Add isDelta test

Author: peterhellberg

gfx_test.go

@@ -0,0 +1,31 @@
+package gfx
+
+import (
+	"math"
+	"testing"
+)
+
+func inDelta(t *testing.T, expected, actual, delta float64) bool {
+	t.Helper()
+
+	if math.IsNaN(expected) && math.IsNaN(actual) {
+		return true
+	}
+
+	if math.IsNaN(expected) {
+		t.Error("Expected must not be NaN")
+		return false
+	}
+
+	if math.IsNaN(actual) {
+		t.Errorf("Expected %v with delta %v, but was NaN", expected, delta)
+		return false
+	}
+
+	if dt := expected - actual; dt < -delta || dt > delta {
+		t.Errorf("Max difference between %v and %v allowed is %v, but difference was %v", expected, actual, delta, dt)
+		return false
+	}
+
+	return true
+}