find starting number for image sequences automatically

io/loadthread.cpp

@@ -290,6 +290,8 @@ bool LoadThread::load_worker(QFile& f, QXmlStreamReader& stream, int type) {
                   f->proxy = (attr.value() == "1");
                 } else if (attr.name() == "proxypath") {
                   f->proxy_path = attr.value().toString();
+                } else if (attr.name() == "startnumber") {
+                  f->start_number = attr.value().toInt();
                 }
               }
 

io/previewgenerator.cpp

@@ -524,7 +524,15 @@ void PreviewGenerator::run() {
 
   QString errorStr;
   bool error = false;
-  int errCode = avformat_open_input(&fmt_ctx_, filename, nullptr, nullptr);
+
+  AVDictionary* format_opts = nullptr;
+
+  // for image sequences that don't start at 0, set the index where it does start
+  if (footage_->start_number > 0) {
+    av_dict_set(&format_opts, "start_number", QString::number(footage_->start_number).toUtf8(), 0);
+  }
+
+  int errCode = avformat_open_input(&fmt_ctx_, filename, nullptr, &format_opts);
   if(errCode != 0) {
     char err[1024];
     av_strerror(errCode, err, 1024);

io/proxygenerator.cpp

@@ -47,9 +47,15 @@ void ProxyGenerator::transcode(const ProxyInfo& info) {
   // set progress to 0
   current_progress = 0.0;
 
+  // for image sequences that don't start at 0, set the index where it does start
+  AVDictionary* format_opts = nullptr;
+  if (footage->start_number > 0) {
+    av_dict_set(&format_opts, "start_number", QString::number(footage->start_number).toUtf8(), 0);
+  }
+
   // open input file
   AVFormatContext* input_fmt_ctx = nullptr;
-  avformat_open_input(&input_fmt_ctx, footage->url.toUtf8(), nullptr, nullptr);
+  avformat_open_input(&input_fmt_ctx, footage->url.toUtf8(), nullptr, &format_opts);
 
   // open output file
   AVFormatContext* output_fmt_ctx = nullptr;