fix(canvas): 预演录制转码 - 奇数分辨率白膜录像按偶数缩放并无音轨导出 MP4

WebGL 录制没有音轨，dpr 1.5 下宽高经常是奇数，libx264 的 yuv420p 会直接失败。转码改为偶数缩放、去掉音轨，并用 ultrafast 单线程适配关闭汇编的 ffmpeg 内核。

web/src/lib/canvas/canvas-video-merge-args.ts

@@ -0,0 +1,9 @@
+// 画布录制转 MP4 的 ffmpeg 参数，纯函数便于单测。
+// 预演台白膜用 WebGL canvas.captureStream 录制，没有音轨，画布在 dpr 1.5 下宽高经常是奇数。
+// @ffmpeg/core 的 libx264 + yuv420p 要求宽高都能被 2 整除，奇数帧会直接失败：
+// "width not divisible by 2" → exit 1，界面只看到「视频转码失败，请重试」。
+// 该内核还关闭了 pthreads 和 asm，默认 medium preset 在视口分辨率下转不动，所以用 ultrafast 单线程。
+
+export function buildRecordingTranscodeArgs(inputName: string, outputName: string): string[] {
+    return ["-i", inputName, "-an", "-vf", "scale=trunc(iw/2)*2:trunc(ih/2)*2", "-c:v", "libx264", "-preset", "ultrafast", "-pix_fmt", "yuv420p", "-movflags", "+faststart", "-threads", "1", outputName];
+}

web/src/lib/canvas/canvas-video-merge.ts

@@ -3,6 +3,7 @@ import ffmpegCoreURL from "@ffmpeg/core?url";
 import ffmpegWasmURL from "@ffmpeg/core/wasm?url";
 import { fetchFile } from "@ffmpeg/util";
 import { getMediaBlob } from "@/services/file-storage";
+import { buildRecordingTranscodeArgs } from "./canvas-video-merge-args";
 
 export type MergeVideoInput = { id: string; url?: string; storageKey?: string };
 export type MergeVideoProgress = { phase: "loading" | "reading" | "encoding"; progress: number };
@@ -40,15 +41,21 @@ export async function transcodeVideoToMp4(input: Blob, onProgress?: (progress: M
     try {
         await ffmpeg.writeFile(inputName, await fetchFile(input));
         onProgress?.({ phase: "encoding", progress: 55 });
-        const exitCode = await ffmpeg.exec([
-            "-i", inputName,
-            "-c:v", "libx264",
-            "-pix_fmt", "yuv420p",
-            "-c:a", "aac",
-            "-movflags", "+faststart",
-            outputName,
-        ]);
-        if (exitCode !== 0) throw new Error("视频转码失败，请重试");
+        const logs: string[] = [];
+        const onLog = ({ message }: { message: string }) => {
+            if (message) logs.push(message);
+        };
+        ffmpeg.on("log", onLog);
+        let exitCode = 1;
+        try {
+            exitCode = await ffmpeg.exec(buildRecordingTranscodeArgs(inputName, outputName));
+        } finally {
+            ffmpeg.off("log", onLog);
+        }
+        if (exitCode !== 0) {
+            console.error("白膜视频转码失败", logs.slice(-12));
+            throw new Error("视频转码失败，请重试");
+        }
         const output = await ffmpeg.readFile(outputName);
         onProgress?.({ phase: "encoding", progress: 100 });
         return new Blob([output as BlobPart], { type: "video/mp4" });

web/test/canvas-video-merge-args.test.ts

@@ -0,0 +1,15 @@
+import { describe, expect, test } from "bun:test";
+
+import { buildRecordingTranscodeArgs } from "../src/lib/canvas/canvas-video-merge-args";
+
+describe("buildRecordingTranscodeArgs", () => {
+    test("奇数画布先裁成偶数，并且不要求音轨", () => {
+        const args = buildRecordingTranscodeArgs("recording.webm", "recording-transcoded.mp4");
+        expect(args).toContain("-an");
+        expect(args).toContain("scale=trunc(iw/2)*2:trunc(ih/2)*2");
+        expect(args).not.toContain("-c:a");
+        expect(args.indexOf("-vf")).toBeGreaterThan(args.indexOf("-i"));
+        expect(args.indexOf("-vf")).toBeLessThan(args.indexOf("recording-transcoded.mp4"));
+        expect(args).toEqual(["-i", "recording.webm", "-an", "-vf", "scale=trunc(iw/2)*2:trunc(ih/2)*2", "-c:v", "libx264", "-preset", "ultrafast", "-pix_fmt", "yuv420p", "-movflags", "+faststart", "-threads", "1", "recording-transcoded.mp4"]);
+    });
+});