waveshare213v2: Implement flicker-free partial updates

Update both image buffers (B/W and red) and configure the controller to
only refresh the changed areas. With this change the "DrawPartial"
function becomes equivalent to "Draw" and is thus marked as deprecated.
It didn't really provide additional functionality before as partial
updates weren't truly implemented.

Signed-off-by: Michael Hanselmann <public@hansmi.ch>

waveshare2in13v2/controller.go

@@ -78,8 +78,15 @@ func configDisplayMode(ctrl controller, mode PartialUpdate, lut LUT) {
 }
 
 func updateDisplay(ctrl controller, mode PartialUpdate) {
+	var displayUpdateFlags byte
+
+	if mode == Partial {
+		// Make use of red buffer
+		displayUpdateFlags = 0b1000_0000
+	}
+
 	ctrl.sendCommand(displayUpdateControl1)
-	ctrl.sendData([]byte{0})
+	ctrl.sendData([]byte{displayUpdateFlags})
 
 	ctrl.sendCommand(displayUpdateControl2)
 	ctrl.sendData([]byte{0xC7})

waveshare2in13v2/controller_test.go

@@ -138,7 +138,7 @@ func TestUpdateDisplay(t *testing.T) {
 			name: "partial",
 			mode: Partial,
 			want: []record{
-				{cmd: displayUpdateControl1, data: []byte{0}},
+				{cmd: displayUpdateControl1, data: []byte{0x80}},
 				{cmd: displayUpdateControl2, data: []byte{0xc7}},
 				{cmd: masterActivation},
 			},

waveshare2in13v2/waveshare213v2.go

@@ -239,30 +239,10 @@ func (d *Dev) Bounds() image.Rectangle {
 	return image.Rect(0, 0, d.opts.Width, d.opts.Height)
 }
 
-// Draw draws the given image to the display.
+// Draw draws the given image to the display. Only the destination area is
+// uploaded. Depending on the update mode the whole display or the destination
+// area is refreshed.
 func (d *Dev) Draw(dstRect image.Rectangle, src image.Image, srcPts image.Point) error {
-	opts := drawOpts{
-		cmd:     writeRAMBW,
-		devSize: image.Pt(d.opts.Width, d.opts.Height),
-		buffer:  d.buffer,
-		dstRect: dstRect,
-		src:     src,
-		srcPts:  srcPts,
-	}
-
-	eh := errorHandler{d: *d}
-
-	drawImage(&eh, &opts)
-
-	if eh.err == nil {
-		updateDisplay(&eh, d.mode)
-	}
-
-	return eh.err
-}
-
-// DrawPartial draws the given image to the display. Display will update only changed pixel.
-func (d *Dev) DrawPartial(dstRect image.Rectangle, src image.Image, srcPts image.Point) error {
 	opts := drawOpts{
 		devSize: image.Pt(d.opts.Width, d.opts.Height),
 		buffer:  d.buffer,
@@ -273,6 +253,7 @@ func (d *Dev) DrawPartial(dstRect image.Rectangle, src image.Image, srcPts image
 
 	eh := errorHandler{d: *d}
 
+	// Keep the two buffers in sync.
 	for _, cmd := range []byte{writeRAMBW, writeRAMRed} {
 		opts.cmd = cmd
 
@@ -290,6 +271,13 @@ func (d *Dev) DrawPartial(dstRect image.Rectangle, src image.Image, srcPts image
 	return eh.err
 }
 
+// DrawPartial draws the given image to the display.
+//
+// Deprecated: Use Draw instead. DrawPartial merely forwards all calls.
+func (d *Dev) DrawPartial(dstRect image.Rectangle, src image.Image, srcPts image.Point) error {
+	return d.Draw(dstRect, src, srcPts)
+}
+
 // Halt clears the display.
 func (d *Dev) Halt() error {
 	return d.Clear(image1bit.On)