Send only the changed dots of a frame to the Nano

Frames were sent as F0 plus one s command per yellow dot (thousands of
characters). During scrolling that overran the Nano's serial buffer, and
half-received frames redrew the whole screen. showBitmap() now sends two B
commands over the changed rectangle (dots off, dots on), or one B command for
a full frame, and the pacing counts the characters sent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/display/frames.cpp

@@ -10,18 +10,15 @@ FlipdotFrames::FlipdotFrames(BROSE9323 &display, PageResources &resources) : _di
 {
 }
 
+// Only the dots that changed are sent (unless redraw: the Nano's image is unknown)
 uint32_t FlipdotFrames::dynamicPage(const LocalTime &time, uint32_t now, bool redraw)
 {
 	uint8_t bitmap[BITMAP_BYTES];
 	uint32_t wait = renderPage(_dynamic, time, now, _resources, bitmap);
 	bool moving = wait != 0;
-	if (!redraw && memcmp(bitmap, _last_dynamic, BITMAP_BYTES) == 0)
-		return wait;
-	// The Nano has no acknowledgement: wait until it has flipped the dots of this frame,
-	// or the next frame overflows its serial buffer
-	uint32_t busy = _flipMs(bitmap, redraw);
-	_drawFullScreen(bitmap, moving);
-	memcpy(_last_dynamic, bitmap, BITMAP_BYTES);
+	// The Nano has no acknowledgement: the next frame waits until it has received this one
+	// and flipped its dots, or the next frame overflows its serial buffer
+	uint32_t busy = _show(bitmap, !redraw, moving);
 	return moving && wait < busy ? busy : wait;
 }
 
@@ -30,14 +27,17 @@ void FlipdotFrames::setDrive(uint16_t flip_time, const DriveSettings &drive)
 	_dot_us = (uint32_t)drive.pulses * (flip_time + drive.flip_gap);
 }
 
-// About how long the Nano takes to receive and show the bitmap (all: every dot flips)
-uint32_t FlipdotFrames::_flipMs(const uint8_t *bitmap, bool all) const
+// Sends the bitmap; returns about how long the Nano takes to receive and show it in ms
+uint32_t FlipdotFrames::_show(const uint8_t *bitmap, bool changes_only, bool instant)
 {
 	uint32_t dots = 0;
 	for (int y = 0; y < BITMAP_HEIGHT; y++)
 		for (int x = 0; x < BITMAP_WIDTH; x++)
-			dots += all || bitmapPixel(bitmap, x, y) != _display.shownPixel(x, y);
-	return FRAME_SEND_TIME + dots * _dot_us / 1000;
+			dots += bitmapPixel(bitmap, x, y) != _display.shownPixel(x, y);
+	size_t chars = _display.showBitmap(bitmap, changes_only, instant);
+	if (chars == 0)
+		return 0;
+	return chars / NANO_CHARS_PER_MS + dots * _dot_us / 1000 + FRAME_MARGIN;
 }
 
 void FlipdotFrames::noWifi(int hour, int minute)
@@ -86,9 +86,7 @@ void FlipdotFrames::setOffBitmap(const uint8_t *bitmap)
 		memset(_off, 0, BITMAP_BYTES);
 }
 
-void FlipdotFrames::_drawFullScreen(const uint8_t *bitmap, bool instant)
+void FlipdotFrames::_drawFullScreen(const uint8_t *bitmap)
 {
-	_display.fillScreen(0);
-	_display.drawBitmap(0, 0, bitmap, BITMAP_WIDTH, BITMAP_HEIGHT, 1);
-	_display.display(false, instant);
+	_show(bitmap, false, false);
 }