diff --git a/internal/graphics/command.go b/internal/graphics/command.go
index 315bd9b86..0717e1b7f 100644
--- a/internal/graphics/command.go
+++ b/internal/graphics/command.go
@@ -27,6 +27,12 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/sync"
 )
 
+// command represents a drawing command.
+//
+// A command for drawing that is created when Image functions are called like DrawImage,
+// or Fill.
+// A command is not immediately executed after created. Instaed, it is queued after created,
+// and executed only when necessary.
 type command interface {
 	Exec(indexOffsetInBytes int) error
 }
diff --git a/internal/restorable/copy.go b/internal/restorable/copy.go
index 2d6271be6..5da90a12b 100644
--- a/internal/restorable/copy.go
+++ b/internal/restorable/copy.go
@@ -23,6 +23,13 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/math"
 )
 
+// CopyImage copies origImg to a new RGBA image.
+//
+// Basically CopyImage just calls draw.Draw.
+// If origImg is a paletted image, an optimized copying method is used.
+//
+// CopyImage is used only from ebiten package but defined in restorable package,
+// because this function needs to be tested but cannot be exposed to Ebiten users.
 func CopyImage(origImg image.Image) *image.RGBA {
 	size := origImg.Bounds().Size()
 	w, h := size.X, size.Y
diff --git a/internal/restorable/doc.go b/internal/restorable/doc.go
new file mode 100644
index 000000000..169e57403
--- /dev/null
+++ b/internal/restorable/doc.go
@@ -0,0 +1,67 @@
+// Copyright 2017 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package restorable offers an Image struct that stores image commands
+// and restores its pixel data from the commands when context lost happens.
+//
+// When a function like DrawImage or Fill is called, an Image tries to record
+// the information for restoring.
+//
+// * Context lost
+//
+// Contest lost is a process that information on GPU memory are removed by OS
+// to make more room on GPU memory.
+// This can happen e.g. when GPU memory usage is high, or just switching applications
+// might cause context lost on mobiles.
+// As Ebiten's image data is on GPU memory, the game can't continue when context lost happens
+// without restoring image information.
+// The package restorable is the package to record information for such restoring.
+//
+// * DrawImage
+//
+// DrawImage function tries to record an item of 'draw image history' in the target image.
+// If a target image is stale or volatile, no item is created.
+// If an item of the history is created,
+// it can be said that the target image depends on the source image.
+// In other words, If A.DrawImage(B, ...) is called,
+// it can be said that the image A depends on the image B.
+//
+// * Fill, ReplacePixels and Dispose
+//
+// These functions are also drawing functions and the target image stores the pixel data
+// instead of draw image history items. There is no dependency here.
+//
+// * Making images stale
+//
+// After any of the drawing functions is called, the target image can't be depended on by
+// any other images. For example, if an image A depends on an image B, and B is changed
+// by a Fill call after that, the image A can't depend on the image B any more.
+// In this case, as the image A is no longer relaiable, the image A becomes 'stale'.
+// As all the stale images are resolved before context lost happens,
+// draw image history items are kept as they are
+// (even if an image C depends on the stale image A, it is still fine).
+//
+// * Stale image
+//
+// A stale image is an image that can't be restored from the recorded information.
+// All stale images must be resolved by reading pixels from GPU before the frame ends.
+// If a source image of DrawImage is a stale image, the target always becomes stale.
+//
+// * Volatile image
+//
+// A volatile image is a special image that is always cleared when a frame starts.
+// For instance, the game screen passed via the update function is a volatile image.
+// A volatile image doesn't have to record the drawing history.
+// If a source image of DrawImage is a volatile image, the target always becomes stale.
+package restorable
diff --git a/internal/restorable/images.go b/internal/restorable/images.go
index 27fa2066f..5e475abc4 100644
--- a/internal/restorable/images.go
+++ b/internal/restorable/images.go
@@ -22,16 +22,20 @@ import (
 // restoringEnabled indicates if restoring happens or not.
 var restoringEnabled = true // This value is overridden at enabled_*.go.
 
+// IsRestoringEnabled returns a boolean value indicating whether
+// restoring process works or not.
 func IsRestoringEnabled() bool {
 	// This value is updated only at init or EnableRestoringForTesting.
 	// No need to lock here.
 	return restoringEnabled
 }
 
+// EnableRestoringForTesting forces to enable restoring for testing.
 func EnableRestoringForTesting() {
 	restoringEnabled = true
 }
 
+// images is a set of Image objects.
 type images struct {
 	images      map[*Image]struct{}
 	lastChecked *Image
@@ -42,6 +46,8 @@ var theImages = &images{
 	images: map[*Image]struct{}{},
 }
 
+// FlushAndResolveStalePixels flushes the queued draw commands and resolves
+// all stale images.
 func FlushAndResolveStalePixels() error {
 	if err := graphics.FlushCommands(); err != nil {
 		return err