Pulled over empty tile caching from coverage-exp branch.

README.md

@@ -9,7 +9,7 @@ on YouTube.
 A live map of an online world can be viewed [here](http://maps.mt.sha-bang.de/).
 
 See [COMPILE](https://bitbucket.org/s_l_teichmann/mtsatellite/src/default/COMPILE.md) how to compile
-MTSatellite. Essentially you need Go 1.3 (or higher) and a GNU/Linux system.
+MTSatellite. Essentially you need Go 1.4 (or higher) and a GNU/Linux system.
 
 See [SETUP](https://bitbucket.org/s_l_teichmann/mtsatellite/src/default/SETUP.md) how to bring
 MTSatellite to life.

common/basetilecreator.go

@@ -6,6 +6,7 @@ package common
 
 import (
 	"image/color"
+	"io/ioutil"
 	"log"
 	"path/filepath"
 	"strconv"
@@ -41,13 +42,16 @@ var tileDepths = [...][2]int16{
 	{-1024, -257},
 	{-1934, -1025}}
 
+var BackgroundColor = color.RGBA{R: 0xff, G: 0xff, B: 0xff, A: 0xff}
+
 type BaseTileCreator struct {
-	client   *RedisClient
-	colors   *Colors
-	renderer *Renderer
-	yOrder   *YOrder
-	baseDir  string
-	update   bool
+	client     *RedisClient
+	colors     *Colors
+	renderer   *Renderer
+	yOrder     *YOrder
+	baseDir    string
+	update     bool
+	emptyImage []byte
 }
 
 func NewBaseTileCreator(
@@ -116,11 +120,25 @@ func (btc *BaseTileCreator) CreateTile(x, z int16, i, j int) error {
 		oareas, nareas = nareas, oareas[0:0]
 	}
 
+	path := filepath.Join(btc.baseDir, strconv.Itoa(i), strconv.Itoa(j)+".png")
+
+	// Empty images are likely to be produced during seeding.
+	if !btc.update && btc.renderer.IsEmpty() {
+		// To avoid redundant encoding cache the resulting empty image.
+		if btc.emptyImage == nil {
+			var err error
+			m := BackgroundImage((tileWidth-2)*16, (tileHeight-2)*16, BackgroundColor)
+			if btc.emptyImage, err = EncodeToMem(m); err != nil {
+				return err
+			}
+		}
+		log.Printf("Writing empty (%d, %d) to file %s\n", x, z, path)
+		return ioutil.WriteFile(path, btc.emptyImage, 0666)
+	}
+
 	image := btc.renderer.CreateShadedImage(
 		16, 16, (tileWidth-2)*16, (tileHeight-2)*16,
-		btc.colors, color.RGBA{R: 0xff, G: 0xff, B: 0xff, A: 0xff})
-
-	path := filepath.Join(btc.baseDir, strconv.Itoa(i), strconv.Itoa(j)+".png")
+		btc.colors, BackgroundColor)
 
 	log.Printf("Writing (%d, %d) to file %s\n", x, z, path)
 

common/image.go

@@ -6,6 +6,7 @@ package common
 
 import (
 	"bufio"
+	"bytes"
 	"errors"
 	"image"
 	"image/png"
@@ -35,6 +36,15 @@ func nextSuffix() string {
 	return strconv.Itoa(int(1e9 + r%1e9))[1:]
 }
 
+func EncodeToMem(img image.Image) ([]byte, error) {
+	var buf bytes.Buffer
+	enc := png.Encoder{png.BestCompression}
+	if err := enc.Encode(&buf, img); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
 func SaveAsPNG(path string, img image.Image) (err error) {
 	var file *os.File
 	if file, err = os.Create(path); err != nil {

common/renderer.go

@@ -8,6 +8,7 @@ import (
 	"container/heap"
 	"image"
 	"image/color"
+	"image/draw"
 	"math"
 )
 
@@ -195,6 +196,10 @@ func (r *Renderer) IsFilled() bool {
 	return true
 }
 
+func (r *Renderer) IsEmpty() bool {
+	return r.SolidBlocks == 0 && r.TransparentBlocks == 0
+}
+
 func (r *Renderer) RenderBlock(block *Block, colors *Colors) (err error) {
 
 	bx := block.Coord.X - r.xOfs
@@ -406,6 +411,12 @@ func safeColor(x int32) uint8 {
 	}
 }
 
+func BackgroundImage(width, height int, bg color.RGBA) *image.RGBA {
+	m := image.NewRGBA(image.Rect(0, 0, width, height))
+	draw.Draw(m, m.Bounds(), &image.Uniform{bg}, image.ZP, draw.Src)
+	return m
+}
+
 func (r *Renderer) CreateShadedImage(
 	xOfs, zOfs, width, height int,
 	cols *Colors, background color.RGBA) *image.RGBA {