Re: [FFmpeg-devel] [PATCH] lavu/stereo3d: change baseline and horizontal FOV to rationals

[Lynne via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

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On 22/06/2024 23:30, James Almer wrote:
> On 6/22/2024 6:26 PM, Lynne via ffmpeg-devel wrote:
>> On 22/06/2024 22:19, James Almer wrote:
>>> On 6/22/2024 5:11 PM, Lynne via ffmpeg-devel wrote:
>>>> This avoids hardcoding any implementation-specific limitiations as
>>>> part of the API, and allows for future expandability.
>>>>
>>>> This also allows API users to more conveniently convert the
>>>> values into floats without hardcoding specific conversion constants.
>>>>
>>>> The draft implementation of this mechanism for the draft of
>>>> AVTransport uses rationals for these particular fields.
>>>>
>>>> The API was committed 2 days ago, so changing these fields now
>>>> is within the realms of acceptable.
>>>> ---
>>>>   fftools/ffprobe.c                            |  4 ++--
>>>>   libavformat/dump.c                           |  9 +++++----
>>>>   libavformat/mov.c                            |  9 ++++++---
>>>>   libavutil/stereo3d.h                         |  8 ++++----
>>>>   tests/ref/fate/matroska-spherical-mono       |  4 ++--
>>>>   tests/ref/fate/matroska-spherical-mono-remux |  8 ++++----
>>>>   tests/ref/fate/matroska-stereo_mode          | 16 ++++++++--------
>>>>   tests/ref/fate/matroska-vp8-alpha-remux      |  4 ++--
>>>>   tests/ref/fate/mov-spherical-mono            |  4 ++--
>>>>   9 files changed, 35 insertions(+), 31 deletions(-)
>>>>
>>>> diff --git a/fftools/ffprobe.c b/fftools/ffprobe.c
>>>> index d7ba980ff9..2da928b7c3 100644
>>>> --- a/fftools/ffprobe.c
>>>> +++ b/fftools/ffprobe.c
>>>> @@ -2546,9 +2546,9 @@ static void print_pkt_side_data(WriterContext *w,
>>>>               print_int("inverted", !!(stereo->flags & 
>>>> AV_STEREO3D_FLAG_INVERT));
>>>>               print_str("view", av_stereo3d_view_name(stereo->view));
>>>>               print_str("primary_eye", 
>>>> av_stereo3d_primary_eye_name(stereo->primary_eye));
>>>> -            print_int("baseline", stereo->baseline);
>>>> +            print_q("baseline", stereo->baseline, '/');
>>>>               print_q("horizontal_disparity_adjustment", stereo- 
>>>> >horizontal_disparity_adjustment, '/');
>>>> -            print_int("horizontal_field_of_view", stereo- 
>>>> >horizontal_field_of_view);
>>>> +            print_q("horizontal_field_of_view", stereo- 
>>>> >horizontal_field_of_view, '/');
>>>>           } else if (sd->type == AV_PKT_DATA_SPHERICAL) {
>>>>               const AVSphericalMapping *spherical = 
>>>> (AVSphericalMapping *)sd->data;
>>>>               print_str("projection", 
>>>> av_spherical_projection_name(spherical->projection));
>>>> diff --git a/libavformat/dump.c b/libavformat/dump.c
>>>> index 61a2c6a29f..ac7d3038ab 100644
>>>> --- a/libavformat/dump.c
>>>> +++ b/libavformat/dump.c
>>>> @@ -262,13 +262,14 @@ static void dump_stereo3d(void *ctx, const 
>>>> AVPacketSideData *sd, int log_level)
>>>>       av_log(ctx, log_level, "%s, view: %s, primary eye: %s",
>>>>              av_stereo3d_type_name(stereo->type), 
>>>> av_stereo3d_view_name(stereo->view),
>>>>              av_stereo3d_primary_eye_name(stereo->primary_eye));
>>>> -    if (stereo->baseline)
>>>> -        av_log(ctx, log_level, ", baseline: %"PRIu32"", stereo- 
>>>> >baseline);
>>>> +    if (stereo->baseline.num && stereo->baseline.den)
>>>> +        av_log(ctx, log_level, ", baseline: %d/%d", stereo- 
>>>> >baseline.num, stereo->baseline.den);
>>>>       if (stereo->horizontal_disparity_adjustment.num && stereo- 
>>>> >horizontal_disparity_adjustment.den)
>>>>           av_log(ctx, log_level, ", horizontal_disparity_adjustment: 
>>>> %d/%d",
>>>>                  stereo->horizontal_disparity_adjustment.num, 
>>>> stereo- >horizontal_disparity_adjustment.den);
>>>> -    if (stereo->horizontal_field_of_view)
>>>> -        av_log(ctx, log_level, ", horizontal_field_of_view: 
>>>> %"PRIu32"", stereo->horizontal_field_of_view);
>>>> +    if (stereo->horizontal_field_of_view.num && stereo- 
>>>> >horizontal_field_of_view.den)
>>>> +        av_log(ctx, log_level, ", horizontal_field_of_view: %d/%d", 
>>>> stereo->horizontal_field_of_view.num,
>>>> +               stereo->horizontal_field_of_view.den);
>>>>       if (stereo->flags & AV_STEREO3D_FLAG_INVERT)
>>>>           av_log(ctx, log_level, " (inverted)");
>>>> diff --git a/libavformat/mov.c b/libavformat/mov.c
>>>> index f08fec3fb6..64cd17e770 100644
>>>> --- a/libavformat/mov.c
>>>> +++ b/libavformat/mov.c
>>>> @@ -6545,7 +6545,8 @@ static int mov_read_eyes(MOVContext *c, 
>>>> AVIOContext *pb, MOVAtom atom)
>>>>       MOVStreamContext *sc;
>>>>       int size, flags = 0;
>>>>       int64_t remaining;
>>>> -    uint32_t tag, baseline = 0;
>>>> +    uint32_t tag;
>>>> +    AVRational baseline = av_make_q(0, 0);
>>>>       enum AVStereo3DView view = AV_STEREO3D_VIEW_PACKED;
>>>>       enum AVStereo3DPrimaryEye primary_eye = AV_PRIMARY_EYE_NONE;
>>>>       AVRational horizontal_disparity_adjustment = { 0, 1 };
>>>> @@ -6642,7 +6643,8 @@ static int mov_read_eyes(MOVContext *c, 
>>>> AVIOContext *pb, MOVAtom atom)
>>>>               avio_skip(pb, 1); // version
>>>>               avio_skip(pb, 3); // flags
>>>> -            baseline = avio_rb32(pb);
>>>> +            baseline.num = avio_rb32(pb);
>>>> +            baseline.den = 1000000; // micrometers
>>>>               break;
>>>>           }
>>>> @@ -6782,7 +6784,8 @@ static int mov_read_hfov(MOVContext *c, 
>>>> AVIOContext *pb, MOVAtom atom)
>>>>               return AVERROR(ENOMEM);
>>>>       }
>>>> -    sc->stereo3d->horizontal_field_of_view = avio_rb32(pb);
>>>> +    sc->stereo3d->horizontal_field_of_view.num = avio_rb32(pb);
>>>> +    sc->stereo3d->horizontal_field_of_view.den = 1000; // thousands 
>>>> of a degree
>>>>       return 0;
>>>>   }
>>>> diff --git a/libavutil/stereo3d.h b/libavutil/stereo3d.h
>>>> index 00a5c3900e..097f6c9455 100644
>>>> --- a/libavutil/stereo3d.h
>>>> +++ b/libavutil/stereo3d.h
>>>> @@ -213,9 +213,9 @@ typedef struct AVStereo3D {
>>>>       /**
>>>>        * The distance between the centres of the lenses of the 
>>>> camera system,
>>>> -     * in micrometers. Zero if unset.
>>>> +     * in meters. Zero if unset.
>>>>        */
>>>> -    uint32_t baseline;
>>>> +    AVRational baseline;
>>>
>>> This is not ok. No reason for distance be a soft float, or allow 
>>> negative values. In the case of the blin box, it's a 32bit unsigned 
>>> value, which will not fit in AVRational.
>>
>> Added an (int32_t) cast locally. This is also how 
>> horizontal_disparity_adjustment is handled, with a uint32_t converted 
>> into a rational.
> 
> No, that one is an int32_t as it can be negative (Ranges -1.0 to 1.0), 
> thus it works fine as AVRational. baseline does not, so it should stay 
> an uint32_t.

I doubt that anyone will use a baseline greater than 2.1 (2^31/2*10^6) 
kilometers, but sure, this part is dropped. I more strongly doubt anyone 
will use a baseline of less than a micrometer, mechanics start to play 
with dice, and atomic force microscopy has coarser resolution than this.

>>
>>>
>>> You can easily convert this in your code by using a 1000000 
>>> denominator if you wish.
>>>
>>>>       /**
>>>>        * Relative shift of the left and right images, which changes 
>>>> the zero parallax plane.
>>>> @@ -224,9 +224,9 @@ typedef struct AVStereo3D {
>>>>       AVRational horizontal_disparity_adjustment;
>>>>       /**
>>>> -     * Horizontal field of view in thousanths of a degree. Zero if 
>>>> unset.
>>>> +     * Horizontal field of view, in degrees. Zero if unset.
>>>>        */
>>>> -    uint32_t horizontal_field_of_view;
>>>> +    AVRational horizontal_field_of_view;
>>>
>>> This one i can't say. Would for example Matroska implement degrees as 
>>> a float, or uint?
>>
>> Likely a float (EBML_FLOAT). This is how mastering metadata and video 
>> projection are handled.
> 
> But Mastering metadata is not in degrees.

We use rationals for everything, not just framerates and ratios.

> Wait for Derek to comment. He knows this field better than me.

Yup, that's the plan.

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