diff --git a/sprite/portable/portable.go b/sprite/portable/portable.go
new file mode 100644
index 0000000..788dd9f
--- /dev/null
+++ b/sprite/portable/portable.go
@@ -0,0 +1,126 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package portable blah blah blah TODO.
+package portable
+
+import (
+	"fmt"
+	"image"
+
+	"code.google.com/p/go.mobile/f32"
+	"code.google.com/p/go.mobile/sprite"
+)
+
+func Engine(dst *image.RGBA) sprite.Engine {
+	return &engine{
+		dst:      dst,
+		nodes:    []*node{nil},
+		sheets:   []image.Image{nil},
+		textures: []*texture{nil},
+	}
+}
+
+type node struct {
+	// TODO: move this into package sprite as Node.EngineFields.RelTransform??
+	relTransform f32.Mat3
+}
+
+type texture struct {
+	img    image.Image
+	bounds image.Rectangle
+}
+
+var _ sprite.Engine = (*engine)(nil)
+
+type engine struct {
+	dst           *image.RGBA
+	nodes         []*node
+	sheets        []image.Image
+	textures      []*texture
+	absTransforms []f32.Mat3
+}
+
+func (e *engine) Register(n *sprite.Node) {
+	if n.EngineFields.Index != 0 {
+		panic("portable: sprite.Node already registered")
+	}
+
+	o := &node{}
+	o.relTransform.Identity()
+
+	e.nodes = append(e.nodes, o)
+	n.EngineFields.Index = int32(len(e.nodes))
+}
+
+func (e *engine) Unregister(n *sprite.Node) {
+	panic("todo")
+}
+
+func (e *engine) LoadSheet(a image.Image) (sprite.Sheet, error) {
+	e.sheets = append(e.sheets, a)
+	return sprite.Sheet(len(e.sheets)), nil
+}
+
+func (e *engine) LoadTexture(s sprite.Sheet, bounds image.Rectangle) (sprite.Texture, error) {
+	if s < 0 || len(e.sheets) <= int(s) {
+		return 0, fmt.Errorf("portable: LoadTexture: sheet %d is out of bounds", s)
+	}
+	e.textures = append(e.textures, &texture{
+		img:    e.sheets[s],
+		bounds: bounds,
+	})
+	return sprite.Texture(len(e.textures)), nil
+}
+
+func (e *engine) UnloadSheet(s sprite.Sheet) error {
+	panic("todo")
+}
+
+func (e *engine) UnloadTexture(x sprite.Texture) error {
+	panic("todo")
+}
+
+func (e *engine) SetTexture(n *sprite.Node, t sprite.Time, x sprite.Texture) {
+	n.EngineFields.Dirty = true // TODO: do we need to propagate dirtiness up/down the tree?
+	n.EngineFields.Texture = x
+}
+
+func (e *engine) SetTransform(n *sprite.Node, t sprite.Time, m f32.Mat3) {
+	n.EngineFields.Dirty = true // TODO: do we need to propagate dirtiness up/down the tree?
+	e.nodes[n.EngineFields.Index].relTransform = m
+}
+
+func (e *engine) Render(scene *sprite.Node, t sprite.Time) {
+	e.absTransforms = e.absTransforms[:0]
+	e.render(scene, t)
+}
+
+func (e *engine) render(n *sprite.Node, t sprite.Time) {
+	if n.Arranger != nil {
+		n.Arranger.Arrange(e, n, t)
+	}
+
+	// Push absTransforms.
+	// TODO: cache absolute transforms and use EngineFields.Dirty?
+	rel := &e.nodes[n.EngineFields.Index].relTransform
+	if len(e.absTransforms) == 0 {
+		e.absTransforms = append(e.absTransforms, *rel)
+	} else {
+		m := f32.Mat3{}
+		m.Mul(rel, &e.absTransforms[len(e.absTransforms)-1]) // TODO: swap args order??
+		e.absTransforms = append(e.absTransforms, m)
+	}
+
+	if x := e.textures[n.EngineFields.Texture]; x != nil {
+		// TODO: draw transformed image onto e.dst.
+	}
+
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		e.render(c, t)
+	}
+
+	// Pop absTransforms.
+	e.absTransforms = e.absTransforms[:len(e.absTransforms)-1]
+}
diff --git a/sprite/sprite.go b/sprite/sprite.go
new file mode 100644
index 0000000..fc82a09
--- /dev/null
+++ b/sprite/sprite.go
@@ -0,0 +1,65 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sprite blah blah blah TODO.
+//
+// Typical main loop:
+//
+//	for each frame {
+//		quantize time.Now() to a sprite.Time
+//		process UI events
+//		modify the scene's nodes and animations (Arranger values)
+//		e.Render(scene, t)
+//	}
+package sprite
+
+import (
+	"image"
+
+	"code.google.com/p/go.mobile/f32"
+)
+
+// TODO: move this to a code.google.com/p/go.mobile/clock package?
+type Time int32
+
+type Arranger interface {
+	Arrange(e Engine, n *Node, t Time)
+}
+
+type Sheet int32
+
+type Texture int32
+
+type Engine interface {
+	Register(n *Node)
+	Unregister(n *Node)
+
+	LoadSheet(a image.Image) (Sheet, error)
+	LoadTexture(s Sheet, bounds image.Rectangle) (Texture, error)
+	UnloadSheet(s Sheet) error
+	UnloadTexture(x Texture) error
+
+	SetTexture(n *Node, t Time, x Texture)
+	SetTransform(n *Node, t Time, m f32.Mat3) // sets transform relative to parent.
+
+	Render(scene *Node, t Time)
+}
+
+type Node struct {
+	Parent, FirstChild, LastChild, PrevSibling, NextSibling *Node
+
+	Arranger Arranger
+
+	// EngineFields contains fields that should only be accessed by Engine
+	// implementations. It is exported because such implementations can be
+	// in other packages.
+	EngineFields struct {
+		// TODO: separate TexDirty and TransformDirty bits?
+		Dirty   bool
+		Index   int32
+		Texture Texture
+	}
+}
+
+// TODO: Node parent/sibling/child-related methods.