feat: video auto fit based on video stream size

src/utils/videoFit.ts

@@ -0,0 +1,94 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { combineLatest, map, type Observable, of, switchMap } from "rxjs";
+import {
+  type LocalParticipant,
+  type RemoteParticipant,
+  Track,
+} from "livekit-client";
+
+import { type ObservableScope } from "../state/ObservableScope.ts";
+import { type Behavior } from "../state/Behavior.ts";
+import {
+  observeInboundRtpStreamStats$,
+  observeOutboundRtpStreamStats$,
+} from "../state/MediaViewModel.ts";
+
+type Size = {
+  width: number;
+  height: number;
+};
+
+export function videoFit$(
+  scope: ObservableScope,
+  videoSize$: Observable<Size | undefined>,
+  tileSize$: Observable<Size | undefined>,
+): Behavior<"cover" | "contain"> {
+  const fit$ = combineLatest([videoSize$, tileSize$]).pipe(
+    map(([videoSize, tileSize]) => {
+      if (!videoSize || !tileSize) {
+        // If we don't have the sizes, default to cover to avoid black bars.
+        // This is a reasonable default as it will ensure the video fills the tile, even if it means cropping.
+        return "cover";
+      }
+      const videoAspectRatio = videoSize.width / videoSize.height;
+      const tileAspectRatio = tileSize.width / tileSize.height;
+
+      // If video is landscape (ratio > 1) and tile is portrait (ratio < 1) or vice versa,
+      // we want to use "contain" (fit) mode to avoid excessive cropping
+      const videoIsLandscape = videoAspectRatio > 1;
+      const tileIsLandscape = tileAspectRatio > 1;
+
+      // If the orientations are the same, use the cover mode (Preserves the aspect ratio, and the image fills the container.)
+      // If they're not the same orientation, use the contain mode (Preserves the aspect ratio, but the image is letterboxed - black bars- to fit within the container.)
+      return videoIsLandscape === tileIsLandscape ? "cover" : "contain";
+    }),
+  );
+
+  return scope.behavior(fit$, "cover");
+}
+
+/**
+ * Helper function to get the video size from a participant.
+ * It observes the participant's video track stats and extracts the frame width and height.
+ * @param participant$ - an Observable of a LocalParticipant or RemoteParticipant, or null if no participant is selected.
+ * @returns an Observable of the video size (width and height) or undefined if the size cannot be determined.
+ */
+export function videoSizeFromParticipant$(
+  participant$: Observable<LocalParticipant | RemoteParticipant | null>,
+): Observable<{ width: number; height: number } | undefined> {
+  return participant$
+    .pipe(
+      // If we have a participant, observe their video track stats. If not, return undefined.
+      switchMap((p) => {
+        if (!p) return of(undefined);
+        if (p.isLocal) {
+          return observeOutboundRtpStreamStats$(p, Track.Source.Camera);
+        } else {
+          return observeInboundRtpStreamStats$(p, Track.Source.Camera);
+        }
+      }),
+    )
+    .pipe(
+      // Extract the frame width and height from the stats. If we don't have valid stats, return undefined.
+      map((stats) => {
+        if (!stats) return undefined;
+        if (
+          // For video tracks, frameWidth and frameHeight should be numbers. If they're not, we can't determine the size.
+          typeof stats.frameWidth !== "number" ||
+          typeof stats.frameHeight !== "number"
+        ) {
+          return undefined;
+        }
+        return {
+          width: stats.frameWidth,
+          height: stats.frameHeight,
+        };
+      }),
+    );
+}