[Libav-user] Mapping / converting QTCapture's to FFmpeg's pixel format

[salsaman]

NV12 is planar Y, followed by packed UV combined.

So to convert to YUV420, the Y plane remains the same, but you need to
move the V values so they are after the U values.

I.e.

Ydata[width * height] + UVdata[widt * height/2] ->

Ydata[width*height] + Udata[width*height/4] + Vdata[width*height/4]

I have never encountered a YUV format with extra rowstride padding, so
most likely that is not a problem.


Regards,
Salsaman.

http://lives.sourceforge.net
https://www.ohloh.net/accounts/salsaman


On Tue, Feb 5, 2013 at 6:47 AM, Alex Cohn <alexcohn at netvision.net.il> wrote:
> On Tue, Feb 5, 2013 at 7:51 AM, Brad O'Hearne <brado at bighillsoftware.com> wrote:
>> I have in my mind that the first one is probably the right pixel format (also
>> the one I need to use), but there's some state of the resulting frame /
>> packet that isn't being set properly I'm guessing, and it is causing to
>> dereference a null pointer somewhere inside libavcodec. Does anyone have any
>> idea what might be causing this? Is there some value I need to set before
>> encoding the video?
>
> Unfortunately, most video codecs support only few (usually, one) pixel
> formats - both as input for encoding and output for decoding. When you
> use ffmpeg command line and specify an incompatible pixel format,
> ffmpeg will perform format conversion for you. But when you use the
> libavcodec library, you must take care of this conversion yourself.
>
> It works (well, fails miserably) as expected when you specify
> AV_PIX_FMT_NV12 (which, as far as I can tell, is the ffmpeg equivalent
> for kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange) for a codec that
> does not support this format. You can use swscale to convert the
> images to AV_PIX_FMT_YUV420P, which is supported by vast majority of
> ffmpeg video encoders.
>
> It does puzzle me, though, that you experience crash when you
> configure your capture with kCVPixelFormatType_420YpCbCr8Planar. How
> do you pass it to avpicture_fill()? If I understand correctly,
> kCVPixelFormatType_420YpCbCr8Planar exposes three fields:
> ComponentInfoY, ComponentInfoCb, and ComponentInfoCr. But ffmpeg
> expects a contiguous byte array of Y pixels (w*h), followed by Cb
> (w/2*h/2).
>
> Therefore, even for conversion from
> kCVPixelFormatType_420YpCbCr8Planar to AV_PIX_FMT_YUV420P you need
> some extra step: create a byte array of size w*h*3/2 and copy the
> three planes to this array. If the source has line padding, you should
> take care of this, too.
>
> Regards,
> Alex
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