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Confused by `VideoDecoderConfig.codedWidth`, does not reflect the actual coded width of the underlying coded frame

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5/5
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Issue 类型
缺陷
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需要澄清
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技术栈
javascript
领域
api, audio-video-rtc

调研方向

先从 WebCodecs 中 VideoDecoderConfig.codedWidth 和 VideoFrame.codedWidth 的定义开始，然后运行所提供的 VideoEncoder/VideoDecoder AVC 复现。将报告的配置尺寸与解码帧的尺寸进行比较；完成的标准是确定该行为是否符合预期，并就规范是否需要修改以及如何修改达成一致。

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描述

In all permutations I have tried ((Chrome, Firefox, Safari) x (Windows, Mac)), the codedWidth and codedHeight properties in the VideoDecoderConfig emitted by VideoEncoder contain the display dimensions of the frame, or in other words, the dimensions with which the encoder was initialized. In my work on Mediabunny, I have relied on this behavior extensively as a means to communicate video dimensions to muxers.

However, I kind of just realized... that behavior is wrong (probably)? The definition of VideoDecoderConfig.codedWidth is:

Width of the VideoFrame in pixels, potentially including non-visible padding, and prior to considering potential ratio adjustments.

Which is identical to the definition of VideoFrame.codedWidth.

So let's say we do this:

const decoder = new VideoDecoder({
	output: (frame) => {
		console.log("Actual coded width:", frame.codedWidth);
		console.log(frame);
	},
	error: console.error,
});

const encoder = new VideoEncoder({
	output: (chunk, meta) => {
		console.log("Reported coded width:", meta.decoderConfig.codedWidth);

		decoder.configure(meta.decoderConfig);
		decoder.decode(chunk);
		decoder.flush();
	},
	error: console.error,
});
encoder.configure({
	codec: 'avc1.64001f',
	width: 650,
	height: 650,
	bitrate: 1e6,
});

const canvas = new OffscreenCanvas(650, 650);
const ctx = canvas.getContext('2d');
ctx.fillStyle = 'red';
ctx.fillRect(100, 100, 200, 200);

const frame = new VideoFrame(canvas, { timestamp: 0 });
encoder.encode(frame);

await encoder.flush();

We encode a 650x650 frame using AVC, which then, when decoded, has a codedWidth of 656. Makes total sense, since the dimensions have been rounded up to the nearest 16x16 macroblock, and 16 divides 656. But, the decoder config reports the codedWidth as 650.

So, same field, same definition in the spec, different values.

While I think it's useful that it contains the display dimensions, I would argue the behavior is not technically correct. Here's what I would expect:

  • codedWidth reflects the actual codedWidth you'd get by decoding the chunks (this makes it consistent, and actually provides new information that you didn't already know based on how you set up the encoder)
  • VideoDecoderConfig is extended with displayWidth, displayHeight, visibleRect, and codedRect, with definitions identical to those on VideoFrame.

What do you guys think? Is the current behavior of codedWidth faulty or intentional? Can this be fixed, or is it too late now since any change might break existing software that relies on the current behavior?

Edit:
My proposed ""change"" to codedWidth would mean that demuxers, when extracting VideoDecoderConfig for VideoDecoder, would need more complicated logic to derive the coded width, since they likely only know the display dimensions from the container metadata. Luckily, the fields are optional, so any demuxer could just omit it. But still, kinda annoying. Perhaps one should go the other route, retconning codedWidth to be defined in the spec as it is currently provided by browsers?

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