Don't switch to autodetected format if source one supports animation but autodetected one doesn't

docs/configuration.md

@@ -132,6 +132,8 @@ imgproxy can use the `Accept` HTTP header to detect if the browser supports AVIF
 
 **📝Note:** imgproxy prefers AVIF over WebP. This means that if both AVIF and WebP detection/enforcement are enabled and the browser supports both of them, AVIF will be used.
 
+**📝Note:** If both the source and the requested image formats support animation and AVIF detection/enforcement is enabled, AVIF won't be used as AVIF sequence is not supported yet.
+
 **📝Note:** When AVIF/WebP support detection is enabled, please take care to configure your CDN or caching proxy to take the `Accept` HTTP header into account while caching.
 
 **⚠️Warning:** Headers cannot be signed. This means that an attacker can bypass your CDN cache by changing the `Accept` HTTP headers. Have this in mind when configuring your production caching setup.

process.go

@@ -34,6 +34,13 @@ func imageTypeGoodForWeb(imgtype imageType) bool {
 		imgtype != imageTypeBMP
 }
 
+func canSwitchFormat(src, dst, want imageType) bool {
+	return imageTypeSaveSupport(want) &&
+		(!vipsSupportAnimation(src) ||
+			(dst != imageTypeUnknown && !vipsSupportAnimation(dst)) ||
+			vipsSupportAnimation(want))
+}
+
 func extractMeta(img *vipsImage, baseAngle int, useOrientation bool) (int, int, int, bool) {
 	width := img.Width()
 	height := img.Height()
@@ -745,18 +752,18 @@ func processImage(ctx context.Context) ([]byte, context.CancelFunc, error) {
 	switch {
 	case po.Format == imageTypeUnknown:
 		switch {
-		case po.PreferAvif && imageTypeSaveSupport(imageTypeAVIF):
+		case po.PreferAvif && canSwitchFormat(imgdata.Type, imageTypeUnknown, imageTypeAVIF):
 			po.Format = imageTypeAVIF
-		case po.PreferWebP && imageTypeSaveSupport(imageTypeWEBP):
+		case po.PreferWebP && canSwitchFormat(imgdata.Type, imageTypeUnknown, imageTypeWEBP):
 			po.Format = imageTypeWEBP
 		case imageTypeSaveSupport(imgdata.Type) && imageTypeGoodForWeb(imgdata.Type):
 			po.Format = imgdata.Type
 		default:
 			po.Format = imageTypeJPEG
 		}
-	case po.EnforceAvif && imageTypeSaveSupport(imageTypeAVIF):
+	case po.EnforceAvif && canSwitchFormat(imgdata.Type, po.Format, imageTypeAVIF):
 		po.Format = imageTypeAVIF
-	case po.EnforceWebP && imageTypeSaveSupport(imageTypeWEBP):
+	case po.EnforceWebP && canSwitchFormat(imgdata.Type, po.Format, imageTypeWEBP):
 		po.Format = imageTypeWEBP
 	}