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Extract slides from a presentation video by comparing the PSNR of consecutive difference of down-sampled frames against a threshold
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| #!/bin/bash | |
| set -e | |
| # Usage: ./extract-slides.sh mylecture.mp4 | |
| # Config variables | |
| SIMILARITY_THRESHOLD=25 # A slide change is assumed iff frames PSNR similarity falls below this value | |
| FRAME_PERIOD=30 # [Seconds] Duration between sampling times | |
| CONVERT_TO_PDF=0 # Set to 1 to make pdf. Might cause errors if imagemagick is not configured properly. See https://askubuntu.com/questions/1081895/trouble-with-batch-conversion-of-png-to-pdf-using-convert | |
| PDF_FILE="${1%.*}-slides.pdf" # Only used if CONVERT_TO_PDF is 1 | |
| OUTPUT_PATH="${1%.*}-slides" # Only used if CONVERT_TO_PDF is 0 | |
| FRAMES_TEMP_DIRECTORY="/tmp/extract-slides/frames/${1%.*}" | |
| SLIDES_TEMP_DIRECTORY="/tmp/extract-slides/slides/${1%.*}" | |
| # Get directories ready | |
| mkdir -p "$FRAMES_TEMP_DIRECTORY" | |
| mkdir -p "$SLIDES_TEMP_DIRECTORY" | |
| rm -f "$FRAMES_TEMP_DIRECTORY"/*.jpg | |
| rm -f "$SLIDES_TEMP_DIRECTORY"/*.jpg | |
| # Slice the video into frames | |
| ffmpeg -i "$1" -vf fps=1/$FRAME_PERIOD "$FRAMES_TEMP_DIRECTORY/%04d.jpg" | |
| # Detect slides | |
| last_frame=NONE | |
| current_slide_number=1 | |
| IFS=$'\n' | |
| for frame in $(ls "$FRAMES_TEMP_DIRECTORY"/*) | |
| do | |
| current_slide_name=$(printf "%04d.jpg" $current_slide_number) | |
| if [ "$last_frame" == "NONE" ] | |
| then | |
| cp "$frame" "$SLIDES_TEMP_DIRECTORY"/$current_slide_name | |
| else | |
| similarity=$(ffmpeg -i "$frame" -i "$last_frame" -filter_complex "psnr" -f null /dev/null 2>&1 | grep PSNR | grep -Po "average:\K(\d+|inf)") | |
| if [ "$similarity" != "inf" ] && [ "$similarity" -lt "$SIMILARITY_THRESHOLD" ] | |
| then | |
| # New slide | |
| current_slide_number=$(($current_slide_number+1)) | |
| current_slide_name=$(printf "%04d.jpg" $current_slide_number) | |
| cp "$frame" "$SLIDES_TEMP_DIRECTORY/$current_slide_name" | |
| else | |
| # Old slide, unchanged or with small updates | |
| cp "$frame" "$SLIDES_TEMP_DIRECTORY/$current_slide_name" | |
| fi | |
| fi | |
| last_frame="$frame" | |
| done | |
| # Convert to pdf | |
| if [ "$CONVERT_TO_PDF" == "1" ] | |
| then | |
| convert $(ls "$SLIDES_TEMP_DIRECTORY"/*) "$PDF_FILE" | |
| else | |
| mkdir -p "$OUTPUT_PATH" | |
| rm -f "$OUTPUT_PATH"/*.jpg | |
| cp "$SLIDES_TEMP_DIRECTORY"/* "$OUTPUT_PATH/" | |
| fi | |
| # Clear temp files | |
| rm -rf "$FRAMES_TEMP_DIRECTORY" | |
| rm -rf "$SLIDES_TEMP_DIRECTORY" |
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