Serve JPEG XL when a request names no format (closes #222)

A /v1/image/ URL whose last segment is a size with no format, such as
800x600 or orig, is served as JPEG XL and signed as jxl, so it shares
the signature of the same URL ending in .jxl. An encrypted URL whose
token holds no format is served as JPEG XL, as encurl.DefaultFormat is
now jxl. The generator page selects JPEG XL by default, and a form
with an empty format, or none, makes a URL whose name ends in .jxl.

The image processor no longer takes an empty format as orig: both
routes give every request a format, so it refuses a request with none
instead of keeping a second default. auto still ends with JPEG.

Model: opus-5-5

README.md

@@ -231,10 +231,11 @@ proxy in front of pixa must pass that header on unchanged. A form body over 1
 MiB is refused with 413. The image routes answer the errors listed for them with
 JSON holding `error`, `status` and `timestamp`.
 
-An image URL has this form:
+An image URL has one of these forms, the second with no format:
 
 ```
 /v1/image/<host>/<path>/<size>.<format>?sig=<signature>&exp=<expiration>&q=<quality>&fit=<fit>
+/v1/image/<host>/<path>/<size>?sig=<signature>&exp=<expiration>&q=<quality>&fit=<fit>
 ```
 
 Images are only fetched from origins using TLS with valid certificates, unless
@@ -245,7 +246,9 @@ A request whose query string cannot be decoded, or gives any parameter more than
 once, is refused with 400.
 
 - `<format>`: one of `orig` (or `original`), `jpeg` (or `jpg`), `png`, `webp`,
-  `avif`, `jxl` (JPEG XL), `gif`, or `auto` (below)
+  `avif`, `jxl` (JPEG XL), `gif`, or `auto` (below). A URL with no format (the
+  second form, with no dot after the size) is served as JPEG XL, the default, as
+  with `jxl`
 - `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)
 - `sig` and `exp`: the signature and its expiry, needed unless the host is
   allowlisted (see Signature Specification)
@@ -321,13 +324,15 @@ nor change what it asks for.
    lasts 30 days, or until `/logout`.
 2. On the generator page, give the source image's URL, the width and height, the
    format, quality and fit, and how long the URL lasts, then submit the form
-   (`POST /generate`). Width and height both empty or `0` keep the original
-   size; if only one of them is empty or `0`, that side is scaled to keep the
-   image's proportions.
+   (`POST /generate`). The format is JPEG XL unless another is chosen; a form
+   sent with an empty format, or none, also makes a JPEG XL URL. Width and
+   height both empty or `0` keep the original size; if only one of them is empty
+   or `0`, that side is scaled to keep the image's proportions.
 3. The page shows the URL, `https://<host>/v1/e/<token>/img.<format>`, and when
    it expires. `<host>` is the host the page was opened on, and the URL starts
    with `http` instead while `debug` is on. The name after the token is ignored
-   and only gives the URL a file extension, `jpg` for `orig` and `auto`.
+   and only gives the URL a file extension, `jpg` for `orig` and `auto`, and
+   `jxl` for a form with no format.
 
 The token holds the source's host, path and query and the size, format, quality,
 fit and expiry, encrypted with a key derived from `signing_key`. The source
@@ -387,8 +392,9 @@ Where:
 - `width` — requested width in pixels, `0` for original
 - `height` — requested height in pixels, `0` for original
 - `format` — output format, one of those listed under Routes, with `original`
-  signed as `orig` and `jpg` as `jpeg`; `auto` is signed as `auto`, not as the
-  format chosen for the request
+  signed as `orig`, `jpg` as `jpeg`, and no format as `jxl`, so a URL with no
+  format has the signature of the same URL ending in `.jxl`; `auto` is signed as
+  `auto`, not as the format chosen for the request
 - `expiration` — the URL's `exp` query parameter, the Unix timestamp when the
   signature expires; a request whose `exp` is not a whole number, an empty
   `exp=` included, is refused with 400

TODO.md

@@ -30,6 +30,15 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-08 JPEG XL is the default output (closes #222): a `/v1/image/` URL
+  whose last segment is a size with no format, such as `800x600` or `orig`, is
+  served as JPEG XL and signed as `jxl`, so it has the signature of the same URL
+  ending in `.jxl`. An encrypted URL whose token holds no format is served as
+  JPEG XL (`encurl.DefaultFormat`). The generator page selects JPEG XL until
+  another format is chosen, and a form with an empty format, or none, makes a
+  JPEG XL URL whose name ends in `.jxl`. The image processor no longer takes an
+  empty format as `orig`: both routes give every request a format, and it
+  refuses a request with none. `auto` still ends with JPEG.
 - 2026-10-08 JPEG XL as an input and output format (part of #222): a source
   whose bytes start with either JPEG XL signature, the bare codestream's `FF 0A`
   or the container's, is detected as `image/jxl`, which the upstream fetch

internal/encurl/encurl.go

@@ -14,7 +14,7 @@ import (
 // Default values for optional fields.
 const (
 	DefaultQuality = 85
-	DefaultFormat  = imgcache.FormatOriginal
+	DefaultFormat  = imgcache.FormatJXL
 	DefaultFitMode = imgcache.FitCover
 
 	// HKDF salt for URL encryption key derivation
@@ -37,7 +37,7 @@ type Payload struct {
 	SourceQuery string               `cbor:"q,omitempty"`  // optional
 	Width       int                  `cbor:"w,omitempty"`  // 0 = original
 	Height      int                  `cbor:"ht,omitempty"` // 0 = original
-	Format      imgcache.ImageFormat `cbor:"f,omitempty"`  // default: orig
+	Format      imgcache.ImageFormat `cbor:"f,omitempty"`  // default: jxl
 	Quality     int                  `cbor:"ql,omitempty"` // default: 85
 	FitMode     imgcache.FitMode     `cbor:"fm,omitempty"` // default: cover
 	ExpiresAt   int64                `cbor:"e,omitempty"`  // 0 = never expires

internal/handlers/auth.go

@@ -369,10 +369,15 @@ func (s *Handlers) buildGeneratedURL(r *http.Request, token, format string) stri
 		scheme = "http"
 	}
 
-	// Determine file extension for the trailing filename
+	// Determine file extension for the trailing filename. A form with no
+	// format makes a token with none, which is served as encurl.DefaultFormat.
 	ext := format
-	if ext == "" || ext == "orig" || ext == "auto" {
-		ext = "jpg" // Default extension
+
+	switch format {
+	case "":
+		ext = string(encurl.DefaultFormat)
+	case "orig", "auto":
+		ext = "jpg"
 	}
 
 	return scheme + "://" + r.Host + "/v1/e/" + url.PathEscape(token) + "/img." + ext

internal/handlers/jpegxl_default_internal_test.go

@@ -0,0 +1,162 @@
+package handlers
+
+import (
+	"fmt"
+	"log/slog"
+	"maps"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/davidbyttow/govips/v2/vips"
+
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/signature"
+)
+
+// requireJPEGXL requires that rec answers 200 with a JPEG XL image.
+func requireJPEGXL(t *testing.T, rec *httptest.ResponseRecorder) {
+	t.Helper()
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d; body %q",
+			rec.Code, http.StatusOK, rec.Body.String())
+	}
+
+	if got := rec.Header().Get("Content-Type"); got != jxlType {
+		t.Errorf("Content-Type = %q, want %s", got, jxlType)
+	}
+
+	if got := vips.DetermineImageType(rec.Body.Bytes()); got != vips.ImageTypeJXL {
+		t.Errorf("body is %s, want jxl", vips.ImageTypes[got])
+	}
+}
+
+// TestImageWithoutFormat_ServesJPEGXL verifies that a /v1/image/ URL whose
+// last segment is a size with no format, 50x50 or orig, answers JPEG XL.
+func TestImageWithoutFormat_ServesJPEGXL(t *testing.T) {
+	t.Parallel()
+
+	route := newImageRoute(t, newPhotoFetcher(t, allowlistedHost))
+
+	for _, size := range []string{"50x50", "orig"} {
+		target := "/v1/image/" + allowlistedHost + photoPath + "/" + size
+		requireJPEGXL(t, sendGet(t, route, target))
+	}
+}
+
+// TestImageWithoutFormat_SignedAsJXL verifies that a /v1/image/ URL with no
+// format is signed as jxl: the signature made for the URL ending in .jxl is
+// accepted for the same URL without .jxl.
+func TestImageWithoutFormat_SignedAsJXL(t *testing.T) {
+	t.Parallel()
+
+	route := newImageRoute(t, newPhotoFetcher(t, signedHost))
+	expires := time.Now().Add(time.Hour)
+
+	sig := signature.New(testSigningKey).Sign(&signature.Request{
+		SourceHost: signedHost,
+		SourcePath: photoPath,
+		Width:      50,
+		Height:     50,
+		Format:     string(imgcache.FormatJXL),
+		Quality:    encurl.DefaultQuality,
+		FitMode:    string(imgcache.FitCover),
+		Expires:    expires,
+	})
+	query := fmt.Sprintf("?sig=%s&exp=%d", sig, expires.Unix())
+
+	for _, size := range []string{"50x50.jxl", "50x50"} {
+		target := "/v1/image/" + signedHost + photoPath + "/" + size + query
+		requireJPEGXL(t, sendGet(t, route, target))
+	}
+}
+
+// TestImageEncWithoutFormat_ServesJPEGXL verifies that an encrypted URL whose
+// token holds no format answers JPEG XL.
+func TestImageEncWithoutFormat_ServesJPEGXL(t *testing.T) {
+	t.Parallel()
+
+	h, srv := newSignedHostServer(t, slog.New(slog.DiscardHandler))
+
+	token, err := h.encGen.Generate(&encurl.Payload{
+		SourceHost: signedHost,
+		SourcePath: photoPath,
+		Width:      50,
+		Height:     50,
+	})
+	if err != nil {
+		t.Fatalf("Generate() error = %v", err)
+	}
+
+	requireJPEGXL(t, getEncToken(srv, token))
+}
+
+// TestGeneratorPage_SelectsJPEGXL verifies that the generator page's format
+// choice is JPEG XL until another is chosen.
+func TestGeneratorPage_SelectsJPEGXL(t *testing.T) {
+	t.Parallel()
+
+	h, srv := newCSRFTestRouter(t)
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", nil)
+	req.AddCookie(newSessionCookie(t, h))
+
+	rec := httptest.NewRecorder()
+	srv.ServeHTTP(rec, req)
+
+	if !strings.Contains(rec.Body.String(), `<option value="jxl" selected>`) {
+		t.Errorf("generator page does not select JPEG XL: %s", rec.Body.String())
+	}
+}
+
+// TestGeneratePost_NoFormat_MakesJPEGXLURL verifies that the generator form
+// sent with an empty format field, or with none, makes a URL whose name ends
+// in .jxl and which answers JPEG XL.
+func TestGeneratePost_NoFormat_MakesJPEGXLURL(t *testing.T) {
+	t.Parallel()
+
+	photo := url.Values{
+		sourceURLField: {"https://" + signedHost + photoPath},
+		widthField:     {"50"},
+		heightField:    {"50"},
+	}
+
+	emptyFormat := maps.Clone(photo)
+	emptyFormat.Set(formatField, "")
+
+	for name, form := range map[string]url.Values{
+		"empty format field": emptyFormat,
+		"no format field":    photo,
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			_, imageSrv := newSignedHostServer(t, slog.New(slog.DiscardHandler))
+
+			rec := generatePost(t, form)
+
+			match := generatedURLPattern.FindStringSubmatch(rec.Body.String())
+			if match == nil {
+				t.Fatalf("generator page shows no URL: %d %s",
+					rec.Code, rec.Body.String())
+			}
+
+			t.Logf("generated URL path: %s", match[1])
+
+			if !strings.HasSuffix(match[1], "/img.jxl") {
+				t.Errorf("generated URL %s does not end in /img.jxl", match[1])
+			}
+
+			imageRec := httptest.NewRecorder()
+			imageSrv.ServeHTTP(imageRec, httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, match[1], nil))
+
+			requireJPEGXL(t, imageRec)
+		})
+	}
+}

internal/imageprocessor/empty_format_internal_test.go

@@ -0,0 +1,21 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+)
+
+// TestImageProcessor_EmptyFormatRefused verifies that a request with no format
+// is refused, as both image routes give every request a format before it is
+// processed.
+func TestImageProcessor_EmptyFormatRefused(t *testing.T) {
+	t.Parallel()
+
+	_, err := New(Params{}).Process(
+		t.Context(), bytes.NewReader(createTestJPEG(t, 20, 20)), &Request{},
+	)
+	if !errors.Is(err, ErrUnsupportedOutputFormat) {
+		t.Errorf("Process() error = %v, want %v", err, ErrUnsupportedOutputFormat)
+	}
+}

internal/imageprocessor/imageprocessor.go

@@ -256,9 +256,9 @@ func (p *ImageProcessor) Process(
 		}
 	}
 
-	// Determine output format
+	// orig is the source's own format; encode refuses an empty format
 	outputFormat := req.Format
-	if outputFormat == FormatOriginal || outputFormat == "" {
+	if outputFormat == FormatOriginal {
 		outputFormat = p.formatFromString(inputFormat)
 	}
 

internal/imgcache/urlparser.go

@@ -38,8 +38,10 @@ func ValidateDimension(name string, value int) error {
 	return nil
 }
 
-// sizeFormatRegex matches patterns like "800x600.webp", "0x0.jpeg", "orig.png"
-var sizeFormatRegex = regexp.MustCompile(`^(\d+)x(\d+)\.(\w+)$|^(orig)\.(\w+)$`)
+// sizeFormatRegex matches patterns like "800x600.webp", "0x0.jpeg", "orig.png",
+// and a size with no format, such as "800x600" or "orig"
+var sizeFormatRegex = regexp.MustCompile(
+	`^(\d+)x(\d+)(?:\.(\w+))?$|^(orig)(?:\.(\w+))?$`)
 
 // ParsedURL contains the parsed components of an image proxy URL.
 type ParsedURL struct {
@@ -56,12 +58,13 @@ type ParsedURL struct {
 }
 
 // ParseImagePath parses the path captured by chi's wildcard:
-// <host>/<path>/<size>.<format>
+// <host>/<path>/<size>.<format>, or <host>/<path>/<size> for JPEG XL
 // This is the primary entry point when using chi routing.
 // Examples:
 //   - cdn.example.com/photos/cat.jpg/800x600.webp
 //   - cdn.example.com/photos/cat.jpg/0x0.jpeg
 //   - cdn.example.com/photos/cat.jpg/orig.png
+//   - cdn.example.com/photos/cat.jpg/800x600
 func ParseImagePath(path string) (*ParsedURL, error) {
 	// Strip leading slash if present (chi may include it)
 	path = strings.TrimPrefix(path, "/")
@@ -212,7 +215,7 @@ func checkPathTraversal(path string) error {
 	return nil
 }
 
-// parseSizeFormat parses strings like "800x600.webp" or "orig.png"
+// parseSizeFormat parses strings like "800x600.webp", "orig.png" or "800x600"
 func parseSizeFormat(s string) (Size, ImageFormat, error) {
 	matches := sizeFormatRegex.FindStringSubmatch(s)
 	if matches == nil {
@@ -254,6 +257,11 @@ func parseSizeFormat(s string) (Size, ImageFormat, error) {
 		return Size{}, "", err
 	}
 
+	// A URL that names no format is served, and signed, as JPEG XL
+	if formatStr == "" {
+		return size, FormatJXL, nil
+	}
+
 	format, err := parseFormat(formatStr)
 	if err != nil {
 		return Size{}, "", err

internal/templates/generator.html

@@ -100,7 +100,7 @@
                         <option value="png" {{if eq .FormFormat "png"}}selected{{end}}>PNG</option>
                         <option value="webp" {{if eq .FormFormat "webp"}}selected{{end}}>WebP</option>
                         <option value="avif" {{if eq .FormFormat "avif"}}selected{{end}}>AVIF</option>
-                        <option value="jxl" {{if eq .FormFormat "jxl"}}selected{{end}}>JPEG XL</option>
+                        <option value="jxl" {{if or (eq .FormFormat "jxl") (eq .FormFormat "")}}selected{{end}}>JPEG XL</option>
                         <option value="gif" {{if eq .FormFormat "gif"}}selected{{end}}>GIF</option>
                     </select>
                 </div>