H264 profile and level_h.264

For h.264 users, how to specify the profile/level seems to be a fairly common question. Profiles define sets of bit stream features a h.264 stream can use. Levels define restrictions on the video resolution, frame rate and some stuff called VBV (Video Buffer Verifier).

H.264 Profiles

H.264 Profiles are discussed in depth on Wikipedia, but to simplify the considerations for the average user, I will focus on the Baseline, Main and High Profiles. Baseline Profile I/P slices Multiple reference frames (–refs <int>, >1 in the x264 CLI) In-loop deblocking CAVLC entropy coding (–no-cabac in the x264 CLI) Main Profile Baseline Profile features mentioned above B slices CABAC entropy coding Interlaced coding – PAFF/MBAFF Weighted prediction High Profile Main Profile features mentioned above 8×8 transform option (–8×8dct in the x264 CLI) Custom quantisation matrices

H.264 Levels

H.264 Levels are also discussed on Wikipedia, though in my opinion, less explicitly and less successfully than for the profiles. For practical use, to specify a Level, a number of constraints have to be met.

The resolutions/frame rates in the following table are merely a guideline – the number of macroblocks per second is the actual restriction. To calculate this:

Macroblocks are 16×16 in H.264 and so to code a frame one can calulate the number of macroblocks per frame as:

ceil( width / 16.0 ) * ceil( height / 16.0 )

Note: The ceil() function rounds up its argument to the next integer. For example, ceil( 10.2 ) returns 11.

Then you need to multiply the number of macroblocks per frame by the number of frames per second to get the number of macroblocks per second.

macroblocks per second = ceil( width / 16.0 ) * ceil( height / 16.0 ) * frame rate

Level

VBV maximum bit rate [1000bits/s]

VBV buffer size [1000bits]

Macroblocks/s

Resolution and frame rate

1

64

175

1485

128×96@30 or 176×144@15

1b

128

350

1485

128×96@30 or 176×144@15

1.1

192

500

3000

176×144@30 or 320×240@10

1.2

384

1000

6000

176×144@60 or 320×240@20

1.3

768

2000

11880

352×288@30

2

2000

2000

11880

352×288@30

2.1

4000

4000

19800

352×288@50

2.2

4000

4000

20250

352×288@50 or 640×480@15

3

10000

10000

40500

720×480@30 or 720×576@25

3.1

14000

14000

108000

1280×720@30

3.2

20000

20000

216000

1280×720@60

4

20000

25000

245760

1920×1088@30 or 2Kx1K@30

4.1

50000

62500

245760

1920×1088@30 or 2Kx1K@30

4.2

50000

62500

522240

1920×1088@60 or or 2Kx1K@60

5

135000

135000

589824

2560×1920@30

5.1

240000

240000

983040

4Kx2K@30 or 4