performance issue with using crop to scroll through a large image
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:$xres:0:black@0.0" \ -pix_fmt monob \ -frames:v 1 \ "$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"
On Fri, Sep 1, 2023 at 4:40 PM <miranda@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:$xres:0:black@0.0" \ -pix_fmt monob \ -frames:v 1 \ "$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" _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-user
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-loop will demux and decode image over and over again.
There is video loop filter that allows to resolve that.
i see - indeed using the loop filter i can now process at 1.6x. thank you! best, miranda
participants (3)
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miranda kastemaa -
miranda＠pulusound.fi
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Paul B Mahol