+ auto mapped = film()->mapped_audio_channels();
+ std::vector<dcp::Channel> extra_active_channels;
+ auto add_if_mapped = [mapped, &extra_active_channels](dcp::Channel channel) {
+ if (std::find(mapped.begin(), mapped.end(), static_cast<int>(channel)) != mapped.end()) {
+ extra_active_channels.push_back(channel);
+ }
+ };
+
+ add_if_mapped(dcp::Channel::HI);
+ add_if_mapped(dcp::Channel::VI);
+ add_if_mapped(dcp::Channel::BSL);
+ add_if_mapped(dcp::Channel::BSR);
+