How to change stream resolution when screencast?

[Jay]

I have a screencast inherited from cricket::VideoCapturer.

When I let IsScreencast() return false, video resolution in the receiver side will change dynamically, and I cannot control it.

If IsScreencast() returns true, video resolution will be always fixed the same as the frame resolution passed in screencast.SignalFrameCaptured(&screencast, frame).

Assert captured screen size is always 1920*1080, and I want the video stream resolution can be 960*540 or others. 

How to change stream resolution when screencast?

I try to use formats and constraints:

```

        vector<cricket::VideoFormat> formats;

        formats.push_back(cricket::VideoFormat(1920, 1080, 100, cricket::FOURCC_ABGR));

        formats.push_back(cricket::VideoFormat(960, 540, 100, cricket::FOURCC_ABGR));

        screencast.SetSupportedFormats(formats);

```
```

        webrtc::FakeConstraints constraints2;

        constraints2.SetMandatoryMaxFrameRate(20);

        constraints2.AddMandatory(webrtc::MediaConstraintsInterface::kMinWidth, 960);

        constraints2.AddMandatory(webrtc::MediaConstraintsInterface::kMinHeight, 540);

        constraints2.AddMandatory(webrtc::MediaConstraintsInterface::kMaxAspectRatio, 960.0/540.0);
        videoSource = mPeerConnectionFactoryPtr->CreateVideoSource(&screencast, &constraints2);

```
or use SDP max-recv-width and imageattr both in sender and receiver

```

a=rtpmap:100 VP8/90000

a=rtcp-fb:100 ccm fir

a=rtcp-fb:100 nack

a=rtcp-fb:100 nack pli

a=rtcp-fb:100 goog-remb

a=rtcp-fb:100 transport-cc

a=fmtp:100 max-recv-width=640;max-recv-height=480

```

```

a=rtpmap:100 VP8/90000

a=setup:active

a=ssrc:3555258440 cname:{95e6e7c3-309b-45b5-975a-5fad567b1fd1}

a=imageattr:100 recv [x=960,y=540] send [x=960,y=540]

```

all are not working.

Did I have something wrong?

[Jay]

I find the code that is impossible to change resolution when set `IsScreencast()` return true.

below it will never get into `video_adapter_.AdaptFrameResolution()`.

```

  if (enable_video_adapter_ && !IsScreencast()) {

    if (!video_adapter_.AdaptFrameResolution(

            width, height, capture_time_ns,

            crop_width, crop_height, out_width, out_height)) {

      // VideoAdapter dropped the frame.

      return false;

    }

    *crop_x = (width - *crop_width) / 2;

    *crop_y = (height - *crop_height) / 2;

  } else { 

```

the workaround is to set `IsScreencast()` always return false even it's a screen cast.