graphics: Add NewVolatileImage (#235)

graphicscontext.go

@@ -43,20 +43,18 @@ func (c *graphicsContext) SetSize(screenWidth, screenHeight int, screenScale flo
 	if c.offscreen != nil {
 		c.offscreen.Dispose()
 	}
-	offscreen, err := NewImage(screenWidth, screenHeight, FilterNearest)
+	offscreen, err := NewVolatileImage(screenWidth, screenHeight, FilterNearest)
 	if err != nil {
 		return err
 	}
-	offscreen.impl.volatile = true
 
 	intScreenScale := int(math.Ceil(screenScale))
 	w := screenWidth * intScreenScale
 	h := screenHeight * intScreenScale
-	offscreen2, err := NewImage(w, h, FilterLinear)
+	offscreen2, err := NewVolatileImage(w, h, FilterLinear)
 	if err != nil {
 		return err
 	}
-	offscreen2.impl.volatile = true
 
 	w = int(float64(screenWidth) * screenScale)
 	h = int(float64(screenHeight) * screenScale)

image.go

@@ -194,7 +194,34 @@ type DrawImageOptions struct {
 //
 // This function is concurrent-safe.
 func NewImage(width, height int, filter Filter) (*Image, error) {
-	img, err := newImageImpl(width, height, filter)
+	img, err := newImageImpl(width, height, filter, false)
+	if err != nil {
+		return nil, err
+	}
+	if err := img.Fill(color.Transparent); err != nil {
+		return nil, err
+	}
+	eimg, err := theImagesForRestoring.add(img)
+	if err != nil {
+		return nil, err
+	}
+	return eimg, nil
+}
+
+// NewVolatileImage returns an empty 'volatile' image.
+// A volatile image is an image that pixels might be lost at the next frame.
+//
+// This is suitable for offscreen images that pixels are changed every frame.
+//
+// Pixels in regular non-volatile images are saved at each end of a frame if necessary
+// and restored automatically on GL context lost.
+// On the other hand, pixels in volatile images are not saved and can be lost
+// on GL context lost.
+// Saving pixels is an expensive operation, and it is desirable to avoid it if possible.
+//
+// This function is concurrent-safe.
+func NewVolatileImage(width, height int, filter Filter) (*Image, error) {
+	img, err := newImageImpl(width, height, filter, true)
 	if err != nil {
 		return nil, err
 	}

imageimpl.go

@@ -41,16 +41,17 @@ type imageImpl struct {
 	m        sync.Mutex
 }
 
-func newImageImpl(width, height int, filter Filter) (*imageImpl, error) {
+func newImageImpl(width, height int, filter Filter, volatile bool) (*imageImpl, error) {
 	img, err := graphics.NewImage(width, height, glFilter(filter))
 	if err != nil {
 		return nil, err
 	}
 	i := &imageImpl{
-		image:  img,
-		width:  width,
-		height: height,
-		filter: filter,
+		image:    img,
+		width:    width,
+		height:   height,
+		filter:   filter,
+		volatile: volatile,
 	}
 	runtime.SetFinalizer(i, (*imageImpl).Dispose)
 	return i, nil