diff --git a/README.md b/README.md
index 9fbeb54..92dd6cd 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,7 @@
 
 [![GoDoc](https://godoc.org/github.com/steambap/captcha?status.svg)](https://godoc.org/github.com/steambap/captcha)
 [![Build Status](https://travis-ci.org/steambap/captcha.svg)](https://travis-ci.org/steambap/captcha)
+[![Go Report Card](https://goreportcard.com/badge/github.com/steambap/captcha)](https://goreportcard.com/report/github.com/steambap/captcha)
 
 </div>
 
diff --git a/captcha.go b/captcha.go
index 1716fd2..2c88a51 100644
--- a/captcha.go
+++ b/captcha.go
@@ -72,7 +72,7 @@ func (data *Data) WriteTo(w io.Writer) error {
 
 func init() {
 	var err error
-	ttfFont, err = freetype.ParseFont(TTF)
+	ttfFont, err = freetype.ParseFont(ttf)
 	if err != nil {
 		panic(err)
 	}
@@ -159,10 +159,10 @@ func drawSineCurve(img *image.NRGBA, opts *Options) {
 
 func randomDarkColor() hsva {
 	hue := float64(rng.Intn(361)) / 360
-	saturation := 0.6 + rng.Float64() * 0.2
-	value := 0.25 + rng.Float64() * 0.2
+	saturation := 0.6 + rng.Float64()*0.2
+	value := 0.25 + rng.Float64()*0.2
 
-	return hsva{h: hue, s:saturation, v:value, a:uint8(255)}
+	return hsva{h: hue, s: saturation, v: value, a: uint8(255)}
 }
 
 func drawText(text string, img *image.NRGBA, opts *Options) error {
diff --git a/example/main.go b/example/main.go
index 05b3f02..4cb66dc 100644
--- a/example/main.go
+++ b/example/main.go
@@ -8,8 +8,8 @@ import (
 )
 
 func main() {
-	http.HandleFunc("/", IndexHandle)
-	http.HandleFunc("/captcha", CaptchaHandle)
+	http.HandleFunc("/", indexHandle)
+	http.HandleFunc("/captcha", captchaHandle)
 	fmt.Println("Server start at port 8080")
 	err := http.ListenAndServe(":8080", nil)
 	if err != nil {
@@ -17,7 +17,7 @@ func main() {
 	}
 }
 
-func IndexHandle(w http.ResponseWriter, _ *http.Request) {
+func indexHandle(w http.ResponseWriter, _ *http.Request) {
 	doc, err := template.ParseFiles("index.html")
 	if err != nil {
 		fmt.Fprint(w, err.Error())
@@ -26,7 +26,7 @@ func IndexHandle(w http.ResponseWriter, _ *http.Request) {
 	doc.Execute(w, nil)
 }
 
-func CaptchaHandle(w http.ResponseWriter, _ *http.Request) {
+func captchaHandle(w http.ResponseWriter, _ *http.Request) {
 	img, err := captcha.New(150, 50)
 	if err != nil {
 		fmt.Fprint(w, nil)
diff --git a/font.go b/font.go
index 6439172..f35b407 100644
--- a/font.go
+++ b/font.go
@@ -3,7 +3,7 @@
 package captcha
 
 // The following is Comismsh TrueType font data.
-var TTF = []byte{
+var ttf = []byte{
 	0x00, 0x01, 0x00, 0x00, 0x00, 0x10, 0x00, 0x40, 0x00, 0x04, 0x00, 0xc0, 0x4f, 0x53, 0x2f, 0x32,
 	0x7f, 0x0c, 0x33, 0xcc, 0x00, 0x01, 0x37, 0xf8, 0x00, 0x00, 0x00, 0x56, 0x50, 0x43, 0x4c, 0x54,
 	0x61, 0x79, 0x01, 0xab, 0x00, 0x01, 0x38, 0x50, 0x00, 0x00, 0x00, 0x36, 0x63, 0x6d, 0x61, 0x70,
diff --git a/fonts/gen.go b/fonts/gen.go
index fbdb431..0ab103f 100644
--- a/fonts/gen.go
+++ b/fonts/gen.go
@@ -21,7 +21,7 @@ func main() {
 	fmt.Fprint(buf, "// DO NOT EDIT. This file is generated.\n\n")
 	fmt.Fprint(buf, "package captcha\n\n")
 	fmt.Fprint(buf, "// The following is Comismsh TrueType font data.\n")
-	fmt.Fprint(buf, "var TTF = []byte{")
+	fmt.Fprint(buf, "var ttf = []byte{")
 	for i, x := range src {
 		if i&15 == 0 {
 			buf.WriteByte('\n')