Apply wind field texture filter server-side

Reduces download size and local cpu time.

README.md

@@ -40,6 +40,6 @@ The wind vector (in m/s) is extracted from the [GRIB2](https://www.nco.ncep.noaa
 
 The elevation and wind vector magnitude range I chose seems to produce similar images as the old official one from 2019 (available at [`www.gstatic.com/pixel/livewallpaper/windy/gfs_wind_1000.jpg`](https://www.gstatic.com/pixel/livewallpaper/windy/gfs_wind_1000.jpg)), and the red/green/blue level curves are similar.
 
-See [`windy.api.pgaskin.net/wind_field.jpg`](https://windy.api.pgaskin.net/wind_field.jpg) for the latest wind field image generated by this [code](./api/windy.go).
+To create the texture passed to the particle system and background shaders, the image is scaled down to 1/4 of the size (i.e., 360x180) using bilinear filtering, then blurred using a gaussian kernel of radius 2. This matches what was done by the original live wallpaper. This filtering is done to smooth out the streamlines and remove local outlier values, resulting in less detailed and rounder wallpaper wind trails. Since the wallpaper still looks good, and is interesting in its own way before this filtering, I'm probably going to add variants with an unfiltered wind field later.
 
-Before being passed to the particle system and background shaders, the image is scaled down to 1/4 of the size (i.e., 360x180) using bilinear filtering, then blurred using a gaussian kernel of radius 2. This matches what was done by the original live wallpaper. This filtering is done to smooth out the streamlines and remove local outlier values, resulting in less detailed and rounder wallpaper wind trails. Since the wallpaper still looks good, and is interesting in its own way before this filtering, I'm probably going to add variants with an unfiltered wind field later.
+See [`windy.api.pgaskin.net/wind_field.jpg`](https://windy.api.pgaskin.net/wind_field.jpg) for the latest wind field image generated by this [code](./api/windy.go), and [`windy.api.pgaskin.net/wind_cache.png?filter=1`](https://windy.api.pgaskin.net/wind_cache.png?filter=1) for the latest filtered texture.

api/main.go

@@ -110,7 +110,7 @@ func main() {
 		Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			if p, ok := strings.CutPrefix(r.URL.Path, "/"); ok {
 				if !strings.ContainsRune(p, '/') {
-					if strings.HasPrefix(p, "wind_field.") {
+					if strings.HasPrefix(p, "wind_field.") || strings.HasPrefix(p, "wind_cache.") {
 						switch r.Method {
 						case http.MethodGet, http.MethodHead:
 							windy.ServeHTTP(w, r)

api/windy.go

@@ -22,6 +22,7 @@ import (
 	"os"
 	"os/exec"
 	"path"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -70,6 +71,10 @@ type WindData struct {
 		Data []byte
 		ETag string
 	}
+	FilteredPNG [1]struct {
+		Data []byte
+		ETag string
+	}
 	Updated time.Time
 	Cycle   gfsCycle
 	Source  string
@@ -116,17 +121,35 @@ func (h *Windy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Last-Modified", data.Updated.UTC().Format(http.TimeFormat))
 
 	var buf []byte
-	switch ext := path.Ext(r.URL.Path); ext {
-	case ".jpg":
+	switch base := path.Base(r.URL.Path); base {
+	case "wind_field.jpg":
 		w.Header().Set("Content-Type", "image/jpeg")
 		w.Header().Set("ETag", data.JPG.ETag)
 		buf = data.JPG.Data
-	case ".png":
-		w.Header().Set("Content-Type", "image/png")
+	case "wind_field.png":
+		w.Header().Set("Content-Type", "image/jpeg")
 		w.Header().Set("ETag", data.PNG.ETag)
 		buf = data.PNG.Data
+	case "wind_cache.png":
+		var v int
+		if ss := r.URL.Query()["filter"]; len(ss) != 1 {
+			http.Error(w, "Exactly one filter type (?filter=) is required.", http.StatusBadRequest)
+			return
+		} else if n, _ := strconv.ParseUint(ss[0], 10, 64); n == 0 {
+			http.Error(w, "Invalid filter type.", http.StatusBadRequest)
+			return
+		} else {
+			v = int(n)
+		}
+		if v > len(data.FilteredPNG) {
+			http.Error(w, "Unsupported filter type.", http.StatusBadRequest)
+			return
+		}
+		w.Header().Set("Content-Type", "image/png")
+		w.Header().Set("ETag", data.FilteredPNG[v-1].ETag)
+		buf = data.FilteredPNG[v-1].Data
 	default:
-		http.Error(w, "No image available for extension "+ext+".", http.StatusNotFound)
+		http.Error(w, "No image available for "+base+".", http.StatusNotFound)
 		return
 	}
 	http.ServeContent(w, r, "", data.Updated, bytes.NewReader(buf))
@@ -187,6 +210,7 @@ loop:
 
 			var output WindData
 			output.Updated = time.Now()
+			output.Updated = time.Date(2023, 11, 2, 4, 0, 0, 0, time.UTC)
 			output.Cycle = gfsCycle(output.Updated)
 
 			var wind [][][2]float64
@@ -247,7 +271,7 @@ loop:
 			for latIdx := range wind {
 				for lngIdx := range wind[latIdx] {
 					s, u, v := decompose(wind[latIdx][lngIdx])
-					img.Set(lngIdx, latIdx, color.RGBA{
+					img.SetRGBA(lngIdx, latIdx, color.RGBA{
 						R: uint8(mapValue(u, -1, 1, 0, 255)),
 						G: uint8(mapValue(v, -1, 1, 0, 255)),
 						B: uint8(mapValue(s, 0, 30, 0, 255)),
@@ -256,24 +280,46 @@ loop:
 				}
 			}
 			logger.Info("generated image, encoding")
+			{
+				var pngBuf bytes.Buffer
+				if err := png.Encode(&pngBuf, img); err != nil {
+					return nil, fmt.Errorf("encode png: %w", err)
+				}
+				output.PNG.Data = pngBuf.Bytes()
 
-			var pngBuf bytes.Buffer
-			if err := png.Encode(&pngBuf, img); err != nil {
-				return nil, fmt.Errorf("encode png: %w", err)
-			}
-			output.PNG.Data = pngBuf.Bytes()
-
-			var jpgBuf bytes.Buffer
-			if err := jpeg.Encode(&jpgBuf, img, &jpeg.Options{Quality: 100}); err != nil {
-				return nil, fmt.Errorf("encode png: %w", err)
-			}
-			output.JPG.Data = jpgBuf.Bytes()
+				var jpgBuf bytes.Buffer
+				if err := jpeg.Encode(&jpgBuf, img, &jpeg.Options{Quality: 100}); err != nil {
+					return nil, fmt.Errorf("encode png: %w", err)
+				}
+				output.JPG.Data = jpgBuf.Bytes()
 
-			pngSha := sha1.Sum(output.PNG.Data)
-			jpgSha := sha1.Sum(output.JPG.Data)
+				pngSha := sha1.Sum(output.PNG.Data)
+				jpgSha := sha1.Sum(output.JPG.Data)
 
-			output.PNG.ETag = "\"" + hex.EncodeToString(pngSha[:]) + "\""
-			output.JPG.ETag = "\"" + hex.EncodeToString(jpgSha[:]) + "\""
+				output.PNG.ETag = "\"" + hex.EncodeToString(pngSha[:]) + "\""
+				output.JPG.ETag = "\"" + hex.EncodeToString(jpgSha[:]) + "\""
+			}
+			for i, filter := range []func(*image.RGBA){
+				func(img *image.RGBA) {
+					bilinear4(img)
+					gaussian5(img)
+				},
+			} {
+				logger.Info("generating pre-filtered texture", "filter", i+1)
+
+				old := img
+				img := image.NewRGBA(old.Rect)
+				img.Pix = slices.Clone(old.Pix)
+				filter(img)
+
+				var buf bytes.Buffer
+				if err := png.Encode(&buf, img); err != nil {
+					return nil, fmt.Errorf("encode png: %w", err)
+				}
+				output.FilteredPNG[i].Data = buf.Bytes()
+				sha := sha1.Sum(output.FilteredPNG[i].Data)
+				output.FilteredPNG[i].ETag = "\"" + hex.EncodeToString(sha[:]) + "\""
+			}
 
 			return &output, nil
 		}); err != nil {
@@ -287,6 +333,83 @@ loop:
 	}
 }
 
+// gaussian5 does a gaussian blur with a 5x5 kernel.
+func gaussian5(img *image.RGBA) {
+	if img.Stride != 4*img.Rect.Max.X {
+		panic("wtf")
+	}
+	tmp := make([]uint8, len(img.Pix))
+	gaussian5k(tmp, img.Pix, img.Rect.Max.X, img.Rect.Max.Y)
+	gaussian5k(img.Pix, tmp, img.Rect.Max.Y, img.Rect.Max.X)
+}
+
+// gaussian5k does a gaussian blur with a 5x5 kernel along the x-axis on a
+// row-major RGBA image, transposing the result.
+func gaussian5k(out, in []uint8, w, h int) {
+	kernel := [...]uint32{
+		// gaussian blur kernel (radius 2)
+		// note: 65536 = 2^16
+		// note: sum is 65534/65536, so it's close enough (it must not be more, though, or pixels will overflow)
+		uint32(math.Trunc(65536 * 0.06136)),
+		uint32(math.Trunc(65536 * 0.24477)),
+		uint32(math.Trunc(65536 * 0.38774)),
+		uint32(math.Trunc(65536 * 0.24477)),
+		uint32(math.Trunc(65536 * 0.06136)),
+	}
+	for y := 0; y < h; y++ {
+		for x := 0; x < w; x++ {
+			var c [4]uint32
+			for ki, k := range kernel {
+				r := (len(kernel) - 1) / 2
+				x := min(max((ki-r)+x, 0), w-1)
+				ip := in[(y*w+x)*len(c):][:len(c):len(c)]
+				for i := range c {
+					c[i] += (uint32(ip[i]) + 1) * k // +1 so 255 = 255 in the result
+				}
+			}
+			p := out[(x*h+y)*len(c):][:len(c):len(c)] // transposed
+			for i := range c {
+				p[i] = uint8(c[i] >> 16)
+			}
+		}
+	}
+}
+
+// bilinear4 scales img by 0.25 using a bilinear filter (essentially a box
+// filter since 4 is a power of 2).
+func bilinear4(img *image.RGBA) {
+	if img.Stride != 4*img.Rect.Max.X {
+		panic("wtf")
+	}
+	tmp := make([]uint8, len(img.Pix)/4)
+	img.Stride /= 4 // 1/4
+	bilinear4k(tmp, img.Pix, img.Rect.Max.X, img.Rect.Max.Y)
+	img.Rect.Max.X /= 4 // 1/4
+	bilinear4k(img.Pix, tmp, img.Rect.Max.Y, img.Rect.Max.X)
+	img.Rect.Max.Y /= 4 // 1/4
+}
+
+// bilinear4k scales img by 0.25 using a bilinear filter along the x-axis on a
+// row-major RGBA image, transposing the result.
+func bilinear4k(out, in []uint8, w, h int) {
+	for y := 0; y < h; y++ {
+		for x := 0; x < w/4; x++ { // 1/4
+			var c [4]uint32
+			for i := 0; i < 4; i++ { // 1/4
+				x := x*4 + i
+				ip := in[(y*w+x)*len(c):][:len(c):len(c)]
+				for i := range c {
+					c[i] += uint32(ip[i])
+				}
+			}
+			p := out[(x*h+y)*len(c):][:len(c):len(c)] // transposed
+			for i := range c {
+				p[i] = uint8(c[i] >> 2) // 1/2^2 == 1/4
+			}
+		}
+	}
+}
+
 // zeroDef returns def if val is zero, and val otherwise.
 func zeroDef[T comparable](val, def T) T {
 	var zero T

app/build.gradle

@@ -14,7 +14,6 @@ android {
         versionName "5"
 
         buildConfigField "String", "WIND_FIELD_API_HOST", "\"windy.api.pgaskin.net\""
-        buildConfigField "long", "WIND_FIELD_SIZE_ESTIMATED", "1500" // kB
         buildConfigField "long", "WIND_FIELD_UPDATE_INTERVAL", "360" // min
         buildConfigField "long", "WIND_FIELD_UPDATE_INTERVAL_MINIMUM", "15" // min
         buildConfigField "boolean", "SAVE_SCREENSHOTS", "false"

app/src/main/java/net/pgaskin/windy/WindField.java

@@ -9,10 +9,8 @@
 
 import com.badlogic.gdx.graphics.Pixmap;
 import com.badlogic.gdx.graphics.Texture;
-import com.badlogic.gdx.math.MathUtils;
 
 import java.io.File;
-import java.io.FileOutputStream;
 import java.io.InputStream;
 import java.nio.file.Files;
 import java.nio.file.StandardCopyOption;
@@ -64,84 +62,20 @@ private static File windCacheFile(Context context, boolean temp) {
     public static void updateCache(Context context, InputStream src) throws Exception {
         Log.i(TAG, "updating cached field pixmap");
 
-        final BitmapFactory.Options cfg = new BitmapFactory.Options();
-        cfg.inPreferredConfig = Bitmap.Config.ARGB_8888;
-        Bitmap img = BitmapFactory.decodeStream(src, null, cfg);
+        Files.copy(src, windCacheFile(context, true).toPath(), StandardCopyOption.REPLACE_EXISTING);
+
+        final Bitmap img = BitmapFactory.decodeFile(windCacheFile(context, true).toString());
         if (img == null) {
             throw new Exception("Failed to decode input bitmap");
         }
-        if (img.getConfig() != Bitmap.Config.ARGB_8888) {
-            throw new Exception("Input bitmap was not decoded as ARGB8888");
-        }
-
-        img = Bitmap.createScaledBitmap(img, img.getWidth() / 4, img.getHeight() / 4, true);
-        img = blur(img);
-
-        try (final FileOutputStream tmp = new FileOutputStream(windCacheFile(context, true))) {
-            if (!img.compress(Bitmap.CompressFormat.PNG, 100, tmp)) {
-                throw new Exception("Failed to encode scaled bitmap");
-            }
-        }
-        Files.move(windCacheFile(context, true).toPath(), windCacheFile(context, false).toPath(), StandardCopyOption.ATOMIC_MOVE);
-
         synchronized (currentBitmapLock) {
             if (currentBitmap != null) {
                 currentBitmap.recycle();
             }
             currentBitmap = img;
             currentSeq.addAndGet(1);
         }
-    }
-
-    private static Bitmap blur(Bitmap src) {
-        final int w = src.getWidth();
-        final int h = src.getHeight();
-        final int[] a = new int[w*h];
-        final int[] b = new int[w*h];
-        src.getPixels(a, 0, w, 0, 0, w, h); // ARGB8888
-        for (int x = 0; x < w; x++) {
-            for (int y = 0; y < h; y++) {
-                blurKernel(a, b, x, y, w, h, false);
-            }
-        }
-        for (int x = 0; x < w; x++) {
-            for (int y = 0; y < h; y++) {
-                blurKernel(b, a, x, y, w, h, true);
-            }
-        }
-        if (!src.isMutable()) {
-            src = Bitmap.createBitmap(w, h, src.getConfig());
-        }
-        src.setPixels(b, 0, w, 0, 0, w, h);
-        return src;
-    }
 
-    private static void blurKernel(int[] in, int[] out, int x, int y, int w, int h, boolean vertical) {
-        final int[] KERNEL = {
-                // gaussian blur kernel (radius 2)
-                // note: 65536 = 2^16
-                // note: sum is 65534/65536, so it's close enough (it must not be more, though, or pixels will overflow)
-                (int)(65536f * 0.06136f),
-                (int)(65536f * 0.24477f),
-                (int)(65536f * 0.38774f),
-                (int)(65536f * 0.24477f),
-                (int)(65536f * 0.06136f),
-        };
-        int r = 0, g = 0, b = 0;
-        for (int ki = 0, i = (vertical?y:x) - (KERNEL.length-1)/2; ki < KERNEL.length; i++, ki++) {
-            int px = vertical ? x : MathUtils.clamp(i, 0, w-1);
-            int py = vertical ? MathUtils.clamp(i, 0, h-1) : y;
-            int pc = in[py*w + px];
-            int kv = KERNEL[ki];
-            r += kv * (0xFF & (pc >>> 16));
-            g += kv * (0xFF & (pc >>> 8));
-            b += kv * (0xFF & (pc));
-        }
-        int pc = 0;
-        pc |= 0xFF << 24;
-        pc |= ((r >>> 16) << 16);
-        pc |= ((g >>> 16) << 8);
-        pc |= ((b >>> 16));
-        out[y*w + x] = pc;
+        Files.move(windCacheFile(context, true).toPath(), windCacheFile(context, false).toPath(), StandardCopyOption.ATOMIC_MOVE);
     }
 }

app/src/main/java/net/pgaskin/windy/WindFieldUpdateService.java

@@ -42,7 +42,7 @@ public boolean onStartJob(JobParameters params) {
                 }
 
                 final NetworkCapabilities cap = this.getSystemService(ConnectivityManager.class).getNetworkCapabilities(net);
-                final URL url = new URL("https", BuildConfig.WIND_FIELD_API_HOST, "/wind_field.jpg");
+                final URL url = new URL("https", BuildConfig.WIND_FIELD_API_HOST, "/wind_cache.png?filter=1");
                 Log.i(TAG, "updating wind field from " + url + " using network " + net + " with capabilities " + cap);
 
                 if (!cap.hasCapability(NetworkCapabilities.NET_CAPABILITY_NOT_VPN)) {
@@ -144,7 +144,7 @@ private static boolean schedule(Context context, int jobID) {
                     throw new IllegalArgumentException("Unknown jobID");
             }
             builder.setRequiredNetworkType(JobInfo.NETWORK_TYPE_UNMETERED);
-            builder.setEstimatedNetworkBytes(BuildConfig.WIND_FIELD_SIZE_ESTIMATED * 1000, 0);
+            builder.setEstimatedNetworkBytes(256 * 1000, 0);
             builder.setBackoffCriteria(BuildConfig.WIND_FIELD_UPDATE_INTERVAL_MINIMUM * 60 * 1000, JobInfo.BACKOFF_POLICY_EXPONENTIAL);
 
             final JobScheduler scheduler = context.getSystemService(JobScheduler.class);