Re: [FFmpeg-devel] [PATCH] avcodec/videotoolboxenc: Add CBR option to H264 and HEVC encoder

[Rick Kern <kernrj@gmail.com>]

On Fri, Aug 26, 2022 at 7:42 AM Sebastian Beckmann <
beckmann.sebastian@outlook.de> wrote:

> Adds an option to use constant bitrate instead of average bitrate to the
> videotoolbox encoders. This is enabled via -constant_bit_rate true.
> macOS 13 is required for this option to work.
>
> Signed-off-by: Sebastian Beckmann <beckmann.sebastian@outlook.de>
> ---
> libavcodec/videotoolboxenc.c | 37 +++++++++++++++++++++++++++++++++---
> 1 file changed, 34 insertions(+), 3 deletions(-)
>
> diff --git a/libavcodec/videotoolboxenc.c b/libavcodec/videotoolboxenc.c
> index 823e5ad94e..9eb6fe09a2 100644
> --- a/libavcodec/videotoolboxenc.c
> +++ b/libavcodec/videotoolboxenc.c
> @@ -101,6 +101,7 @@ static struct{
>     CFStringRef kVTCompressionPropertyKey_RealTime;
>     CFStringRef kVTCompressionPropertyKey_TargetQualityForAlpha;
>     CFStringRef
> kVTCompressionPropertyKey_PrioritizeEncodingSpeedOverQuality;
> +    CFStringRef kVTCompressionPropertyKey_ConstantBitRate;
>
>     CFStringRef
> kVTVideoEncoderSpecification_EnableHardwareAcceleratedVideoEncoder;
>     CFStringRef
> kVTVideoEncoderSpecification_RequireHardwareAcceleratedVideoEncoder;
> @@ -164,6 +165,7 @@ static void loadVTEncSymbols(){
>             "TargetQualityForAlpha");
>     GET_SYM(kVTCompressionPropertyKey_PrioritizeEncodingSpeedOverQuality,
>             "PrioritizeEncodingSpeedOverQuality");
> +    GET_SYM(kVTCompressionPropertyKey_ConstantBitRate, "ConstantBitRate");
>
>
> GET_SYM(kVTVideoEncoderSpecification_EnableHardwareAcceleratedVideoEncoder,
>             "EnableHardwareAcceleratedVideoEncoder");
> @@ -236,6 +238,7 @@ typedef struct VTEncContext {
>     int realtime;
>     int frames_before;
>     int frames_after;
> +    bool constant_bit_rate;
>
>     int allow_sw;
>     int require_sw;
> @@ -1073,12 +1076,22 @@ static bool vtenc_qscale_enabled(void)
>     return !TARGET_OS_IPHONE && TARGET_CPU_ARM64;
> }
>
> +// constant bit rate only on Macs with Apple Silicon running macOS 13
> (Ventura) or newer
> +static bool vtenc_constant_bit_rate_enabled(void)
>

It's possible that Apple adds support for CBR on iOS or Intel-based Macs in
the future, so it would be more future-proof to avoid hard-coding here.
Then you could always try to set the property, and print an error message
if it returns an error.  See PrioritizeEncodingSpeedOverQuality for an
example.

I see the same hard-coded logic is used in vtenc_qscale_enabled(). Thanks
for following the patterns in this file, because that's usually the right
thing to do. However, the qscale issue slipped through the cracks and also
needs to be changed.


> +{
> +    if (__builtin_available(macOS 13, *))

+        return !TARGET_OS_IPHONE && TARGET_CPU_ARM64;
> +    else
> +        return false;
> +}
> +
> static int vtenc_create_encoder(AVCodecContext   *avctx,
>                                 CMVideoCodecType codec_type,
>                                 CFStringRef      profile_level,
>                                 CFNumberRef      gamma_level,
>                                 CFDictionaryRef  enc_info,
>                                 CFDictionaryRef  pixel_buffer_info,
> +                                bool constant_bit_rate,
>                                 VTCompressionSessionRef *session)
> {
>     VTEncContext *vtctx = avctx->priv_data;
> @@ -1122,6 +1135,11 @@ static int vtenc_create_encoder(AVCodecContext
>  *avctx,
>         return AVERROR_EXTERNAL;
>     }
>
> +    if (constant_bit_rate && !vtenc_constant_bit_rate_enabled()) {
> +        av_log(avctx, AV_LOG_ERROR, "Error: -constant_bit_rate true not
> available for encoder.\n");
> +        return AVERROR_EXTERNAL;
> +    }
> +
>     if (avctx->flags & AV_CODEC_FLAG_QSCALE) {
>         quality = quality >= 100 ? 1.0 : quality / 100;
>         quality_num = CFNumberCreate(kCFAllocatorDefault,
> @@ -1139,9 +1157,16 @@ static int vtenc_create_encoder(AVCodecContext
>  *avctx,
>                                       &bit_rate);
>         if (!bit_rate_num) return AVERROR(ENOMEM);
>
> -        status = VTSessionSetProperty(vtctx->session,
> -
> kVTCompressionPropertyKey_AverageBitRate,
> -                                      bit_rate_num);
> +        if (constant_bit_rate) {
> +            status = VTSessionSetProperty(vtctx->session,
> +
> compat_keys.kVTCompressionPropertyKey_ConstantBitRate,
> +                                          bit_rate_num);
>

For example, you could check status here, and output a message about no CBR
support.


> +        } else {
> +            status = VTSessionSetProperty(vtctx->session,
> +
> kVTCompressionPropertyKey_AverageBitRate,
> +                                          bit_rate_num);
> +        }
> +
>         CFRelease(bit_rate_num);
>     }
>
> @@ -1530,6 +1555,7 @@ static int vtenc_configure_encoder(AVCodecContext
> *avctx)
>                                   gamma_level,
>                                   enc_info,
>                                   pixel_buffer_info,
> +                                  vtctx->constant_bit_rate,
>                                   &vtctx->session);
>
> init_cleanup:
> @@ -2532,6 +2558,7 @@ static int vtenc_populate_extradata(AVCodecContext
>  *avctx,
>                                   gamma_level,
>                                   enc_info,
>                                   pixel_buffer_info,
> +                                  vtctx->constant_bit_rate,
>                                   &vtctx->session);
>     if (status)
>         goto pe_cleanup;
> @@ -2727,6 +2754,8 @@ static const AVOption h264_options[] = {
>
>     { "a53cc", "Use A53 Closed Captions (if available)", OFFSET(a53_cc),
> AV_OPT_TYPE_BOOL, {.i64 = 1}, 0, 1, VE },
>
> +    { "constant_bit_rate", "Require constant bit rate (macOS 13 or
> newer)", OFFSET(constant_bit_rate), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1,
> VE },
> +
>     COMMON_OPTIONS
>     { NULL },
> };
> @@ -2760,6 +2789,8 @@ static const AVOption hevc_options[] = {
>
>     { "alpha_quality", "Compression quality for the alpha channel",
> OFFSET(alpha_quality), AV_OPT_TYPE_DOUBLE, { .dbl = 0.0 }, 0.0, 1.0, VE },
>
> +    { "constant_bit_rate", "Require constant bit rate (macOS 13 or
> newer)", OFFSET(constant_bit_rate), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1,
> VE },
> +
>     COMMON_OPTIONS
>     { NULL },
> };
> --
> 2.37.0 (Apple Git-136)
>
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