in range(frames):
+            rgba = np.zeros((height, width, 4), dtype=np.uint8)
+            rgba[..., 0] = 200
+            rgba[..., 1] = 50 + i * 30
+            rgba[..., 2] = 80
+            rgba[..., 3] = (i + 1) * 50
+            frame = av.VideoFrame.from_ndarray(rgba, format="rgba")
+            frame = frame.reformat(format="yuva420p")
+            for packet in stream.encode(frame):
+                container.mux(packet)
+        for packet in stream.encode(None):
+            container.mux(packet)
+    return buf.getvalue()
+
+
+def _per_frame_alpha_means_libvpx(source) -> list:
+    means = []
+    with av.open(source, mode="r") as container:
+        stream = container.streams.video[0]
+        decoder = CodecContext.create("libvpx-vp9", "r")
+        for packet in container.demux(stream):
+            for frame in decoder.decode(packet):
+                rgba = frame.to_ndarray(format="rgba")
+                means.append(float(rgba[..., 3].mean()))
+    return means
+
+
+def test_save_to_auto_preserves_vp9_alpha_via_stream_copy():
+    """Save Video with format=auto, codec=auto stream-copies a WebM/VP9+alpha file
+    without touching the alpha plane. This is the assumption that the consolidated
+    'Bria Remove Video Background' node relies on when background_color='Transparent'
+    (which makes Bria return a webm_vp9 url with a real alpha channel)."""
+    raw = _make_webm_vp9_with_alpha()
+    original_means = _per_frame_alpha_means_libvpx(io.BytesIO(raw))
+    assert len(original_means) > 0, "test fixture failed to produce frames"
+    assert original_means == sorted(original_means), (
+        "test fixture alpha did not encode as monotonically increasing"
+    )
+
+    video = VideoFromFile(io.BytesIO(raw))
+    out_buf = io.BytesIO()
+    video.save_to(out_buf, format=VideoContainer.AUTO, codec=VideoCodec.AUTO)
+
+    out_buf.seek(0)
+    saved_means = _per_frame_alpha_means_libvpx(out_buf)
+
+    assert len(saved_means) == len(original_means)
+    for original, saved in zip(original_means, saved_means):
+        assert original == pytest.approx(saved, abs=EPSILON), (
+            f"alpha lost across stream-copy round-trip: {original} -> {saved}"
+        )