test render script can cut without desync

230101/setup.sh

@@ -0,0 +1,24 @@
+#!/bin/busybox ash
+set -euo pipefail
+
+for i in 1 2 3; do rm "C0$i.svg" || true; rm "C0$i.mp4" || true; done
+
+function write_svg() {
+	echo '<svg viewBox="0 0 1920 1080" width="1920" height="1080">'
+	echo '  <rect fill="'"$1"'" x="0" y="0" width="1920" height="1080"/>'
+	echo '</svg>'
+}
+
+write_svg blue  >C01.svg
+write_svg green >C02.svg
+write_svg red   >C03.svg
+
+function render_svg() {
+	ffmpeg -hide_banner -vaapi_device /dev/dri/renderD128 \
+		-loop 1 -i "$1" \
+		-filter_complex "[0]format=nv12,hwupload[v];aevalsrc=sin($3*2*PI*t):s=48000[a]" \
+		-map "[v]" -map "[a]" \
+		-c:v h264_vaapi -r 25 -t 4 -b:a 128000 "$2"
+}
+
+for i in 1 2 3; do render_svg "C0$i.svg" "C0$i.mp4" $(($i*40+400)); done