inky: fix blend precision, height quirk, and minor cleanups

- Fix color blending truncation in blend(): compute the full weighted
    sum as float64 before casting to uint8, avoiding double truncation
  - Fix UC8159 on IMPRESSION57 incorrectly reporting height as 447;
    correct it to 448 in DetectOpts
  - Add Opts.String() for human-readable option printing
  - Refactor update() if/else chain to a switch statement
  - Add TestDevImpression_BlendPrecision to cover the blend fix

Signed-off-by: Çağlar Onur <caglar@10ur.org>

inky/impression.go

@@ -283,16 +283,11 @@ func (d *DevImpression) blend() color.Palette {
 
 	pr := make([]color.Color, 0)
 	for i := range satPalette {
-		rs, gs, bs := uint8(float64(satPalette[i].R)*sat),
-			uint8(float64(satPalette[i].G)*sat),
-			uint8(float64(satPalette[i].B)*sat)
+		r := uint8(float64(satPalette[i].R)*sat + float64(dscPalette[i].R)*(1.0-sat))
+		g := uint8(float64(satPalette[i].G)*sat + float64(dscPalette[i].G)*(1.0-sat))
+		b := uint8(float64(satPalette[i].B)*sat + float64(dscPalette[i].B)*(1.0-sat))
 
-		rd, gd, bd :=
-			uint8(float64(dscPalette[i].R)*(1.0-sat)),
-			uint8(float64(dscPalette[i].G)*(1.0-sat)),
-			uint8(float64(dscPalette[i].B)*(1.0-sat))
-
-		pr = append(pr, color.RGBA{rs + rd, gs + gd, bs + bd, dscPalette[i].A})
+		pr = append(pr, color.RGBA{r, g, b, dscPalette[i].A})
 	}
 
 	if d.Dev.model == IMPRESSION73SPECTRA6 {
@@ -567,6 +562,7 @@ func (d *DevImpression) resetEC() error {
 	if err := d.sendCommand(el673PSR, []byte{0x5F, 0x69}); err != nil {
 		return err
 	}
+
 	if err := d.sendCommand(el673BTST1, []byte{0x40, 0x1F, 0x1F, 0x2C}); err != nil {
 		return err
 	}
@@ -576,6 +572,7 @@ func (d *DevImpression) resetEC() error {
 	if err := d.sendCommand(el673BTST2, []byte{0x6F, 0x1F, 0x17, 0x17}); err != nil {
 		return err
 	}
+
 	if err := d.sendCommand(el673POFS, []byte{0x00, 0x54, 0x00, 0x44}); err != nil {
 		return err
 	}
@@ -602,12 +599,14 @@ func (d *DevImpression) resetEC() error {
 }
 
 func (d *DevImpression) update(pix []uint8) error {
-	if d.model == IMPRESSION73 {
+	switch d.model {
+	case IMPRESSION73:
 		return d.updateAC(pix)
-	} else if d.model == IMPRESSION73SPECTRA6 {
+	case IMPRESSION73SPECTRA6:
 		return d.updateEC(pix)
-	}
+	default:
 		return d.updateUC(pix)
+	}
 }
 
 func (d *DevImpression) updateUC(pix []uint8) error {
@@ -710,10 +709,12 @@ func (d *DevImpression) wait(dur time.Duration) {
 		log.Printf("Err: %s", err)
 		return
 	}
+
 	if d.busy.Read() == gpio.High {
 		time.Sleep(dur)
 		return
 	}
+
 	// Wait for rising edges (Low -> High) or the timeout.
 	tEnd := time.Now().Add(dur)
 	edgeDur := dur

inky/impression_test.go

@@ -0,0 +1,48 @@
+// Copyright 2019 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package inky
+
+import (
+	"image/color"
+	"testing"
+)
+
+func TestDevImpression_BlendPrecision(t *testing.T) {
+	// Saturation level that triggers truncation if not handled correctly.
+	// For example, if rs=10.9 and rd=5.9:
+	// Old: uint8(10.9) + uint8(5.9) = 10 + 5 = 15
+	// New: uint8(10.9 + 5.9) = uint8(16.8) = 16
+
+	// We need to simulate this with real palette values and saturation.
+	// Let's use a simplified case.
+	// satPalette[0].R = 109, dscPalette[0].R = 59, saturation = 10%
+	// rs = 109 * 0.1 = 10.9
+	// rd = 59 * 0.9 = 53.1
+	// Old: 10 + 53 = 63
+	// New: uint8(10.9 + 53.1) = uint8(64.0) = 64
+
+	d := &DevImpression{
+		saturation: 10,
+	}
+	d.Dev.model = IMPRESSION73 // This uses sc7 and dsc
+
+	// Override sc7 and dsc for the test up to at least common length
+	oldSc7 := sc7
+	oldDsc := dsc
+	defer func() {
+		sc7 = oldSc7
+		dsc = oldDsc
+	}()
+
+	sc7 = []color.NRGBA{{R: 109, G: 0, B: 0, A: 255}}
+	dsc = []color.NRGBA{{R: 59, G: 0, B: 0, A: 255}}
+
+	p := d.blend()
+	c := p[0].(color.RGBA)
+
+	if c.R != 64 {
+		t.Errorf("Expected R=64, got %d. Precision loss detected!", c.R)
+	}
+}

inky/opts.go

@@ -97,6 +97,10 @@ func DetectOpts(bus i2c.Bus) (*Opts, error) {
 		options.Model = WHAT
 	case 14:
 		options.Model = IMPRESSION57
+		// Some UC8159's report Height as 447 so fix that.
+		if options.Height == 447 {
+			options.Height = 448
+		}
 	case 15, 16:
 		options.Model = IMPRESSION4
 	case 20:
@@ -124,3 +128,7 @@ func readEep(bus i2c.Bus) ([]byte, error) {
 
 	return data, nil
 }
+
+func (o *Opts) String() string {
+	return fmt.Sprintf("Resolution: %dx%d Model: %s Model Color: %s Border Color: %s PCB Variant: %d Display Variant: %d", o.Width, o.Height, o.Model, o.ModelColor, o.BorderColor, o.PCBVariant, o.DisplayVariant)
+}