internal/ui: optimize GPU usages when the screen doesn't have to be updated

This change skips rendering when 1) the screen is not cleared every frame
(`SetScreenClearedEveryFrame(false)`) and 2) Draw doesn't draw anything
onto the screen. The GPU usages decreased on some machines (e.g. GPU usage
was 10% with an empty Ebitengine project and became 2-3 % on a Windows
machine).

Updates #2341

examples/skipdraw/main.go

@@ -0,0 +1,107 @@
+// Copyright 2022 The Ebitengine 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.
+
+//go:build example
+// +build example
+
+package main
+
+import (
+	"bytes"
+	"image"
+	_ "image/jpeg"
+	"log"
+	"math"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
+	"github.com/hajimehoshi/ebiten/v2/examples/resources/images"
+	"github.com/hajimehoshi/ebiten/v2/inpututil"
+)
+
+const (
+	screenWidth  = 640
+	screenHeight = 480
+)
+
+var (
+	gophersImage *ebiten.Image
+)
+
+type Game struct {
+	count      int
+	input      bool
+	introShown bool
+}
+
+func (g *Game) Update() error {
+	g.count++
+	g.input = len(inpututil.AppendPressedKeys(nil)) > 0
+	if inpututil.IsKeyJustPressed(ebiten.KeyF) {
+		ebiten.SetFullscreen(!ebiten.IsFullscreen())
+	}
+	return nil
+}
+
+func (g *Game) Draw(screen *ebiten.Image) {
+	// If there is no inpnut, skip draw. Ebitengine skips GPU usages in this case.
+	if !g.input && g.introShown {
+		return
+	}
+
+	screen.Clear()
+
+	w, h := gophersImage.Size()
+	op := &ebiten.DrawImageOptions{}
+
+	// Move the image's center to the screen's upper-left corner.
+	// This is a preparation for rotating. When geometry matrices are applied,
+	// the origin point is the upper-left corner.
+	op.GeoM.Translate(-float64(w)/2, -float64(h)/2)
+
+	// Rotate the image. As a result, the anchor point of this rotate is
+	// the center of the image.
+	op.GeoM.Rotate(float64(g.count%360) * 2 * math.Pi / 360)
+
+	// Move the image to the screen's center.
+	op.GeoM.Translate(screenWidth/2, screenHeight/2)
+
+	screen.DrawImage(gophersImage, op)
+
+	ebitenutil.DebugPrint(screen, "Press any keys to keep animations.\nPress F to switch fullscreen.")
+
+	g.introShown = true
+}
+
+func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
+	return screenWidth, screenHeight
+}
+
+func main() {
+	ebiten.SetWindowResizingMode(ebiten.WindowResizingModeEnabled)
+	ebiten.SetScreenClearedEveryFrame(false)
+
+	// Decode an image from the image file's byte slice.
+	img, _, err := image.Decode(bytes.NewReader(images.Gophers_jpg))
+	if err != nil {
+		log.Fatal(err)
+	}
+	gophersImage = ebiten.NewImageFromImage(img)
+
+	ebiten.SetWindowSize(screenWidth, screenHeight)
+	ebiten.SetWindowTitle("Skip Draw (Ebitengine Demo)")
+	if err := ebiten.RunGame(&Game{}); err != nil {
+		log.Fatal(err)
+	}
+}

gameforui.go

@@ -147,7 +147,7 @@ func (g *gameForUI) DrawOffscreen() error {
 	return nil
 }
 
-func (g *gameForUI) DrawScreen() {
+func (g *gameForUI) DrawFinalScreen() {
 	scale, offsetX, offsetY := g.ScreenScaleAndOffsets()
 	var geoM GeoM
 	geoM.Scale(scale, scale)

internal/graphicscommand/command.go

@@ -168,7 +168,8 @@ func (q *commandQueue) Flush(graphicsDriver graphicsdriver.Graphics, endFrame bo
 
 // flush must be called the main thread.
 func (q *commandQueue) flush(graphicsDriver graphicsdriver.Graphics, endFrame bool) (err error) {
-	if len(q.commands) == 0 {
+	// If endFrame is true, Begin/End should be called to ensure the framebuffer is swapped.
+	if len(q.commands) == 0 && !endFrame {
 		return nil
 	}
 

internal/graphicsdriver/metal/graphics_darwin.go

@@ -199,11 +199,15 @@ func (g *Graphics) SetVertices(vertices []float32, indices []uint16) error {
 }
 
 func (g *Graphics) flushIfNeeded(present bool) {
-	if g.cb == (mtl.CommandBuffer{}) {
+	if g.cb == (mtl.CommandBuffer{}) && !present {
 		return
 	}
 	g.flushRenderCommandEncoderIfNeeded()
 
+	if present && g.screenDrawable == (ca.MetalDrawable{}) {
+		g.screenDrawable = g.view.nextDrawable()
+	}
+
 	if !g.view.presentsWithTransaction() && present && g.screenDrawable != (ca.MetalDrawable{}) {
 		g.cb.PresentDrawable(g.screenDrawable)
 	}

internal/ui/context.go

@@ -38,7 +38,7 @@ type Game interface {
 	Layout(outsideWidth, outsideHeight int) (int, int)
 	Update() error
 	DrawOffscreen() error
-	DrawScreen()
+	DrawFinalScreen()
 	ScreenScaleAndOffsets() (scale, offsetX, offsetY float64)
 }
 
@@ -53,6 +53,8 @@ type context struct {
 	// The following members must be protected by the mutex m.
 	outsideWidth  float64
 	outsideHeight float64
+
+	skipCount int
 }
 
 func newContext(game Game) *context {
@@ -63,7 +65,7 @@ func newContext(game Game) *context {
 
 func (c *context) updateFrame(graphicsDriver graphicsdriver.Graphics, outsideWidth, outsideHeight float64, deviceScaleFactor float64, ui *userInterfaceImpl) error {
 	// TODO: If updateCount is 0 and vsync is disabled, swapping buffers can be skipped.
-	return c.updateFrameImpl(graphicsDriver, clock.UpdateFrame(), outsideWidth, outsideHeight, deviceScaleFactor, ui)
+	return c.updateFrameImpl(graphicsDriver, clock.UpdateFrame(), outsideWidth, outsideHeight, deviceScaleFactor, ui, false)
 }
 
 func (c *context) forceUpdateFrame(graphicsDriver graphicsdriver.Graphics, outsideWidth, outsideHeight float64, deviceScaleFactor float64, ui *userInterfaceImpl) error {
@@ -74,14 +76,14 @@ func (c *context) forceUpdateFrame(graphicsDriver graphicsdriver.Graphics, outsi
 		n = 2
 	}
 	for i := 0; i < n; i++ {
-		if err := c.updateFrameImpl(graphicsDriver, 1, outsideWidth, outsideHeight, deviceScaleFactor, ui); err != nil {
+		if err := c.updateFrameImpl(graphicsDriver, 1, outsideWidth, outsideHeight, deviceScaleFactor, ui, true); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
-func (c *context) updateFrameImpl(graphicsDriver graphicsdriver.Graphics, updateCount int, outsideWidth, outsideHeight float64, deviceScaleFactor float64, ui *userInterfaceImpl) (err error) {
+func (c *context) updateFrameImpl(graphicsDriver graphicsdriver.Graphics, updateCount int, outsideWidth, outsideHeight float64, deviceScaleFactor float64, ui *userInterfaceImpl, forceDraw bool) (err error) {
 	if err := theGlobalState.error(); err != nil {
 		return err
 	}
@@ -142,14 +144,14 @@ func (c *context) updateFrameImpl(graphicsDriver graphicsdriver.Graphics, update
 	}
 
 	// Draw the game.
-	if err := c.drawGame(graphicsDriver); err != nil {
+	if err := c.drawGame(graphicsDriver, forceDraw); err != nil {
 		return err
 	}
 
 	return nil
 }
 
-func (c *context) drawGame(graphicsDriver graphicsdriver.Graphics) error {
+func (c *context) drawGame(graphicsDriver graphicsdriver.Graphics, forceDraw bool) error {
 	if (c.offscreen.imageType == atlas.ImageTypeVolatile) != theGlobalState.isScreenClearedEveryFrame() {
 		w, h := c.offscreen.width, c.offscreen.height
 		c.offscreen.MarkDisposed()
@@ -162,20 +164,35 @@ func (c *context) drawGame(graphicsDriver graphicsdriver.Graphics) error {
 	if theGlobalState.isScreenClearedEveryFrame() {
 		c.offscreen.clear()
 	}
+
+	c.offscreen.dirty = false
 	if err := c.game.DrawOffscreen(); err != nil {
 		return err
 	}
 
-	if graphicsDriver.NeedsClearingScreen() {
-		// This clear is needed for fullscreen mode or some mobile platforms (#622).
-		c.screen.clear()
+	const maxSkipCount = 3
+
+	if !forceDraw && !theGlobalState.isScreenClearedEveryFrame() && !c.offscreen.dirty {
+		if c.skipCount < maxSkipCount {
+			c.skipCount++
+		}
+	} else {
+		c.skipCount = 0
 	}
 
-	c.game.DrawScreen()
+	// If the offscreen is not updated and the framebuffers don't have to be updated, skip rendering to save GPU power.
+	if c.skipCount < maxSkipCount {
+		if graphicsDriver.NeedsClearingScreen() {
+			// This clear is needed for fullscreen mode or some mobile platforms (#622).
+			c.screen.clear()
+		}
 
-	// The final screen is never used as the rendering source.
-	// Flush its buffer here just in case.
-	c.screen.flushBufferIfNeeded()
+		c.game.DrawFinalScreen()
+
+		// The final screen is never used as the rendering source.
+		// Flush its buffer here just in case.
+		c.screen.flushBufferIfNeeded()
+	}
 
 	return nil
 }

internal/ui/image.go

@@ -45,6 +45,8 @@ type Image struct {
 	bigOffscreenBuffer      *Image
 	bigOffscreenBufferBlend graphicsdriver.Blend
 	bigOffscreenBufferDirty bool
+
+	dirty bool
 }
 
 func NewImage(width, height int, imageType atlas.ImageType) *Image {
@@ -68,9 +70,12 @@ func (i *Image) MarkDisposed() {
 	i.mipmap.MarkDisposed()
 	i.mipmap = nil
 	i.dotsBuffer = nil
+	i.dirty = false
 }
 
 func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices []float32, indices []uint16, blend graphicsdriver.Blend, dstRegion, srcRegion graphicsdriver.Region, subimageOffsets [graphics.ShaderImageCount - 1][2]float32, shader *Shader, uniforms [][]float32, evenOdd bool, canSkipMipmap bool, antialias bool) {
+	i.dirty = true
+
 	if antialias {
 		// Flush the other buffer to make the buffers exclusive.
 		i.flushDotsBufferIfNeeded()
@@ -141,6 +146,8 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices [
 }
 
 func (i *Image) WritePixels(pix []byte, x, y, width, height int) {
+	i.dirty = true
+
 	if width == 1 && height == 1 {
 		// Flush the other buffer to make the buffers exclusive.
 		i.flushBigOffscreenBufferIfNeeded()