Select all frames in input:

select

The example above is the same as:

select=1

Skip all frames:

select=0

Select only I-frames:

select='eq(pict_type\,I)'

Select one frame every 100:

select='not(mod(n\,100))'

Select only frames contained in the 10-20 time interval:

select=between(t\,10\,20)

Select only I-frames contained in the 10-20 time interval:

select=between(t\,10\,20)*eq(pict_type\,I)

Select frames with a minimum distance of 10 seconds:

select='isnan(prev_selected_t)+gte(t-prev_selected_t\,10)'

Use aselect to select only audio frames with samples number > 100:

aselect='gt(samples_n\,100)'

Create a mosaic of the first scenes:

ffmpeg -i video.avi -vf select='gt(scene\,0.4)',scale=160:120,tile -frames:v 1 preview.png

Comparing scene against a value between 0.3 and 0.5 is generally a sane choice.

Send even and odd frames to separate outputs, and compose them:

select=n=2:e='mod(n, 2)+1' [odd][even]; [odd] pad=h=2*ih [tmp]; [tmp][even] overlay=y=h

Select useful frames from an ffconcat file which is using inpoints and outpoints but where the source files are not intra frame only.

ffmpeg -copyts -vsync 0 -segment_time_metadata 1 -i input.ffconcat -vf select=concatdec_select -af aselect=concatdec_select output.avi