Rewallpaper makes a wallpaper from mpd's currently playing music and set's that as desktop's background!

rewallpaper.sh

#!/bin/bash

# configurations
RW_MUSIC=~/music
RW_CACHE=~/.cache/rewallpaper
RW_DEFAULT=~/.wallpaper.jpg
RW_WIDTH=1366
RW_HEIGHT=768
RW_MARGIN=60

# default values
RW_SONG=''
RW_WALL=''
RW_DB="$RW_CACHE/cache.db"
RW_TMP1="$( mktemp --suffix=.jpg )"
RW_TMP2="$( mktemp --suffix=.jpg )"

# functions
function rw_init {
    # initialize environment

    rw_reset

    mkdir -p "$RW_CACHE"

    if [ ! -f "$RW_DB" ]; then
        echo 'CREATE TABLE CACHED_FILES(FILE TEXT NOT NULL,
        MD5 CHAR(32) NOT NULL);' | sqlite3 "$RW_DB"
    fi
}

function rw_generate {
    # generate a wallpaper for $RW_SONG and set that as wallpaper

    # look for cached wallpaper
    ART_MD5=$( echo "SELECT MD5 FROM CACHED_FILES WHERE FILE = '$RW_SONG'" | sqlite3 "$RW_DB" )
    if [ "$ART_MD5" ]; then
        rw_set $ART_MD5
        return
    fi

    # extract cover art
    [ -f "$RW_TMP1" ] && rm "$RW_TMP1"
    ALB_COVER="$(dirname "$RW_SONG")/cover.jpg"
    if [ -f "$ALB_COVER" ]; then
        cp "$ALB_COVER" "$RW_TMP1"
    elif ! ffmpeg -i "$RW_SONG" "$RW_TMP1" > /dev/null 2>&1; then
        rw_reset
        return
    fi
    ART_MD5=$( md5sum "$RW_TMP1" | head -c 32 )
    echo "INSERT INTO CACHED_FILES VALUES ('$RW_SONG', '$ART_MD5');" | sqlite3 "$RW_DB"

    # generate wallpaper if not cached
    CACHED_WALL="$RW_CACHE/$ART_MD5.jpg"
    if [ ! -f "$CACHED_WALL" ]; then
        # create blured background
        convert "$RW_TMP1" -resize ${RW_WIDTH}x$RW_WIDTH \
            -gravity center -extent ${RW_WIDTH}x$RW_HEIGHT \
            -blur 0x25 "$RW_TMP2"

        # calculate cover size
        NEWSIZE=$(( $RW_HEIGHT - 2 * $RW_MARGIN ))

        # merge
        composite \( "$RW_TMP1" -resize ${NEWSIZE}x$NEWSIZE \) \
            -gravity northeast -geometry +$RW_MARGIN+$RW_MARGIN \
            "$RW_TMP2" "$CACHED_WALL"
    fi

    # set wallpaper
    rw_set $ART_MD5
}

function rw_set {
    # Set wallpaper to $1.jpg. $1 is MD5 hash of song's cover art
    # (not it's wallpaper).
    rw_change "$RW_CACHE/$1.jpg"
}

function rw_reset {
    # Restore default wallpaper
    rw_change "$RW_DEFAULT"
}

function rw_change {
    # Set $1 as current wallpaper. Change if you are using a different method
    # (e.g. feh instead of pcmanfm

    # Make sure pcmanfm daemon is already started
    while ! pgrep pcmanfm > /dev/null; do sleep 1; done
    pcmanfm -w $@
}

function rw_changed {
    # if there is a song in playing or paused state, generate a wallpaper for
    # it. set the default wallpaper otherwise.

    STATUS="$( mpc status -f %file% )"

    if echo "$STATUS" | grep -q '\[p\(laying\|aused\)\]'; then
        RW_SONG="$RW_MUSIC/$( echo -e "$STATUS" | head -n 1 )"
        rw_generate
        return
    fi

    rw_reset
}

function rw_main {
    # main precedure. acts like an event loop listenning for mpd events.

    rw_init

    mpc idleloop | while read JUNK; do rw_changed; done
}

rw_main