fix(effects): contrast zero gain

Author: fumiama

effects.go

@@ -52,6 +52,9 @@ func (dc *Context) Contrast(per int) {
 		}
 	case 100 < per && per <= 200: // 增益
 		gain := 200 - per
+		if gain == 0 {
+			gain = 1
+		}
 		for i, v := range dc.im.Pix {
 			if i%4 == 3 { // alpha
 				continue

effects_test.go

@@ -13,10 +13,15 @@ func TestBrightness(t *testing.T) {
 	}
 	dc := NewContextForImage(im)
 	dc.Brightness(-50)
-	if err := saveImage(dc, "TestBrightness"); err != nil {
+	if err := saveImage(dc, "TestBrightness-50"); err != nil {
 		t.Fatal(err)
 	}
 	checkHash(t, dc, "<gg.Context 30e0b039be47de836a9fb4c8b9825ce8>")
+	dc.Brightness(70) // 70-50=20
+	if err := saveImage(dc, "TestBrightness+20"); err != nil {
+		t.Fatal(err)
+	}
+	checkHash(t, dc, "<gg.Context d8aaf7a3d0aeb11dce3adc72ae9fb464>")
 }
 
 func TestContrast(t *testing.T) {
@@ -26,8 +31,13 @@ func TestContrast(t *testing.T) {
 	}
 	dc := NewContextForImage(im)
 	dc.Contrast(-50)
-	if err := saveImage(dc, "TestContrast"); err != nil {
+	if err := saveImage(dc, "TestContrast-50"); err != nil {
 		t.Fatal(err)
 	}
 	checkHash(t, dc, "<gg.Context e6f161e094f0d95603aad891c51c0b6b>")
+	dc.Contrast(100)
+	if err := saveImage(dc, "TestContrast+100"); err != nil {
+		t.Fatal(err)
+	}
+	checkHash(t, dc, "<gg.Context e1608d9268930ac4f80bd5cb82af927a>")
 }