FFmpeg on iPhone - Modifying Video Orientation

问题:

I'm messing with h264 videos loaded with FFmpeg on the iPhone 3GS. The problem is any videos recorded in "Portrait" orientation have a transformation matrix applied to them causing them to display rotated 90 degrees counter-clock.

From what I understand thus far, I just need to modify the transform matrix in the 'tkhd' atom. The problem is I am having trouble accessing or modifying this data. I checked out the FFmpeg implementation for:

static int mov_read_tkhd(MOVContext *c, ByteIOContext *pb, MOVAtom atom)

which clearly shows how the matrix is accessed in avformat but when I try to access the header bytes using the same functions I am not getting any rational values. Even if I were to successfully pull the matrix I'm not sure how to replace it? FFmpeg has functions for retrieving and appending to the track header but nothing for replace it seems?

Any help would be greatly appreciated.

Thanks, Matt.

回答1:

To those who cannot get the v filter option to work, after some searching I found an alternative option which works for my build:

ffmpeg -i in.avi -vf "transpose=1" out.avi

this flips the video 90 degrees clockwise. hope this helps all those on ubuntu struggling like I was!! :)

回答2:

I just had a patch accepted on the FFMPEG git master branch which should help all of us in adjusting for the iPhone's orientation and subsequent transform applied to the .mov file it sends out. FFMPEG will now send back in metadata:

'rotate'=90,180,or 270 depending on how many degrees you need to turn the image clockwise to get the true rotation.

回答3:

Portrait video recorded in iPhone 3GS is strangely rotated in landscape, and only quicktime plays it correctly, players like VLC etc play the portrait mode videos in landscape because it is encoded that way! Maybe apple have set some bit so that quicktime identifies and sets the orientation properly.

You can use -vfilters option of ffmpeg to rotate the video. It is available in version r21242. You have to patch it on your version of ffmpeg.

configure the build by using --enable-vfilters

and you can use the option:

ffmpeg -vfilters "rotate=90" -i input.mp4 output.mp4

to rotate the input movie.

回答4:

the movie is recorded directly with the orientation of the hardware camera. when you turn it the camera still records with the same orientation and the same straight write to file.

What determines the orientation of the video is the Transform matrix

The matrix is set dependent on the iphone's orientation. And this is what will determine if you have to rotate the video. And that rotation could be 90 degrees and it could be 180 degrees. It depends on the phone orientation.

After you get an idea of what the matrix is and how it relates to the image you will be able to determine the orientation of the video.