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Video coding configuration

Video coding configuration#

The video coding configuration is set through the following arguments

  • --intra_period

  • --p_period

Cool-chic is a video codec implementing 3 different type of frames

  • I (intra) frames have no reference. They can be used for image coding

  • P (inter) frames have a single reference.

  • B (inter) frames have two references

Cool-chic always encodes a Group Of Pictures (GOP), consisting in 1 intra frame followed by 0 to 255 inter frame(s).

Tip

Coding an image is achieved by using a single-frame GOP with only an intra frame.

Intra period#

A GOP starts with an intra frame and followed by an arbitrary number of inter (P or B) frames. The number of frames in the GOP and the number of frames encoded by Cool-chic is thus: intra period + 1:

# This codes 9 frames from the video
# number_of_coded_frames = number_of_frames_in_gop = intra_period + 1
(venv) python3 src/encode.py --intra_period=8 --input=xxx.yuv

P-period#

Both GOPs below have the same --intra_period=8 but have a different --p_period

# A low-delay P configuration
# I0 ---> P1 ---> P2 ---> P3 ---> P4 ---> P5 ---> P6 ---> P7 ---> P8
python3 src/encode.py --intra_period=8 --p_period=1 --input=xxx.yuv

# A hierarchichal Random Access configuration
# I0 -----------------------------------------------------> P8
# \-------------------------> B4 <-------------------------/
#  \----------> B2 <---------/ \----------> B6 <----------/
#   \--> B1 <--/ \--> B3 <--/   \--> B5 <--/  \--> B7 <--/
python3 src/encode.py --intra_period=8 --p_period=8 --input=xxx.yuv

Note the introduction of the --p_period argument in the command line. This sets the distance between the IO frame and the successive P frames. Reducing the temporal distance alleviates the motion compensation process, which can be useful. It can also enables more complex GOP structure such as:

# There is no more prediction from I0 to P8. Instead the GOP in split in
# half so that there is no inter frame with reference further than --p_period

# I0 -----------------------> P4 ------------------------> P8
#  \----------> B2 <---------/ \----------> B6 <----------/
#   \--> B1 <--/ \--> B3 <--/   \--> B5 <--/  \--> B7 <--/
python3 src/encode.py --intra_period=8 --p_period=4 --input=xxx.yuv

Image coding#

To achieve plain image coding i.e. compression a single standalone intra frame simply set the intra period and P period to zero.

# Image coding
python3 src/encode.py --intra_period=0 --p_period=0 --input=xxx.png
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