fix(rtc-node): size NV12 and I420A chroma planes from the chroma height

.changeset/fix-nv12-i420a-plane-length.md

@@ -0,0 +1,5 @@
+---
+'@livekit/rtc-node': patch
+---
+
+Size the chroma planes of NV12 and I420A frames from the chroma height when copying them out of native memory

packages/livekit-rtc/src/video_frame.test.ts

@@ -0,0 +1,28 @@
+// SPDX-FileCopyrightText: 2026 LiveKit, Inc.
+//
+// SPDX-License-Identifier: Apache-2.0
+import { VideoBufferType } from '@livekit/rtc-ffi-bindings';
+import { describe, expect, it } from 'vitest';
+import { getPlaneLength } from './video_frame.js';
+
+describe('getPlaneLength', () => {
+  it.each([
+    // 8x4: luma 32, each chroma plane 4x2 = 8
+    [VideoBufferType.I420, 8, 4, 32 + 8 * 2],
+    [VideoBufferType.I420A, 8, 4, 32 * 2 + 8 * 2],
+    [VideoBufferType.NV12, 8, 4, 32 + 8 * 2],
+    // 4x8: luma 32, each chroma plane 2x4 = 8
+    [VideoBufferType.I420, 4, 8, 32 + 8 * 2],
+    [VideoBufferType.I420A, 4, 8, 32 * 2 + 8 * 2],
+    [VideoBufferType.NV12, 4, 8, 32 + 8 * 2],
+    // odd dimensions round the chroma planes up: 5x3 -> 3x2
+    [VideoBufferType.I420A, 5, 3, 15 * 2 + 6 * 2],
+    [VideoBufferType.NV12, 5, 3, 15 + 6 * 2],
+  ])('sizes buffer type %i at %ix%i as %i bytes', (type, width, height, expected) => {
+    expect(getPlaneLength(type, width, height)).toBe(expected);
+  });
+
+  it('sizes a 1280x720 NV12 frame from its chroma height', () => {
+    expect(getPlaneLength(VideoBufferType.NV12, 1280, 720)).toBe(1_382_400);
+  });
+});

packages/livekit-rtc/src/video_frame.ts

@@ -103,7 +103,8 @@ export class VideoFrame {
   }
 }
 
-const getPlaneLength = (type: VideoBufferType, width: number, height: number): number => {
+/** @internal */
+export const getPlaneLength = (type: VideoBufferType, width: number, height: number): number => {
   const chromaWidth = Math.trunc((width + 1) / 2);
   const chromaHeight = Math.trunc((height + 1) / 2);
   switch (type) {
@@ -118,13 +119,13 @@ const getPlaneLength = (type: VideoBufferType, width: number, height: number): n
     case VideoBufferType.I420:
       return width * height + chromaWidth * chromaHeight * 2;
     case VideoBufferType.I420A:
-      return width * height * 2 + chromaWidth * chromaWidth * 2;
+      return width * height * 2 + chromaWidth * chromaHeight * 2;
     case VideoBufferType.I422:
       return width * height + chromaWidth * height * 2;
     case VideoBufferType.I010:
       return width * height * 2 + chromaWidth * chromaHeight * 4;
     case VideoBufferType.NV12:
-      return width * height + chromaWidth * chromaWidth * 2;
+      return width * height + chromaWidth * chromaHeight * 2;
   }
 };