Convert video to jpg and back using ffmpeg

ffmpeg_video_jpg.md

Video and JPG conversion using FFMPEG

Grab one frame from video

#!/bin/sh -x
# $1 - input video file, e.g. video.mp4
# $2 - timestamp, e.g. 00:33
# $3 - output image file, e.g. output.jpg
ffmpeg -ss $2 -i $1 -vframes 1 -q:v 2 $3

Grab all frames from video

#!/bin/sh -x
# $1 - input file, e.g. input.mp4
ffmpeg -i "$1" -q:v 2 %06d.jpg

Combine jpg files into one video

ffmpeg -f image2 -framerate 30 -i %06d.jpg -c:v libx264 out.mp4

@ankurbhatia24
Edit: read your link after writing below. What I suggested is what the person report as an issue. I guess I don't know.

My first bullet was asking what you want to do in the first place. It looks to me you want to modify just <2> of the video and not anywhere else. May be there is another road. For example, instead of concat, get all frames for the entire video, then make the modification to <2> frames, then assemble all frames back to a video.

@rlan
My first bullet was asking what you want to do in the first place. It looks to me you want to modify just <2> of the video and not anywhere else. May be there is another road. For example, instead of concat, get all frames for the entire video, then make the modification to <2> frames, then assemble all frames back to a video.

So for this above, you are exactly right. I can write all frames, modify <2> and make a full video out of all frames. But here my question is that why is the color/luma difference happening and how can we solve that.

Lets simplify the problem: 1. take a video, 2. write the frames 3. read the frames to make it a video.
Now: the starting video and the new encoded video will have slight luma differences. I want to know why is this happening and how can i solve for this. I am not a ffmpeg pro so need help here. My gut feel is that while writing the frames (png) to disk colorspace conversion from yuv420p to rgb takes place and reverse happens when we encode the frames back to mp4. So is there some parameter which i am missing while encoding back, etc?

@ankurbhatia24 I see you want to understand the cause of the difference. I don't know the answer. In fact more questions come up in my head. Similar to you, I would start with color space of video vs images. Also I would have a rigorous way to measure luma difference, eg bits in files or some luma intensity definition. If you end up doing the analysis, I would be interested in the findings.