[FFmpeg-devel] [PATCH V7 4/6] lavu: add side data AV_FRAME_DATA_BOUNDING_BOXES

Fri Apr 9 18:10:19 EEST 2021

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> Guo, Yejun
> Sent: 2021å¹´4æœˆ9æ—¥ 20:57
> To: FFmpeg development discussions and patches
> <ffmpeg-devel at ffmpeg.org>
> Subject: Re: [FFmpeg-devel] [PATCH V7 4/6] lavu: add side data
> AV_FRAME_DATA_BOUNDING_BOXES
> 
> 
> 
> > -----Original Message-----
> > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> > Lynne
> > Sent: 2021å¹´4æœˆ9æ—¥ 18:03
> > To: FFmpeg development discussions and patches
> > <ffmpeg-devel at ffmpeg.org>
> > Subject: Re: [FFmpeg-devel] [PATCH V7 4/6] lavu: add side data
> > AV_FRAME_DATA_BOUNDING_BOXES
> >
> > Apr 9, 2021, 06:12 by yejun.guo at intel.com:
> >
> > >
> > >
> > >> -----Original Message-----
> > >> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf
> Of
> > Lynne
> > >> Sent: 2021å¹´4æœˆ9æ—¥ 0:57
> > >> To: FFmpeg development discussions and patches
> > <ffmpeg-devel at ffmpeg.org>
> > >> Subject: Re: [FFmpeg-devel] [PATCH V7 4/6] lavu: add side data
> > >> AV_FRAME_DATA_BOUNDING_BOXES
> > >>
> > >
> > > First of all, thanks for the quick replies, I see, all the
> > discussions/comments are to
> > > make this patch better, thank you.
> > >
> > >> >> >
> > >> >> >> >> > +
> > >> >> >> >> > +typedef struct AVBoundingBoxHeader {
> > >> >> >> >> > +    /**
> > >> >> >> >> > +     * Information about how the bounding box is
> > generated.
> > >> >> >> >> > +     * for example, the DNN model name.
> > >> >> >> >> > +     */
> > >> >> >> >> > +    char source[128];
> > >> >> >> >> > +
> > >> >> >> >> > +    /**
> > >> >> >> >> > +     * The size of frame when it is detected.
> > >> >> >> >> > +     */
> > >> >> >> >> > +    int frame_width;
> > >> >> >> >> > +    int frame_height;
> > >> >> >> >> >
> > >> >> >> >>
> > >> >> >> >> Why? This side data is attached to AVFrames only, where we
> > >> >> >> >> already have width and height.
> > >> >> >> >>
> > >> >> >> >
> > >> >> >> > The detection result will be used by other filters, for example,
> > >> >> >> > dnn_classify (see
> > https://github.com/guoyejun/ffmpeg/tree/classify).
> > >> >> >> >
> > >> >> >> > The filter dnn_detect detects all the objects (cat, dog,
> person ...)
> > in a
> > >> >> >> > frame, while dnn_classify classifies one detected object (for
> > example,
> > >> >> person)
> > >> >> >> > for its attribute (for example, emotion, etc.)
> > >> >> >> >
> > >> >> >> > The filter dnn_classify have to check if the frame size is
> changed
> > after
> > >> >> >> > it is detected, to handle the below filter chain:
> > >> >> >> > dnn_detect -> scale -> dnn_classify
> > >> >> >> >
> > >> >> >>
> > >> >> >> This doesn't look good. Why is dnn_classify needing to know
> > >> >> >> the original frame size at all?
> > >> >> >>
> > >> >> >
> > >> >> > For example, the original size of the frame is 100*100, and
> > dnn_detect
> > >> >> > detects a face at place (10, 10) -> (30, 40), such data will be saved
> in
> > >> >> > AVBoundingBox.top/left/right/bottom.
> > >> >> >
> > >> >> > Then, the frame is scaled into 50*50.
> > >> >> >
> > >> >> > Then, dnn_classify is used to analyze the emotion of the face, it
> > needs to
> > >> >> > know the frame size (100*100) when it is detected, otherwise, it
> > does not
> > >> >> > work with just (10,10), (30,40) and 50*50.
> > >> >> >
> > >> >>
> > >> >> Why can't the scale filter also rescale this side data as well?
> > >> >>
> > >> >
> > >> > I'm afraid that we could not make sure all such filters (including
> filters
> > in the
> > >> > future) to do the rescale. And in the previous discussion, I got to
> > know that
> > >> > 'many other existing side-data types are invalidated by scaling'.
> > >> >
> > >> > So, we need frame_width and frame_height here.
> > >> >
> > >>
> > >> No, you don't. You just need to make sure filters which change
> > resolution
> > >> or do cropping also change the side data parameters.
> > >> It's called maintainership. As-is, this won't even work with cropping,
> > >> only with basic aspect ratio preserving scaling.
> > >> For the lack of a better term, this is a hack.
> > >>
> > >
> > > As discussed in previous email, for the frame size change case,
> > dnn_classify
> > > (and other filters which use the detection result, for example drawbox)
> > can
> > > just output a warning message to tell user what happens, and don't do
> > the
> > > classification, otherwise, it will give a wrong/weird result which makes
> > the
> > > user confused.
> > >
> > >>
> > >> I would accept just specifying that if the frame dimensions are
> > >> altered in any way, the side-data is no longer valid and it's up
> > >> to users to figure that out by out of bound coordinates.
> > >> This is what we currently do with video_enc_params.
> > >>
> > >
> > > frame_width/frame_height is not perfect (for the cases such as: scale
> > down
> > > + crop + scale up to the same size), but it provides more info than the
> > checking
> > > of 'out of bound coordinates'. There are many other possible issues
> > when the
> > > coordinates are within the frame.
> > >
> > > If we think we'd better not let user get more info from the warning
> > message,
> > > I'm ok to remove them.
> > >
> > > I'll remove them if there's another comment supporting the removal,
> and
> > > there's no objection.
> > >
> >
> > We definitely shouldn't include variables in public API structs
> > that only serve to print a warning if they don't match.
> 
> Not just 'print a warning', it also impacts the behavior of dnn_classify.
> 
> > Especially one that's fragile and flawed like this one.
> > Users should know that scaling or altering a frame could break
> > this side data, and filters could detect obvious out of bounds
> > results and report them.
> 
> I'll remove them since it is user's responsibility.
> 
> >
> > In the meantime, the main scaling and cropping filters could
> > receive separate patches to preserve metadata at a later data.
> > This is how the avframe cropping field work - they're just metadata,
> > and cropping/scaling filters update those.
> >
> >
> > >> >> >> >> > diff --git a/libavutil/frame.h b/libavutil/frame.h
> > >> >> >> >> > index a5ed91b20a..41e22de02a 100644
> > >> >> >> >> > --- a/libavutil/frame.h
> > >> >> >> >> > +++ b/libavutil/frame.h
> > >> >> >> >> > @@ -198,6 +198,13 @@ enum AVFrameSideDataType {
> > >> >> >> >> >  * Must be present for every frame which should have
> film
> > grain
> > >> >> applied.
> > >> >> >> >> >  */
> > >> >> >> >> >  AV_FRAME_DATA_FILM_GRAIN_PARAMS,
> > >> >> >> >> > +
> > >> >> >> >> > +    /**
> > >> >> >> >> > +     * Bounding boxes for object detection and
> > classification, the
> > >> >> data is
> > >> >> >> a
> > >> >> >> >> AVBoundingBoxHeader
> > >> >> >> >> > +     * followed with an array of AVBoudingBox, and
> > >> >> >> >> AVBoundingBoxHeader.nb_bboxes is the number
> > >> >> >> >> > +     * of array element.
> > >> >> >> >> > +     */
> > >> >> >> >> > +    AV_FRAME_DATA_BOUNDING_BOXES,
> > >> >> >> >> >  };
> > >> >> >> >> >
> > >> >> >> >>
> > >> >> >> >> Finally, why call it a Bounding Box? It's not descriptive at all.
> > >> >> >> >> How about "Object Classification"? It makes much more
> sense,
> > it's
> > >> >> >> >> exactly what this is. So AVObjectClassification,
> > AVObjectClassification,
> > >> >> >> >> AV_FRAME_DATA_OBJECT_CLASSIFICATION and so on.
> > >> >> >> >>
> > >> >> >> >
> > >> >> >> > In object detection papers, bounding box is usually used.
> > >> >> >> > We'd better use the same term, imho, thanks.
> > >> >> >> >
> > >> >> >>
> > >> >> >> Not in this case, API users won't have any idea what this is or
> > what
> > >> >> >> it's for. This is user-facing code after all.
> > >> >> >> Papers in fields can get away with overloading language, but
> > we're
> > >> >> >> trying to make a concise API. Object classification makes sense
> > >> >> >> because this is exactly what this is.
> > >> >> >>
> > >> >> >
> > >> >> > The term bounding box is dominating the domain, for example,
> > even
> > >> >> > HEVC spec uses this term, see page 317 of
> > >> >> >
> > >> >>
> > >>
> >
> https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-H.265-201911-I
> > !!P
> > >> >> DF-E&type=items
> > >> >> >
> > >> >> > also copy some here for your convenient.
> > >> >> > ar_bounding_box_top[ ar_object_idx[ i ] ] u(16)
> > >> >> > ar_bounding_box_left[ ar_object_idx[ i ] ] u(16)
> > >> >> > ar_bounding_box_width[ ar_object_idx[ i ] ] u(16)
> > >> >> > ar_bounding_box_height[ ar_object_idx[ i ] ] u(16)
> > >> >> >
> > >> >> > I would prefer to use bounding box.
> > >> >> >
> > >> >>
> > >> >> It's for an entirely different thing, and like I said, it's just an
> > overloaded
> > >> >> language because they can get away. We're trying to be generic.
> > >> >> This side data is for detecting _and_ classifying objects. In fact,
> most
> > of
> > >> >> the structure is dedicated towards classifying. If you'd like users to
> > actually
> > >> >> use this, give it a good name and don't leave them guessing what
> > this
> > >> >> structure is by throwing vague jargon some other paper or spec
> has
> > >> >> because it's close enough.
> > >> >>
> > >> >
> > >> > all the people in the domain accepts bounding box, they can
> > understand this
> > >> > struct name easily and clearly, they might be bothered if we use
> > another
> > >> name.
> > >> >
> > >> > btw, AVObjectClassification confuses people who just want object
> > detection.
> > >> >
> > >>
> > >> As I said, the name "bounding box" makes no sense once it gets
> > overloaded
> > >> with object classification.
> > >>
> > >
> > > dnn_detect creates an array of 'bounding box' for all detected objects,
> > and
> > > dnn_classify assigns attributes for a set of bounding boxes (with same
> > object
> > > id). 'bounding box' serves both detection and classification properly.
> > >
> > >
> > >> Object classification is still the main use of the filters,
> > >> because the original proposal was to have all of this info be
> > ffmpeg-private,
> > >> which would forbid simple object detection.
> > >>
> > >
> > > The original proposal is to add it as side data which is ffmpeg-public,
> and
> > then,
> > > we spent much time discussing/trying with ffmpeg-private as an
> > temporary
> > > method, and since it is not good to be temporary, we now switch back
> to
> > > ffmpeg-public.
> > >
> > > During the whole period, we don't have any intention to
> > > 'forbid simple object detection', not quite understand your point here.
> > >
> > >
> > >> So I still maintain this should be called "Object classification". I'd
> accept
> > >> "Object detection" as well, but definitely not "bounding box".
> > >>
> > >
> > > imho, ' Object detection' and ' Object classification' are worse, they just
> > > describe one aspect of the struct. The users might just use filter
> > dnn_detect,
> > > they might use filters dnn_detect + dnn_classify.
> > >
> >
> > The whole reason why we have this side data is to both detect
> > _and_ classify. Keyword being _both_. Hence object detection
> > and object classification are much better names.
> > I am opposed to merging this without a name change.
> 
> I understand it is not a good name with codec background, but it is a good
> (possible best) name with object detection background.
> 
> To make things move forward, I'll change the name to 'object detection'
> for both dnn_detect and dnn_classify, they would be:
> AV_FRAME_DATA_OJBECT_DETECTION
> AVObjectDetection
> AVObjectDetectionHeader

I tried to change the code with this new name, but feel like it is not
straight-forward during the coding.

Pedro, who initialized DNN module, just commented on the naming, maybe
I can just wait for more several days to get more comments on the naming,
thanks.