[x264-devel] Is it normal that a non-IDR slice is sometimes larger than an IDR slice?

[Kuu Miyazaki]

Hello

I'm attempting to capture images from my laptop's built-in camera and
encode them into h.264 stream using libx264.

I was able to get h264 stream so I wrapped it in MP4 moof/mdat
sequence. But when I play it back, I can observe a significant
artifact.

(Attached is the broken MP file.)

It could have nothing to do with libx264, but I wanted to ask one thing.

I can observe the size of non-IDR slices sometimes excesses that of IDR slice.
But is it normal? I don't have enough knowledge of the codec.

Bellow are the size of the first 10 samples (I omitted SPS/PPS and SEI.)

=====
NAL unit [0] -----
  i_ref_idc = HIGHEST
  i_type = IDR slice
  i_payload = 5524

NAL unit [1] -----
  i_ref_idc = HIGH
  i_type = non-IDR slice
  i_payload = 2171

NAL unit [2] -----
  i_ref_idc = HIGH
  i_type = non-IDR slice
  i_payload = 9108

NAL unit [3] -----
  i_ref_idc = HIGH
  i_type = non-IDR slice
  i_payload = 3128

NAL unit [4] -----
  i_ref_idc = HIGH
  i_type = non-IDR slice
  i_payload = 8788

NAL unit [5] -----
  i_ref_idc = HIGH
  i_type = non-IDR slice
  i_payload = 7294

NAL unit [6] -----
  i_ref_idc = HIGH
  i_type = non-IDR slice
  i_payload = 7245

NAL unit [7] -----
  i_ref_idc = HIGH
  i_type = non-IDR slice
  i_payload = 5512

NAL unit [8] -----
  i_ref_idc = HIGH
  i_type = non-IDR slice
  i_payload = 5384

NAL unit [9] -----
  i_ref_idc = HIGH
  i_type = non-IDR slice
  i_payload = 7467

=====

The steps are:
1) Capture 1280 x 720 32-bit BGRA image at 30fps.
2) Convert the image into 640 x 360 YUV420p using libswscale.
3) Pass the image to x264_encoder_encode().
4) Repeat step 1-3 30 times.
5) Obtain the NAL units by passing NULL to x264_encoder_encode().

The source code is available here:

  https://github.com/kuu/node-media-capture

(All the x264-related code is in
src/native/platform/osx/facetime_camera_controller.m)

Thanks in advance for your help.

Kuu
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