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

[Paul B Mahol]

On Fri, Sep 1, 2023 at 4:40 PM <miranda at pulusound.fi> wrote:

> 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" \
>

-loop will demux and decode image over and over again.

There is video loop filter that allows  to resolve that.


>      -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"
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