added opacity to mark squares (#148)

Author: notnil

Diff for: image/README.md

@@ -24,8 +24,8 @@ image.SVG(file, pos.Board())
 The default colors, shown in the example SVG below, are (235, 209, 166) for light squares and (165, 117, 81) for dark squares.  The light and dark squares can be customized using the SquareColors() option. 
 
 ```go
-white := color.RGBA{255, 255, 255, 1}
-gray := color.RGBA{120, 120, 120, 1}
+white := color.RGBA{255, 255, 255, 255}
+gray := color.RGBA{120, 120, 120, 255}
 sqrs := image.SquareColors(white, gray)
 image.SVG(file, pos.Board(), sqrs)
 ```
@@ -35,7 +35,7 @@ image.SVG(file, pos.Board(), sqrs)
 MarkSquares is designed to be used as an optional argument to the SVG function.  It marks the given squares with the color.  A possible usage includes marking squares of the previous move.
 
 ```go
-yellow := color.RGBA{255, 255, 0, 1}
+yellow := color.RGBA{255, 255, 0, 51}
 mark := image.MarkSquares(yellow, chess.D2, chess.D4)
 image.SVG(file, pos.Board(), mark)
 ```
@@ -82,7 +82,7 @@ func main() {
     }
 
     // write board SVG to file
-    yellow := color.RGBA{255, 255, 0, 1}
+    yellow := color.RGBA{255, 255, 0, 51}
     mark := image.MarkSquares(yellow, chess.D2, chess.D4)
     arrows := image.MarkArrows(image.Arrow(chess.D2, chess.D4))
     if err := image.SVG(f, pos.Board(), mark, arrows); err != nil {

Diff for: image/image.go

@@ -152,7 +152,9 @@ func (e *Encoder) EncodeSVG(b *chess.Board) error {
 			canvas.Rect(x, y, sqWidth, sqHeight, "fill: "+colorToHex(c))
 			markColor, ok := e.marks[sq]
 			if ok {
-				canvas.Rect(x, y, sqWidth, sqHeight, "fill-opacity:0.2;fill: "+colorToHex(markColor))
+				_, _, _, a := markColor.RGBA()
+				opacity := float64(a) / 0xffff
+				canvas.Rect(x, y, sqWidth, sqHeight, fmt.Sprintf("fill-opacity:%f;fill: %s", opacity, colorToHex(markColor)))
 			}
 			// draw piece
 			p := boardMap[sq]

Diff for: image/image_test.go

@@ -12,8 +12,8 @@ import (
 	"github.com/notnil/chess/image"
 )
 
-const expectedMD5 = "a2ee66ca19e4c347aec41371c1ca07f8"
-const expectedMD5Black = "ce4d4e033a50678898c62928b8e0a15c"
+const expectedMD5 = "625c63ef80796a3485004952a18c9b25"
+const expectedMD5Black = "1988104209a5401be2f645967fadec78"
 const expectedMD5KnightsAndDiagonalArrows = "9c95aa56cec67be2ceee141f259753f7"
 
 func TestSVG(t *testing.T) {
@@ -24,7 +24,7 @@ func TestSVG(t *testing.T) {
 	if err := pos.UnmarshalText([]byte(fenStr)); err != nil {
 		t.Error(err)
 	}
-	mark := image.MarkSquares(color.RGBA{255, 255, 0, 1}, chess.D2, chess.D4)
+	mark := image.MarkSquares(color.RGBA{255, 255, 0, 100}, chess.D2, chess.D4)
 	arrows := image.MarkArrows(image.Arrow(chess.D2, chess.D4))
 	if err := image.SVG(buf, pos.Board(), mark, arrows); err != nil {
 		t.Error(err)
@@ -46,7 +46,7 @@ func TestSVGFromBlack(t *testing.T) {
 	if err := pos.UnmarshalText([]byte(fenStr)); err != nil {
 		t.Error(err)
 	}
-	mark := image.MarkSquares(color.RGBA{255, 255, 0, 1}, chess.D2, chess.D4)
+	mark := image.MarkSquares(color.RGBA{255, 255, 0, 51}, chess.D2, chess.D4)
 	arrows := image.MarkArrows(image.Arrow(chess.D2, chess.D4).WithColor(color.Black))
 	per := image.Perspective(chess.Black)
 	if err := image.SVG(buf, pos.Board(), mark, arrows, per); err != nil {