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RocketNuts
RocketNuts

Reputation: 11110

jpeg colors worse than png when extracting frames with ffmpeg?

When extracting still frames from a video at a specific time mark, like this:

ffmpeg -i foobar.mp4 -vframes 1 -ss 4:20 -q:v 1 example.png

I noticed using PNG or JPG results in different colors.
(Note that the -q:v 1 indicates maximum image quality)

Here are some examples:

  1. JPG vs PNG
  2. JPG vs PNG
  3. JPG vs PNG

In general, the JPG shots seem to be slightly darker and less saturated than the PNGs.

When checking with exiftool or imagemagick's identify, both images use sRGB color space and no ICC profile.

Any idea what's causing this? Or which of these two would be 'correct'?

I also tried saving screenshots with my video player (MPlayerX), in both JPG and PNG. In that case, the frame dumps in either format look exactly the same, and they look mostly like ffmpeg's JPG stills.

Upvotes: 3

Views: 3399

Answers (2)

Gyan
Gyan

Reputation: 92928

This is related to the video range or levels. Video stores color as luma and chroma i.e. brightness and color difference and due to legacy reasons from the days of analogue signals, black and white are not represented as 0 and 255 in a 8-bit encoding but as 16 and 235 respectively. The video stream should normally be flagged that this is the case, since one can also store video where 0 and 255 are B and W respectively. If the file isn't flagged or flagged wrongly, then some rendering or conversion functions can produce the wrong results. But we can force FFmpeg to interpret the input one way or the other.

Use

ffmpeg -i foobar.mp4 -vframes 1 -ss 4:20 -q:v 1 -src_range 0 -dst_range 1 example.png/jpg

This tells FFmpeg to assume studio or limited range and to output to full range. The colours still won't be identical due to color encoding conversion but the major change should disappear.

Upvotes: 3

MAP
MAP

Reputation: 862

I don't know about ffmpeg specifically. But, in general, JPEG images can have compression that lowers the quality slightly in exchange for a large reduction in file size. Most programs that can write JPEG files will have a switch (or however they take options) which sets the "quality" or "compression" or something like that. I don't seem to have an ffmpeg on any of the half dozen machines I have open, or I'd tell you what I thought the right one was.

Upvotes: 0

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