Two h.264 framerates for the price of one

decimate264.py

#!/usr/bin/python3
import sys

KEYINT = 30

# Encode your input like this (example):
# x264 --crf 20 --keyint 30 --min-keyint 30 --bframes 1 --no-b-adapt <input> -o <output.h264>

# Produces display order IBPBPBP...BPBPP|IBPBPBP...BPBPP|...
# Decoding (file) order  IPBPBPB...PBPBP|IPBPBPB...PBPBP|...

# Loads whole file into memory. Fine for PoC...
d = open(sys.argv[1], "rb").read()
out = open(sys.argv[2], "wb")

CODES = { 0x67: "+", 0x68: "I", 0x41: "P", 0x01: "B" }

ctr = 0
for nal in d.split(b"\x00\x00\x00\x01"):
    if not nal:
        continue
    if nal[0] == 0x68: # Reset on I frame
        ctr = 0
    elif nal[0] in (0x41, 0x01):
        ctr += 1
    print("%2d: %02x [%s] (%d)" % (ctr, nal[0], CODES.get(nal[0],"?"), len(nal)))
    if nal[0] == 0x01: # drop B frames
        continue
    if nal[0] == 0x41 and ctr == (KEYINT - 1): # drop the last P frame
        continue
    out.write(b"\x00\x00\x00\x01" + nal)