Consistently cache display device Bounds()

Improve Draw() performance.

devices/apa102/apa102.go

@@ -78,6 +78,7 @@ func New(p spi.Port, o *Opts) (*Dev, error) {
 		numPixels:   o.NumPixels,
 		rawBuf:      buf,
 		pixels:      buf[4 : 4+4*o.NumPixels],
+		rect:        image.Rect(0, 0, o.NumPixels, 1),
 	}, nil
 }
 
@@ -96,11 +97,12 @@ type Dev struct {
 	// It can be changed, it will take effect on the next Draw() or Write() call.
 	Temperature uint16
 
-	s         spi.Conn //
+	s         spi.Conn        //
-	l         lut      // Updated at each .Write() call.
+	l         lut             // Updated at each .Write() call.
-	numPixels int      //
+	numPixels int             //
-	rawBuf    []byte   // Raw buffer sent over SPI. Cached to reduce heap fragmentation.
+	rawBuf    []byte          // Raw buffer sent over SPI. Cached to reduce heap fragmentation.
-	pixels    []byte   // Double buffer of pixels, to enable partial painting via Draw(). Effectively points inside rawBuf.
+	pixels    []byte          // Double buffer of pixels, to enable partial painting via Draw(). Effectively points inside rawBuf.
+	rect      image.Rectangle // Device bounds
 }
 
 func (d *Dev) String() string {
@@ -115,7 +117,7 @@ func (d *Dev) ColorModel() color.Model {
 
 // Bounds implements devices.Display. Min is guaranteed to be {0, 0}.
 func (d *Dev) Bounds() image.Rectangle {
-	return image.Rectangle{Max: image.Point{X: d.numPixels, Y: 1}}
+	return d.rect
 }
 
 // Draw implements devices.Display.
@@ -123,7 +125,7 @@ func (d *Dev) Bounds() image.Rectangle {
 // Using something else than image.NRGBA is 10x slower. When using image.NRGBA,
 // the alpha channel is ignored.
 func (d *Dev) Draw(r image.Rectangle, src image.Image, sp image.Point) {
-	r = r.Intersect(d.Bounds())
+	r = r.Intersect(d.rect)
 	srcR := src.Bounds()
 	srcR.Min = srcR.Min.Add(sp)
 	if dX := r.Dx(); dX < srcR.Dx() {

devices/ssd1306/ssd1306.go

@@ -189,7 +189,7 @@ func (d *Dev) Bounds() image.Rectangle {
 // It discards any failure.
 func (d *Dev) Draw(r image.Rectangle, src image.Image, sp image.Point) {
 	var next []byte
-	if img, ok := src.(*image1bit.VerticalLSB); ok && r == d.Bounds() && src.Bounds() == d.rect && sp.X == 0 && sp.Y == 0 {
+	if img, ok := src.(*image1bit.VerticalLSB); ok && r == d.rect && src.Bounds() == d.rect && sp.X == 0 && sp.Y == 0 {
 		// Exact size, full frame, image1bit encoding: fast path!
 		next = img.Pix
 	} else {

experimental/devices/nrzled/nrzled.go

@@ -82,6 +82,7 @@ func New(p gpiostream.PinOut, opts *Opts) (*Dev, error) {
 			Bits: make([]byte, opts.NumPixels*3*opts.Channels),
 			LSBF: false,
 		},
+		rect: image.Rect(0, 0, opts.NumPixels, 1),
 	}, nil
 }
 
@@ -92,6 +93,7 @@ type Dev struct {
 	channels  int                  // Number of channels per pixel
 	b         gpiostream.BitStream // NRZ encoded bits; cached to reduce heap fragmentation
 	buf       []byte               // Double buffer of RGB/RGBW pixels; enables partial Draw()
+	rect      image.Rectangle      // Device bounds
 }
 
 func (d *Dev) String() string {
@@ -126,7 +128,7 @@ func (d *Dev) ColorModel() color.Model {
 
 // Bounds implements devices.Display. Min is guaranteed to be {0, 0}.
 func (d *Dev) Bounds() image.Rectangle {
-	return image.Rectangle{Max: image.Point{X: d.numPixels, Y: 1}}
+	return d.rect
 }
 
 // Draw implements devices.Display.
@@ -138,7 +140,7 @@ func (d *Dev) Bounds() image.Rectangle {
 // A back buffer is kept so that partial updates are supported, albeit the full
 // LED strip is updated synchronously.
 func (d *Dev) Draw(r image.Rectangle, src image.Image, sp image.Point) {
-	r = r.Intersect(d.Bounds())
+	r = r.Intersect(d.rect)
 	srcR := src.Bounds()
 	srcR.Min = srcR.Min.Add(sp)
 	if dX := r.Dx(); dX < srcR.Dx() {