diff --git a/gitweb/gitweb.perl b/gitweb/gitweb.perl
index f8021da967..3d62019e1f 100755
--- a/gitweb/gitweb.perl
+++ b/gitweb/gitweb.perl
@@ -549,7 +549,12 @@ sub evaluate_path_info {
 		'history',
 	);
 
-	my ($refname, $pathname) = split(/:/, $path_info, 2);
+	# we want to catch
+	# [$hash_parent_base[:$file_parent]..]$hash_parent[:$file_name]
+	my ($parentrefname, $parentpathname, $refname, $pathname) =
+		($path_info =~ /^(?:(.+?)(?::(.+))?\.\.)?(.+?)(?::(.+))?$/);
+
+	# first, analyze the 'current' part
 	if (defined $pathname) {
 		# we got "branch:filename" or "branch:dir/"
 		# we could use git_get_type(branch:pathname), but:
@@ -564,7 +569,13 @@ sub evaluate_path_info {
 			$input_params{'action'} ||= "tree";
 			$pathname =~ s,/$,,;
 		} else {
-			$input_params{'action'} ||= "blob_plain";
+			# the default action depends on whether we had parent info
+			# or not
+			if ($parentrefname) {
+				$input_params{'action'} ||= "blobdiff_plain";
+			} else {
+				$input_params{'action'} ||= "blob_plain";
+			}
 		}
 		$input_params{'hash_base'} ||= $refname;
 		$input_params{'file_name'} ||= $pathname;
@@ -584,6 +595,27 @@ sub evaluate_path_info {
 			$input_params{'hash'} ||= $refname;
 		}
 	}
+
+	# next, handle the 'parent' part, if present
+	if (defined $parentrefname) {
+		# a missing pathspec defaults to the 'current' filename, allowing e.g.
+		# someproject/blobdiff/oldrev..newrev:/filename
+		if ($parentpathname) {
+			$parentpathname =~ s,^/+,,;
+			$parentpathname =~ s,/$,,;
+			$input_params{'file_parent'} ||= $parentpathname;
+		} else {
+			$input_params{'file_parent'} ||= $input_params{'file_name'};
+		}
+		# we assume that hash_parent_base is wanted if a path was specified,
+		# or if the action wants hash_base instead of hash
+		if (defined $input_params{'file_parent'} ||
+			grep { $_ eq $input_params{'action'} } @wants_base) {
+			$input_params{'hash_parent_base'} ||= $parentrefname;
+		} else {
+			$input_params{'hash_parent'} ||= $parentrefname;
+		}
+	}
 }
 evaluate_path_info();