--- big --- snip ----

    IMGENC_CMD=(ffmpeg -f yuv4mpegpipe -r $FRAME_RATE -i - -s $VIDSIZE \
    $SS_FFMOPTS $HQ_FLAGS -r $FRAME_RATE -bf 2 $NR -y "$WORK_DIR/slideshow.m2v")

    # make a fifo to send raw yuv to, to be encoded by ffmpeg
    mkfifo "$WORK_DIR/enc.fifo"

    "${IMGENC_CMD[@]}" 2>/dev/null < "$WORK_DIR/enc.fifo" &
    exec 3> "$WORK_DIR/enc.fifo"

    if [[ $EFFECT = "fade" ]]; then
        [[ $TV_STANDARD = "ntsc" ]] && fade_frames=15 || fade_frames=12
        fade_incr=$(bc -l <<< "scale=2; 100 / $fade_frames") >&2
    elif [[ $EFFECT = "crossfade" ]]; then
        [[ $TV_STANDARD = "ntsc" ]] && fade_frames=30 || fade_frames=25
        fade_incr=$(bc -l <<< "scale=2; 100 / $fade_frames") >&2
    fi

    TOYUV_FADE_CMD=(ppmtoy4m -v 0 -n $fade_frames -A 10:11 -I p -r -S 420mpeg2)
    TOYUV_STILLS_CMD=(ppmtoy4m -n $MIX_VFRAMES -A 10:11 -I p -S 420mpeg2 -r)
    # transitions
    echo >&2

    for f in ${!MIX_IN[@]}; do
        base_ppm="$WORK_DIR/${f}.ppm"
        if [[ $EFFECT = "crossfade" ]]; then
            if ((f == (${#MIX_IN[@]} - 1) )); then
                overlay_ppm="$WORK_DIR/0.ppm"
            else
                overlay_ppm="$WORK_DIR/$((f+1)).ppm"
            fi
        fi
#        spin "Encoding and fading  ${base_ppm##*/}" >&2
        # encode fadein frames - keep yuv header
        if [[ $EFFECT = "fade" ]]; then
            # yuvcorrect (remove headers) only if we are not on 1st slide fadein
            ((f==0)) && YUVCORRECT="cat"
            do_transitions fadein
            # encode transition frames
            cat "$WORK_DIR"/animenu/*.ppm |
            "${TOYUV_FADE_CMD[@]}" 2>/dev/null | $YUVCORRECT 2>/dev/null
            rm -f "$WORK_DIR"/animenu/*.ppm
        fi
        # encode still frames
        # need yuvcorrect if for fade style (remove headers)
        [[ $EFFECT = "fade" ]] && YUVCORRECT="yuvcorrect -T NO_HEADER"
        if [[ $EFFECT = "crossfade" ]]; then
            # encoding 1st slide
            if ((f==0)); then
                YUVCORRECT="cat"
            else # subsequent slides and fades need header removed
                YUVCORRECT="yuvcorrect -T NO_HEADER"
            fi
        fi
        "${TOYUV_STILLS_CMD[@]}" "${MIX_IN[f]}" 2>/dev/null | ${YUVDENOISE[f]} |
        ${YUVMEDIAN[f]} | $YUVCORRECT 2> /dev/null
        # now we need yuvcorrect for everything
        YUVCORRECT="yuvcorrect -T NO_HEADER"
        # do a transition to the next slide
        if [[ $EFFECT = "crossfade" ]]; then
            do_transitions crossfade
        elif [[ $EFFECT = "fade" ]]; then
            do_transitions fadeout
        fi
        # encode transition frames
        cat "$WORK_DIR"/animenu/*.ppm |
        "${TOYUV_FADE_CMD[@]}" 2>/dev/null | $YUVCORRECT 2> /dev/null
        SS_PID=$!
        echo "SS_PID is $SS_PID" >&2
        rm -f "$WORK_DIR"/animenu/*.ppm
        # do crossfade looping to 1st slide if doing crossfade effect
        if (($f == (${#MIX_IN[@]} - 1) )); then
            if [[ $EFFECT = "crossfade" ]]; then
                base_ppm=${MIX_IN[0]}
                do_transitions crossfade
                # encode transition frames
                cat "$WORK_DIR"/animenu/*.ppm |
                "${TOYUV_FADE_CMD[@]}" 2>/dev/null | $YUVCORRECT 2> /dev/null
                rm -f "$WORK_DIR"/animenu/*.ppm
            fi
        fi
        while kill -0 $SS_PID &>/dev/null; do
            sleep 0.5
            spin "Encoding and fading  ${base_ppm##*/}   $SPINNER" >&2
        done
    done >&3
    # close the pipe
    exec 3>&-

--- big --- snip ---