Offset ffprobe when outputting frame data

Question

I've successfully managed to extract OCR data from frames using ffprobe with the following command:

ffprobe -loglevel quiet -hide_banner -select_streams v:0 -show_entries frame_tags=lavfi.ocr.text -f lavfi "movie=movie.mov,ocr" -of flat

It neatly outputs the info on one line like this:

frames.frame.0.tags.lavfi_ocr_text="EE 6:\n_ 00:59:35.00\n"
frames.frame.1.tags.lavfi_ocr_text="ET =:\na Clo Koy\n"
frames.frame.2.tags.lavfi_ocr_text="EE =:\na loony\n"
frames.frame.3.tags.lavfi_ocr_text="=:\na 00:59:35.03\n"
frames.frame.4.tags.lavfi_ocr_text="U6:\na 00:59:35.04\n"
frames.frame.5.tags.lavfi_ocr_text="6:\nal 00:59:35.05\n"
frames.frame.6.tags.lavfi_ocr_text="EE Zz:\nim 00:59:35.06\n"
frames.frame.7.tags.lavfi_ocr_text="=:\nim ll¢kosc hc Lon OYA\n"
frames.frame.8.tags.lavfi_ocr_text="=:\nim 00:59:35.08\n"
frames.frame.9.tags.lavfi_ocr_text="Zz:\nim 00:59:35.09\n"
frames.frame.10.tags.lavfi_ocr_text="=:\nim 00:59:35.10\n"

However, since scanning each frame is a resource intensive operation and depending on how much text is on screen it can sometimes take several seconds per frame I created another probe to detect the first (almost) completely black frame:

ffprobe -loglevel quiet -hide_banner -select_stream v:0 -show_entries frame_tags=lavfi.blackframe.pblack -f lavfi "movie=movie.mov,blackframe" -of flat | head -n1

This command outputs the following:

frames.frame.240.tags.lavfi_blackframe_pblack="99"

I managed to strip off all the stuff except for the "240" number I need. What I want now is to use this number to offset the starting frame for the first command so that its output will start on that frame like this:

frames.frame.240.tags.lavfi_ocr_text="EE 6:\n_ 00:59:35.00\n"
frames.frame.241.tags.lavfi_ocr_text="ET =:\na Clo Koy\n"
frames.frame.242.tags.lavfi_ocr_text="EE =:\na loony\n"
frames.frame.243.tags.lavfi_ocr_text="=:\na 00:59:35.03\n"
frames.frame.244.tags.lavfi_ocr_text="U6:\na 00:59:35.04\n"
frames.frame.245.tags.lavfi_ocr_text="6:\nal 00:59:35.05\n"
frames.frame.246.tags.lavfi_ocr_text="EE Zz:\nim 00:59:35.06\n"
frames.frame.247.tags.lavfi_ocr_text="=:\nim ll¢kosc hc Lon OYA\n"
frames.frame.248.tags.lavfi_ocr_text="=:\nim 00:59:35.08\n"
frames.frame.249.tags.lavfi_ocr_text="Zz:\nim 00:59:35.09\n"
frames.frame.250.tags.lavfi_ocr_text="=:\nim 00:59:35.10\n"

I've already tried creating JPG's with ffmpeg that I can then use with ffprobe but this means creating temp files and stuff. I want to prevent this if possible. I also looked at the -read_intervals option but this doesn't seem to work at all.

I'm running macOS Mojave 10.14.3 with ffprobe:

ffprobe version 4.1.1 Copyright (c) 2007-2019 the FFmpeg developers
  built with Apple LLVM version 10.0.0 (clang-1000.11.45.5)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.1.1 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags='-I/Library/Java/JavaVirtualMachines/openjdk-11.0.2.jdk/Contents/Home/include -I/Library/Java/JavaVirtualMachines/openjdk-11.0.2.jdk/Contents/Home/include/darwin' --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libtesseract --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-videotoolbox --disable-libjack --disable-indev=jack --enable-libaom --enable-libsoxr
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Simple multimedia streams analyzer
usage: ffprobe [OPTIONS] [INPUT_FILE]

Can what I want be done using ffprobe alone?

Answer 1

Turns out the solution was quite simple.

I added the trim parameter to the lavfi filter with the start_pts option and also changed some of the output options.

Full command:

ffprobe -loglevel error -hide_banner -select_streams v:0 -show_frames -show_entries frame=lavfi.ocr.text -f lavfi -i "movie=movie.mov,trim=start_pts=20,ocr" -of flat

It now outputs the following:

frames.frame.0.tags.lavfi_ocr_text="=:\nim 00:59:35.20\n"
frames.frame.1.tags.lavfi_ocr_text="=:\nim 00:59:35.21\n"
frames.frame.2.tags.lavfi_ocr_text="=:\nim lelehkosc clon)\n"
frames.frame.3.tags.lavfi_ocr_text="=:\nim 00:59:35.23\n"
frames.frame.4.tags.lavfi_ocr_text="Zz:\nim 00:59:35.24\n"
frames.frame.5.tags.lavfi_ocr_text="i\nim 00:59:36.00\n"
frames.frame.6.tags.lavfi_ocr_text="=:\nim 00:59:36.01\n"
frames.frame.7.tags.lavfi_ocr_text="=:\nim 00:59:36.02\n"
frames.frame.8.tags.lavfi_ocr_text="=:\nim 00:59:36.03\n"
frames.frame.9.tags.lavfi_ocr_text="=:\nim 00:59:36.04\n"
frames.frame.10.tags.lavfi_ocr_text="=:\nim 00:59:36.05\n"

It still starts at "frame.0" but these are not the frame positions of the original video as you can see from the detected timecode which now starts at 00:59:35.20 instead of 00:59:35.00.

Hope this helps others.