[FFmpeg-user] performance issue with using crop to scroll through a large image

[miranda at pulusound.fi]

hi,

i have written a script[1] using ffmpeg 5.1.2 to generate scrolling 
waveform videos from audio files. i first use showwavespic to write the 
full waveform to an image file, the width of which depends on the 
duration of the audio. i then use crop with a time-varying x coordinate 
to scroll through the image.

my problem is that the processing speed of the image-to-video step 
appears to slow down significantly with longer audio files, i.e. wider 
waveform images. for example, the second ffmpeg invocation on my machine 
processes 10 seconds of audio at ~1x speed, but 5 minutes of audio at 
only ~0.1x speed.

why is this? i expected that after the waveform image is initially 
loaded into memory, the time complexity of crop would depend only on the 
width and height of the cropped area. is there a better way to achieve 
this?

best,
miranda


[1]

#!/usr/bin/env bash
audio_file=$1
xres=1080
yres=1080
downscale=64
fps=60

num_secs=$(soxi -D "$audio_file")
num_samples=$(soxi -s "$audio_file")
img_xres=$((num_samples / downscale))
img_yres="$yres"
img_file="$audio_file.bmp"
vid_file="$audio_file.mkv"

# create an image of the full waveform
ffmpeg \
     -i "$audio_file" \
     -filter_complex 
"[0:a]aformat=channel_layouts=mono,showwavespic=s=${img_xres}x${img_yres}:colors=white:draw=full,format=rgba,pad=$((img_xres 
+ xres)):$yres:$xres:0:black at 0.0" \
     -pix_fmt monob \
     -frames:v 1 \
     "$img_file"

# create a video scrolling through the waveform
ffmpeg \
     -loop 1 -framerate "$fps" -i "$img_file" \
     -i "$audio_file" \
     -vf "crop=w=$xres:h=$yres:x=t*(iw-ow)/$num_secs:y=0" \
     -shortest \
     -r "$fps" \
     -c:v libx264 \
     -c:a copy \
     -pix_fmt yuv420p \
     "$vid_file"