ci: threshold screenshots locally before OCR in the desktop ui helper

In run 38702315040 the app started under Xvfb and showed onboarding, but tesseract's single global threshold, pulled down by the black screen outside the window and the illustration, lost the grey text and the white labels on the blue buttons, so "Create your profile" was never found. A local adaptive threshold (-lat) on the enlarged grayscale image and on its negative turns both into dark text on white. Each shot now also keeps the two images OCR read, in UI_OUT/ocr.

scripts/ci/desktop-linux/ui

@@ -6,7 +6,7 @@
 # a log of every command (steps.log) go to UI_OUT; the app's data stays in UI_STATE between starts.
 #
 #   ui start <app dir>            start Xvfb, openbox and <app dir>/bin/simplex, and wait for its window
-#   ui shot <name>                save UI_OUT/NN-<name>.png and NN-<name>.txt, and print the text on screen
+#   ui shot <name>                save UI_OUT/NN-<name>.png, NN-<name>.txt and the images OCR read (UI_OUT/ocr), and print the text on screen
 #   ui text                       print the text on screen, one phrase per line after its centre x,y
 #   ui wait <text> [secs]         wait up to secs (30) until the text is on screen (case and punctuation ignored)
 #   ui click <x> <y>              click at x,y
@@ -83,12 +83,14 @@ start() {
   xdotool getwindowgeometry "$(head -n 1 "$state/window")"
 }
 
-# ocr <png> lines | ocr <png> find <text>: OCR of the enlarged grayscale image and of its negative, for light text on coloured buttons
+# ocr <png> lines | ocr <png> find <text>: OCR of the enlarged grayscale image and of its negative, for light text on coloured buttons.
+# A threshold per neighbourhood keeps grey text and button labels, which one threshold for the whole screen loses.
 ocr() {
   local png=$1
   shift
-  convert "$png" -colorspace Gray -resize 200% "$state/ocr/pos.png"
-  convert "$state/ocr/pos.png" -negate "$state/ocr/neg.png"
+  convert "$png" -colorspace Gray -resize 200% "$state/ocr/gray.png"
+  convert "$state/ocr/gray.png" -lat 60x60-5% "$state/ocr/pos.png"
+  convert "$state/ocr/gray.png" -negate -lat 60x60-5% "$state/ocr/neg.png"
   tesseract "$state/ocr/pos.png" "$state/ocr/pos" --psm 11 tsv > /dev/null 2>&1
   tesseract "$state/ocr/neg.png" "$state/ocr/neg" --psm 11 tsv > /dev/null 2>&1
   python3 "$here/screen-text.py" 2 "$@" "$state/ocr/pos.tsv" "$state/ocr/neg.tsv"
@@ -114,6 +116,9 @@ shot() {
   f=$(printf '%s/%02d-%s' "$UI_OUT" $((n + 1)) "$1")
   import -window root "$f.png"
   ocr "$f.png" lines > "$f.txt"
+  mkdir -p "$UI_OUT/ocr"
+  cp "$state/ocr/pos.png" "$UI_OUT/ocr/$(basename "$f")-pos.png"
+  cp "$state/ocr/neg.png" "$UI_OUT/ocr/$(basename "$f")-neg.png"
   echo "::group::text on screen: $(basename "$f")"
   cat "$f.txt"
   echo "::endgroup::"