diff --git a/custom.go b/custom.go
new file mode 100644
index 0000000..175e9fe
--- /dev/null
+++ b/custom.go
@@ -0,0 +1,65 @@
+package images4
+
+// Threshold multiplication coefficients for func CustomSimilar.
+// When all values equal 1.0 func CustomSimilar is equivalent
+// to func Similar. By setting those values less than 1, similarity
+// comparison becomes stricter (more precise). Values larger than 1
+// will generalize more and show more false positives. When uncertain,
+// setting all coefficients to 1.0 is the safe starting point.
+type CustomCoefficients struct {
+	Y    float64 // Luma (grayscale information).
+	Cb   float64 // Chrominance b (color information).
+	Cr   float64 // Chrominance r (color information).
+	Prop float64 // Proportion tolerance (how similar are image borders).
+}
+
+// CustomSimilar is like Similar, except it allows changing default
+// thresholds by multiplying them. The practically useful range of
+// the coefficients is [0, 1.0), but can be equal or larger than 1
+// if necessary. All coefficients set to 0 correspond to identical images,
+// for example an image file copy. All coefficients equal to 1 make func
+// CustomSimilar equivalent to func Similar.
+func CustomSimilar(iconA, iconB IconT, coeff CustomCoefficients) bool {
+
+	if !customPropSimilar(iconA, iconB, coeff) {
+		return false
+	}
+	if !customEucSimilar(iconA, iconB, coeff) {
+		return false
+	}
+	return true
+}
+
+func customPropSimilar(iconA, iconB IconT, coeff CustomCoefficients) bool {
+	return PropMetric(iconA, iconB) <= thProp*coeff.Prop
+}
+
+func customEucSimilar(iconA, iconB IconT, coeff CustomCoefficients) bool {
+
+	m1, m2, m3 := EucMetric(iconA, iconB)
+
+	return m1 <= thY*coeff.Y &&
+		m2 <= thCbCr*coeff.Cb &&
+		m3 <= thCbCr*coeff.Cr
+}
+
+// Similar90270 works like Similar, but also considers rotations of ±90°.
+// Those are rotations users might reasonably often do.
+func CustomSimilar90270(iconA, iconB IconT, coeff CustomCoefficients) bool {
+
+	if CustomSimilar(iconA, iconB, coeff) {
+		return true
+	}
+
+	// iconB rotated 90 degrees.
+	if CustomSimilar(iconA, Rotate90(iconB), coeff) {
+		return true
+	}
+
+	// As if iconB was rotated 270 degrees.
+	if CustomSimilar(Rotate90(iconA), iconB, coeff) {
+		return true
+	}
+
+	return false
+}
diff --git a/custom_test.go b/custom_test.go
new file mode 100644
index 0000000..17e3455
--- /dev/null
+++ b/custom_test.go
@@ -0,0 +1,72 @@
+package images4
+
+import (
+	"path"
+	"testing"
+)
+
+func TestCustomSimilar(t *testing.T) {
+
+	// Proportions test.
+
+	i1, _ := Open(path.Join("testdata", "euclidean", "distorted.jpg"))
+	i2, _ := Open(path.Join("testdata", "euclidean", "large.jpg"))
+
+	icon1 := Icon(i1)
+	icon2 := Icon(i2)
+
+	if Similar(icon1, icon2) {
+		t.Errorf("distorted.jpg is NOT similar to large.jpg")
+	}
+
+	if !CustomSimilar(icon1, icon2, CustomCoefficients{1, 1, 1, 10}) {
+		t.Errorf("distorted.jpg IS similar to large.jpg, assuming proportion differences are widely tolerated.")
+	}
+
+	// Euclidean tests.
+
+	i1, _ = Open(path.Join("testdata", "custom", "1.jpg"))
+	i2, _ = Open(path.Join("testdata", "custom", "2.jpg"))
+
+	icon1 = Icon(i1)
+	icon2 = Icon(i2)
+
+	if !Similar(icon1, icon2) {
+		t.Errorf("1.jpg is GENERALLY similar to 2.jpg")
+	}
+
+	// Luma.
+	if CustomSimilar(icon1, icon2, CustomCoefficients{0, 1, 1, 1}) {
+		t.Errorf("1.jpg is NOT IDENTICAL to 2.jpg")
+	}
+
+	// Luma.
+	if CustomSimilar(icon1, icon2, CustomCoefficients{0.4, 1, 1, 1}) {
+		t.Errorf("1.jpg is similar to 2.jpg, BUT NOT VERY SIMILAR")
+	}
+
+	// Chrominance b.
+	if CustomSimilar(icon1, icon2, CustomCoefficients{1, 0.1, 1, 1}) {
+		t.Errorf("1.jpg is similar to 2.jpg, BUT NOT VERY SIMILAR")
+	}
+
+	// Chrominance c.
+	if CustomSimilar(icon1, icon2, CustomCoefficients{1, 1, 0.1, 1}) {
+		t.Errorf("1.jpg is similar to 2.jpg, BUT NOT VERY SIMILAR")
+	}
+
+	// Image comparison to itself (or its own copy).
+
+	if !CustomSimilar(icon1, icon1, CustomCoefficients{0, 0, 0, 0}) {
+		t.Errorf("1.jpg IS IDENTICAL to itself")
+	}
+
+	if !CustomSimilar(icon1, icon1, CustomCoefficients{0.5, 0.5, 0.5, 0.5}) {
+		t.Errorf("1.jpg IS IDENTICAL to itself")
+	}
+
+	if !CustomSimilar(icon1, icon1, CustomCoefficients{1, 1, 1, 1}) {
+		t.Errorf("1.jpg IS IDENTICAL to itself")
+	}
+
+}
diff --git a/testdata/custom/1.jpg b/testdata/custom/1.jpg
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diff --git a/testdata/custom/2.jpg b/testdata/custom/2.jpg
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