add webp support (decode only)

if any transformation is requested, webp images will be encoded and
served as jpeg or png, defaulting to jpeg if no format is specified.

Fixes #88

README.md

@@ -4,7 +4,7 @@ imageproxy is a caching image proxy server written in go.  It features:
 
  - basic image adjustments like resizing, cropping, and rotation
  - access control using host whitelists or request signing (HMAC-SHA256)
- - support for jpeg, png, and gif image formats (including animated gifs)
+ - support for jpeg, png, webp (decode only), and gif image formats (including animated gifs)
  - on-disk caching, respecting the cache headers of the original images
  - easy deployment, since it's pure go
 
@@ -197,6 +197,15 @@ However, you can use the `scaleUp` command-line flag to allow this to happen:
 
     imageproxy -scaleUp true
 
+### WebP support ###
+
+Imageproxy can proxy remote webp images, but they will be served in either jpeg
+or png format (this is because the golang webp library only support decoding)
+if any transformation is requested.  If no format is specified, imageproxy will
+use jpeg by default.  If no transformation is requested (for example, if you
+are just using imageproxy as an SSL proxy) then the original webp image will be
+served as-is without any format conversion.
+
 ## Deploying ##
 
 You can build and deploy imageproxy using any standard go toolchain, but here's

imageproxy.go

@@ -308,7 +308,8 @@ func (t *TransformingTransport) RoundTrip(req *http.Request) (*http.Response, er
 	fmt.Fprintf(buf, "%s %s\n", resp.Proto, resp.Status)
 	resp.Header.WriteSubset(buf, map[string]bool{
 		"Content-Length": true,
-		"Content-Type":   opt.Format != "",
+		// exclude Content-Type header if the format may have changed during transformation
+		"Content-Type": opt.Format != "" || resp.Header.Get("Content-Type") == "image/webp",
 	})
 	fmt.Fprintf(buf, "Content-Length: %d\n\n", len(img))
 	buf.Write(img)

transform.go

@@ -23,6 +23,7 @@ import (
 	"image/png"
 
 	"github.com/disintegration/imaging"
+	_ "golang.org/x/image/webp" // register webp format
 	"willnorris.com/go/gifresize"
 )
 
@@ -62,7 +63,7 @@ func Transform(img []byte, opt Options) ([]byte, error) {
 		if err != nil {
 			return nil, err
 		}
-	case "jpeg":
+	case "jpeg", "webp": // default to encoding webp as jpeg
 		quality := opt.Quality
 		if quality == 0 {
 			quality = defaultQuality