Bugs and Suggestions

Fix, part 1/4: get_format (FFMpegBinding/Sources/FFMpegAVCodecContext.m)

FFmpeg requires get_format to return a format from the list it offers. When VideoToolbox is not offered (HEVC 12-bit, H.264 9/12-bit) or its init failed, returning the software format lets the compiled-in h264/hevc software decoder take over.

#import "libavutil/pixdesc.h"

static enum AVPixelFormat getPreferredPixelFormat(__unused AVCodecContext *ctx, const enum AVPixelFormat *pix_fmts) {
enum AVPixelFormat fallback = AV_PIX_FMT_NONE;
for (const enum AVPixelFormat *p = pix_fmts; *p != AV_PIX_FMT_NONE; p++) {
if (*p == AV_PIX_FMT_VIDEOTOOLBOX) {
return AV_PIX_FMT_VIDEOTOOLBOX;
}
const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(*p);
if (desc && !(desc->flags & AV_PIX_FMT_FLAG_HWACCEL)) {
fallback = *p;
}
}
return fallback;
}

- (void)setupHardwareAccelerationIfPossible {
if (av_hwdevice_ctx_create(&_impl->hw_device_ctx, AV_HWDEVICE_TYPE_VIDEOTOOLBOX, nil, nil, 0) < 0) {
return;
}
_impl->get_format = getPreferredPixelFormat;
}

Fix, part 2/4: reducing 10/12-bit frames to 8-bit (FFMpegBinding/Sources/FFMpegAVFrame.m, needs libavutil/pixdesc.h)

static enum AVPixelFormat eightBitFormatForHighBitDepthFormat(enum AVPixelFormat format) {
const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(format);
if (!desc || (desc->flags & (AV_PIX_FMT_FLAG_HWACCEL | AV_PIX_FMT_FLAG_RGB | AV_PIX_FMT_FLAG_BE | AV_PIX_FMT_FLAG_BITSTREAM | AV_PIX_FMT_FLAG_PAL))) return AV_PIX_FMT_NONE;
if (!(desc->flags & AV_PIX_FMT_FLAG_PLANAR) || desc->nb_components < 3) return AV_PIX_FMT_NONE;
for (int i = 0; i < desc->nb_components; i++) {
if (desc->comp[i].plane != i || desc->comp[i].step != 2 || desc->comp[i].shift != 0 || desc->comp[i].depth <= 8 || desc->comp[i].depth > 16) return AV_PIX_FMT_NONE;
}
bool a = (desc->flags & AV_PIX_FMT_FLAG_ALPHA) != 0;
if (desc->log2_chroma_w == 1 && desc->log2_chroma_h == 1) return a ? AV_PIX_FMT_YUVA420P : AV_PIX_FMT_YUV420P;
if (desc->log2_chroma_w == 1 && desc->log2_chroma_h == 0) return a ? AV_PIX_FMT_YUVA422P : AV_PIX_FMT_YUV422P;
if (desc->log2_chroma_w == 0 && desc->log2_chroma_h == 0) return a ? AV_PIX_FMT_YUVA444P : AV_PIX_FMT_YUV444P;
return AV_PIX_FMT_NONE;
}

New FFMpegAVFrame API:
- isHighBitDepthPlanarYUV: eightBitFormatForHighBitDepthFormat(format) != NONE
- initWithEightBitLayoutOfFrame: allocates a frame of that 8-bit format and size (av_frame_get_buffer)
- reduceBitDepthIntoFrame: for each plane (chroma sized with AV_CEIL_RSHIFT by log2_chroma_w/h), shift = depth - 8, dst = min(255, (src + (1 << (shift - 1))) >> shift); then copies color_range.

Fix, part 3/4: using it in MediaPlayer/Sources/FFMpegMediaVideoFrameDecoder.swift

private var reducedBitDepthFrame: FFMpegAVFrame?

private func eightBitFrame(_ frame: FFMpegAVFrame) -> FFMpegAVFrame? {
guard frame.isHighBitDepthPlanarYUV else {
return frame
}
if let existing = self.reducedBitDepthFrame, frame.reduceBitDepth(into: existing) {
return existing
}
guard let created = FFMpegAVFrame(eightBitLayoutOf: frame), frame.reduceBitDepth(into: created) else {
return nil
}
self.reducedBitDepthFrame = created
return created
}

At both call sites (convertVideoFrameToImage and convertVideoFrame):

- guard let frame = self.planarFrameFor420Conversion(frame) else {
+ guard let frame = self.eightBitFrame(frame).flatMap(self.planarFrameFor420Conversion) else {

The existing 4:2:2/4:4:4 -> 4:2:0 fold then handles the 8-bit frame. There is no tone mapping, so HDR10 shows with SDR contrast instead of black. Converting a 4K frame takes about 4 ms at -Os.

Fix, part 4/4: MPEG-4 Part 2 esds (MediaPlayer/Sources/FFMpegMediaFrameSourceContextHelpers.swift)

static func mpeg4EsdsAtom(_ extradata: Data) -> Data {
if extradata.count > 5 && extradata.prefix(4) == Data([0, 0, 0, 0]) && extradata[extradata.startIndex + 4] == 0x03 {
return extradata
}
func descriptor(_ tag: UInt8, _ payload: Data) -> Data {
let size = payload.count
var result = Data([tag])
result.append(contentsOf: [UInt8(0x80 | ((size >> 21) & 0x7f)), UInt8(0x80 | ((size >> 14) & 0x7f)), UInt8(0x80 | ((size >> 7) & 0x7f)), UInt8(size & 0x7f)])
result.append(payload)
return result
}
var decoderConfig = Data([0x20, 0x11, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])
decoderConfig.append(descriptor(0x05, extradata))
var esDescriptor = Data([0, 0, 0])
esDescriptor.append(descriptor(0x04, decoderConfig))
esDescriptor.append(descriptor(0x06, Data([0x02])))
var atom = Data([0, 0, 0, 0])
atom.append(descriptor(0x03, esDescriptor))
return atom
}

In createFormatDescriptionFromMpeg4CodecData:
- atoms.setObject(extradata as NSData, forKey: "esds" as NSString)
+ atoms.setObject(mpeg4EsdsAtom(extradata) as NSData, forKey: "esds" as NSString)

Tests and what is not covered

I added FFMpegVideoDecodingTests to //submodules/MediaPlayer:UniversalMediaPlayerTests. It reads 12-frame 192x108 fixtures through FFMpegFileReader. On master: h264_8bit with hw=true where VideoToolbox can't initialise gives 0 frames; h264_high10, hevc_main10, hevc_main12 and av1_10bit in software give 0 frames each; hevc_main12 with hw=true gives 0; 8->10-bit gives 4 frames and stops at 0.1 s; for mpeg4_part2, VTDecompressionSessionCreate returns -12909. With the fix all of them decode. I'm happy to share the full diff, test file and fixtures if useful.

Not covered:
- VP9 profile 2 (10/12-bit): libvpx is built with --disable-vp9-highbitdepth (third-party/libvpx/build-libvpx-bazel.sh), so it rejects these streams before decoding.
- RGB (gbrp) and monochrome (gray) frames.
- AV1 when the VideoToolbox AV1 decoder rejects a stream (e.g. 4:4:4): the native av1 decoder has no software fallback, so dav1d would be needed.
- MPEG-4 Part 2 in AVI.

Admin DogOct 11

Are the videos working if they are saved inside the Photos app?

Yes. The affected videos play normally after saving them to the Photos app, and only Telegram's in-app player shows the black or frozen picture. That matches the root cause: VideoToolbox can decode these files, but Telegram's FFmpeg-based player path drops the decoded frames (details and fix in the comments above).

ans08 L

Do you have a channel with sample files to share as link?

Yes: https://t.me/tg_video_decode_samples

Each post is one sample file, captioned with what it is and what happens:
- h264_8bit.mp4: control, plays fine
- h264_high10.mp4: H.264 High 10, black
- hevc_main12.mp4: HEVC Main 12, black
- av1_10bit.mp4: AV1 10-bit, black on devices without AV1 hardware (iPhone before 15 Pro, M1/M2 Macs)
- mpeg4_part2.mp4 and resolution.mp4 (real-world file): MPEG-4 Part 2, black
- h264_8bit_then_10bit.mp4: freezes when the stream switches to 10-bit

The two MPEG-4 files were sent as files, because the web client could not preview them.

Show next 2 comments