graphics: Improve clearing the offscreen performance

graphicscontext.go

@@ -88,7 +88,8 @@ func (c *graphicsContext) Update(afterFrameUpdate func()) error {
 		return err
 	}
 	for i := 0; i < updateCount; i++ {
-		restorable.ClearVolatileImages()
+		c.offscreen.fill(0, 0, 0, 0)
+
 		setRunningSlowly(i < updateCount-1)
 		if err := hooks.Run(); err != nil {
 			return err

internal/restorable/image.go

@@ -115,25 +115,6 @@ func (i *Image) makeStale() {
 	i.stale = true
 }
 
-// clearIfVolatile clears the image if the image is volatile.
-func (i *Image) clearIfVolatile() {
-	if !i.volatile {
-		return
-	}
-	i.basePixels = nil
-	i.drawImageHistory = nil
-	i.stale = false
-	if i.image == nil {
-		panic("not reached")
-	}
-
-	// TODO: ReplacePixels is bad in terms of performance. Use DrawImage if possible.
-	// Note that using DrawImage with *graphics.Image directly is dangerous since the image
-	// is never restored from context lost.
-	w, h := i.image.Size()
-	i.image.ReplacePixels(make([]byte, 4*w*h), 0, 0, w, h)
-}
-
 // ReplacePixels replaces the image pixels with the given pixels slice.
 func (i *Image) ReplacePixels(pixels []byte, x, y, width, height int) {
 	w, h := i.image.Size()

internal/restorable/images.go

@@ -73,13 +73,6 @@ func Restore() error {
 	return theImages.restore()
 }
 
-// ClearVolatileImages clears volatile images.
-//
-// ClearVolatileImages is intended to be called at the start of a frame.
-func ClearVolatileImages() {
-	theImages.clearVolatileImages()
-}
-
 // add adds img to the images.
 func (i *images) add(img *Image) {
 	i.m.Lock()
@@ -195,15 +188,6 @@ func (i *images) restore() error {
 	return nil
 }
 
-// clearVolatileImages clears the volatile images.
-func (i *images) clearVolatileImages() {
-	i.m.Lock()
-	defer i.m.Unlock()
-	for img := range i.images {
-		img.clearIfVolatile()
-	}
-}
-
 // InitializeGLState initializes the GL state.
 func InitializeGLState() error {
 	return graphics.ResetGLState()