palette/moreland: fix floating-point arithmetic error in Palette

Fixes #798.

Signed-off-by: Eng Zer Jun <[email protected]>

Author: Juneezee

palette/moreland/luminance.go

@@ -176,10 +176,9 @@ func (l luminance) Palette(n int) palette.Palette {
 		l.SetMax(1)
 	}
 	delta := (l.max - l.min) / float64(n-1)
-	var v float64
 	c := make([]color.Color, n)
 	for i := range n {
-		v = l.min + float64(delta*float64(i))
+		v := min(l.min+float64(delta*float64(i)), l.max)
 		var err error
 		c[i], err = l.At(v)
 		if err != nil {

palette/moreland/luminance_test.go

@@ -161,3 +161,11 @@ func BenchmarkLuminance_At(b *testing.B) {
 		})
 	}
 }
+
+func Test_luminance_Palette(t *testing.T) {
+	// https://github.com/gonum/plot/issues/798
+	colors := Kindlmann()
+	colors.SetMin(0.3402859786606234)
+	colors.SetMax(15.322841335211892)
+	colors.Palette(15)
+}

palette/moreland/smooth.go

@@ -173,7 +173,7 @@ func (p smoothDiverging) Palette(n int) palette.Palette {
 	delta := (p.max - p.min) / float64(n-1)
 	c := make([]color.Color, n)
 	for i := range c {
-		v := p.min + float64(delta*float64(i))
+		v := min(p.min+float64(delta*float64(i)), p.max)
 		var err error
 		c[i], err = p.At(v)
 		if err != nil {

palette/moreland/smooth_test.go

@@ -246,3 +246,11 @@ func similar(a, b color.Color, tolerance float64) bool {
 	}
 	return true
 }
+
+func Test_smoothDiverging_Palette(t *testing.T) {
+	// https://github.com/gonum/plot/issues/798
+	colors := SmoothBlueRed()
+	colors.SetMin(0.3402859786606234)
+	colors.SetMax(15.322841335211892)
+	colors.Palette(15)
+}