vcomp/lavc: added defaults for OpenH264

Acually there is quite a few that can be set:
1. default compression (witout RC) works quite good
2. bitrate as well (if the bitrate isn't too low)
3. quality controlled RC doesn't seem to work good

Slice considerations:
- 32 (default) triggers a warning for FullHD and OpenH264 will use
16 -> this is OK
- slices=0 would disable multithreading
- setting max_nal_size (with slices=0) would help as well, but it is
not clear what is the "good" value

Not yet added to CI (libopenh264 is included in Ubuntu 22.04, now using
20.04 for the build).

src/video_compress/libavcodec.cpp

@@ -42,6 +42,7 @@
 #include <cinttypes>
 #include <cmath>
 #include <cstdint>
+#include <cstring>                        // for strcmp, strlen, strstr, strchr
 #include <list>
 #include <map>
 #include <regex>
@@ -808,9 +809,11 @@ static void set_cqp(struct AVCodecContext *codec_ctx, int requested_cqp) {
         }
         codec_ctx->flags |= AV_CODEC_FLAG_QSCALE;
 
-        if (strcmp(codec_ctx->codec->name, "mjpeg") == 0) {
+        if (strcmp(codec_ctx->codec->name, "mjpeg") == 0 ||
+            strcmp(codec_ctx->codec->name, "libopenh264") == 0) {
                 codec_ctx->qmin = codec_ctx->qmax = cqp;
-                LOG(LOG_LEVEL_INFO) << MOD_NAME "Setting mjpeg qmin/qmax to " << cqp <<  "\n";
+                MSG(INFO, "Setting %s qmin/qmax to %d\n",
+                    codec_ctx->codec->name, cqp);
         } else if (strstr(codec_ctx->codec->name, "_qsv") != nullptr) {
                 codec_ctx->global_quality = cqp;
                 LOG(LOG_LEVEL_INFO) << MOD_NAME "Setting QSV global_quality to " << cqp <<  "\n";
@@ -855,7 +858,7 @@ bool set_codec_ctx_params(struct state_video_compress_libav *s, AVPixelFormat pi
                 if (check_av_opt_set<double>(s->codec_ctx->priv_data, "crf", crf)) {
                         log_msg(LOG_LEVEL_INFO, "[lavc] Setting CRF to %.2f.\n", crf);
                 }
-        } else {
+        } else if (strcmp(s->codec_ctx->codec->name, "libopenh264") != 0) {
                 set_bitrate = true;
         }
         if (set_bitrate || s->params.requested_bitrate > 0) {
@@ -1957,6 +1960,26 @@ static void configure_svt(AVCodecContext *codec_ctx, struct setparam_param *para
         }
 }
 
+void
+configure_libopenh264(AVCodecContext *codec_ctx,
+                      struct setparam_param *param)
+{
+        if (param->requested_bitrate != 0 || param->requested_bpp != 0) {
+                check_av_opt_set(codec_ctx->priv_data, "rc_mode", "bitrate");
+        } else if (param->requested_cqp != -1) {
+                MSG(WARNING, "Using quality RC with OpenH264 is currently not recoomended "
+                             "- seems like it performs worse than bitrate or none.\n");
+                check_av_opt_set(codec_ctx->priv_data, "rc_mode", "quality");
+        } else { // FFmpeg sets quality by default
+                check_av_opt_set(codec_ctx->priv_data, "rc_mode", "off");
+                return;
+        }
+        MSG(WARNING, "Setting frame skipping for OpenH264 when bitrate/quality "
+                     "is set. Do not set if not desired.\n");
+        check_av_opt_set(codec_ctx->priv_data, "allow_skip_frames", 1);
+        // slices - must be either >1 to enable multithreading (32 default now)
+}
+
 static void setparam_h264_h265_av1(AVCodecContext *codec_ctx, struct setparam_param *param)
 {
         if (regex_match(codec_ctx->codec->name, regex(".*_amf"))) {
@@ -1985,6 +2008,8 @@ static void setparam_h264_h265_av1(AVCodecContext *codec_ctx, struct setparam_pa
                 configure_rav1e(codec_ctx, param);
         } else if (strstr(codec_ctx->codec->name, "libsvt") == codec_ctx->codec->name) {
                 configure_svt(codec_ctx, param);
+        } else if (strcmp(codec_ctx->codec->name, "libopenh264") == 0) {
+                configure_libopenh264(codec_ctx, param);
         } else {
                 log_msg(LOG_LEVEL_WARNING, "[lavc] Warning: Unknown encoder %s. Using default configuration values.\n", codec_ctx->codec->name);
         }