waveshare213v2: Keep display contents in shadow buffer

This is necessary for partial updates of the display--not just
incremental updates of the full display, but updating part of the
display in general.

On the horizontal axis only whole bytes can be written, so updates need
to be aligned to 8 pixels. One option would be to require the caller of
Dev.Draw to only operate at multiples of 8. Another would be to read
back the contents of bytes to be updated partially and combine the data.
According to the datasheet the controller supports that (command 0x27).

The simplest option, however, is to maintain a shadow buffer in the
driver. Updates are applied to the buffer from which only the changed
destination rectangle is sent to the display. The permanent cost is
a bit more than 4kB of memory for a 122x250 display while saving on
memory allocations for a temporary image buffer on updates.

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

waveshare2in13v2/drawing.go

@@ -54,6 +54,7 @@ func setMemoryArea(ctrl controller, area image.Rectangle) {
 type drawOpts struct {
 	cmd     byte
 	devSize image.Point
+	buffer  *image1bit.VerticalLSB
 	dstRect image.Rectangle
 	src     image.Image
 	srcPts  image.Point
@@ -63,15 +64,9 @@ type drawSpec struct {
 	// Destination on display in pixels, normalized to fit into actual size.
 	DstRect image.Rectangle
 
-	// Size of memory area to write; horizontally in bytes, vertically in
-	// pixels.
+	// Area to send to device; horizontally in bytes (thus aligned to
+	// 8 pixels), vertically in pixels.
 	MemRect image.Rectangle
-
-	// Size of image buffer, horizontally aligned to multiples of 8 pixels.
-	BufferSize image.Point
-
-	// Destination rectangle within image buffer.
-	BufferRect image.Rectangle
 }
 
 func (o *drawOpts) spec() drawSpec {
@@ -83,12 +78,6 @@ func (o *drawOpts) spec() drawSpec {
 		s.DstRect.Min.X/8, s.DstRect.Min.Y,
 		(s.DstRect.Max.X+7)/8, s.DstRect.Max.Y,
 	)
-	s.BufferSize = image.Pt(s.MemRect.Dx()*8, s.MemRect.Dy())
-	s.BufferRect = image.Rectangle{
-		Min: image.Point{X: s.DstRect.Min.X - (s.MemRect.Min.X * 8)},
-		Max: image.Point{Y: s.DstRect.Dy()},
-	}
-	s.BufferRect.Max.X = s.BufferRect.Min.X + s.DstRect.Dx()
 
 	return s
 }
@@ -101,8 +90,7 @@ func drawImage(ctrl controller, opts *drawOpts) {
 		return
 	}
 
-	img := image1bit.NewVerticalLSB(image.Rectangle{Max: s.BufferSize})
-	draw.Src.Draw(img, s.BufferRect, opts.src, opts.srcPts)
+	draw.Src.Draw(opts.buffer, s.DstRect, opts.src, opts.srcPts)
 
 	setMemoryArea(ctrl, s.MemRect)
 
@@ -110,12 +98,12 @@ func drawImage(ctrl controller, opts *drawOpts) {
 
 	rowData := make([]byte, s.MemRect.Dx())
 
-	for y := 0; y < img.Bounds().Dy(); y++ {
+	for y := s.MemRect.Min.Y; y < s.MemRect.Max.Y; y++ {
 		for x := 0; x < len(rowData); x++ {
 			rowData[x] = 0
 
 			for bit := 0; bit < 8; bit++ {
-				if img.BitAt((x*8)+bit, y) {
+				if opts.buffer.BitAt(((s.MemRect.Min.X+x)*8)+bit, y) {
 					rowData[x] |= 0x80 >> bit
 				}
 			}

waveshare2in13v2/drawing_test.go

@@ -27,26 +27,24 @@ func TestDrawSpec(t *testing.T) {
 			name: "smaller than display",
 			opts: drawOpts{
 				devSize: image.Pt(100, 200),
+				buffer:  image1bit.NewVerticalLSB(image.Rect(0, 0, 120, 210)),
 				dstRect: image.Rect(17, 4, 25, 8),
 			},
 			want: drawSpec{
 				DstRect: image.Rect(17, 4, 25, 8),
 				MemRect: image.Rect(2, 4, 4, 8),
-				BufferSize: image.Pt(16, 4),
-				BufferRect: image.Rect(1, 0, 9, 4),
 			},
 		},
 		{
 			name: "larger than display",
 			opts: drawOpts{
 				devSize: image.Pt(100, 200),
+				buffer:  image1bit.NewVerticalLSB(image.Rect(0, 0, 100, 200)),
 				dstRect: image.Rect(-20, 50, 125, 300),
 			},
 			want: drawSpec{
 				DstRect: image.Rect(0, 50, 100, 200),
 				MemRect: image.Rect(0, 50, 13, 200),
-				BufferSize: image.Pt(13*8, 150),
-				BufferRect: image.Rect(0, 0, 100, 150),
 			},
 		},
 	} {
@@ -77,6 +75,7 @@ func TestDrawImage(t *testing.T) {
 			opts: drawOpts{
 				cmd:     writeRAMBW,
 				devSize: image.Pt(64, 64),
+				buffer:  image1bit.NewVerticalLSB(image.Rect(0, 0, 64, 64)),
 				dstRect: image.Rect(17, 4, 41, 8),
 				src:     &image.Uniform{image1bit.On},
 				srcPts:  image.Pt(0, 0),
@@ -98,6 +97,7 @@ func TestDrawImage(t *testing.T) {
 			opts: drawOpts{
 				cmd:     writeRAMBW,
 				devSize: image.Pt(80, 120),
+				buffer:  image1bit.NewVerticalLSB(image.Rect(0, 0, 80, 120)),
 				dstRect: image.Rect(0, 0, 80, 120),
 				src:     &image.Uniform{image1bit.On},
 				srcPts:  image.Pt(33, 44),

waveshare2in13v2/waveshare213v2.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"image"
 	"image/color"
+	"image/draw"
 	"time"
 
 	"periph.io/x/conn/v3"
@@ -62,6 +63,8 @@ type Dev struct {
 	rst  gpio.PinOut
 	busy gpio.PinIn
 
+	buffer *image1bit.VerticalLSB
+
 	opts *Opts
 }
 
@@ -139,9 +142,15 @@ func New(p spi.Port, dc, cs, rst gpio.PinOut, busy gpio.PinIn, opts *Opts) (*Dev
 		cs:   cs,
 		rst:  rst,
 		busy: busy,
+		buffer: image1bit.NewVerticalLSB(image.Rectangle{
+			Max: image.Pt((opts.Width+7)/8*8, opts.Height),
+		}),
 		opts: opts,
 	}
 
+	// Default color
+	draw.Src.Draw(d.buffer, d.buffer.Bounds(), &image.Uniform{image1bit.On}, image.Point{})
+
 	return d, nil
 }
 
@@ -176,8 +185,10 @@ func (d *Dev) Init(partialUpdate PartialUpdate) error {
 func (d *Dev) Clear(color color.Color) error {
 	eh := errorHandler{d: *d}
 
-	clearDisplay(&eh, image.Pt(d.opts.Width, d.opts.Height),
-		image1bit.BitModel.Convert(color).(image1bit.Bit))
+	c := image1bit.BitModel.Convert(color).(image1bit.Bit)
+	draw.Src.Draw(d.buffer, d.buffer.Bounds(), &image.Uniform{c}, image.Point{})
+
+	clearDisplay(&eh, image.Pt(d.opts.Width, d.opts.Height), c)
 
 	if eh.err == nil {
 		eh.err = d.turnOnDisplay()
@@ -201,6 +212,7 @@ func (d *Dev) Draw(dstRect image.Rectangle, src image.Image, srcPts image.Point)
 	opts := drawOpts{
 		cmd:     writeRAMBW,
 		devSize: image.Pt(d.opts.Width, d.opts.Height),
+		buffer:  d.buffer,
 		dstRect: dstRect,
 		src:     src,
 		srcPts:  srcPts,
@@ -221,6 +233,7 @@ func (d *Dev) Draw(dstRect image.Rectangle, src image.Image, srcPts image.Point)
 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,
 		dstRect: dstRect,
 		src:     src,
 		srcPts:  srcPts,