Add API functions to set and retrieve frame times

Setting a custom frame time will result in stable animation speeds if projectM does not render at real-time speeds, e.g. while encoding a video from an audio file as fast as possible.

src/api/include/projectM-4/parameters.h

@@ -45,6 +45,38 @@ PROJECTM_EXPORT void projectm_set_texture_search_paths(projectm_handle instance,
                                                        const char** texture_search_paths,
                                                        size_t count);
 
+/**
+ * @brief Sets a user-specified frame time in fractional seconds.
+ *
+ * Setting this to any value equal to or larger than zero will make projectM use this time value for
+ * animations instead of the system clock. Any value less than zero will use the system time instead,
+ * which is the default behavior.
+ *
+ * This method can be used to render visualizations at non-realtime frame rates, e.g. encoding a video
+ * as fast as projectM can render frames.
+ *
+ * While switching back and forth between system and user time values is possible, it will cause
+ * visual artifacts in the rendering as the time value will make large jumps between frames. Thus,
+ * it is recommended to stay with one type of timing value.
+ *
+ * If using this feature, it is further recommended to set the time to 0.0 on the first frame.
+ *
+ * @param instance The projectM instance handle.
+ * @param seconds_since_first_frame Any value >= 0 to use user-specified timestamps, values < 0 will use the system clock.
+ */
+PROJECTM_EXPORT void projectm_set_frame_time(projectm_handle instance, double seconds_since_first_frame);
+
+/**
+ * @brief Returns the fractional seconds time value used rendering the last frame.
+ * @note This will not return the value set with projectm_set_frame_time, but the actual time
+ *       used to render the last frame. If a user-specified frame time was set, this value is
+ *       returned. Otherwise, the frame time measured via the system clock will be returned.
+ * @param instance The projectM instance handle.
+ * @return Time elapsed since projectM was started, or the value of the user-specified time value used
+ *         to render the last frame.
+ */
+PROJECTM_EXPORT double projectm_get_last_frame_time(projectm_handle instance);
+
 /**
  * @brief Sets the beat sensitivity.
  *

src/libprojectM/ProjectM.cpp

@@ -281,6 +281,16 @@ auto ProjectM::PresetLocked() const -> bool
     return m_presetLocked;
 }
 
+void ProjectM::SetFrameTime(double secondsSinceStart)
+{
+    m_timeKeeper->SetFrameTime(secondsSinceStart);
+}
+
+double ProjectM::GetFrameTime()
+{
+    return m_timeKeeper->GetFrameTime();
+}
+
 void ProjectM::SetBeatSensitivity(float sensitivity)
 {
     m_beatSensitivity = std::min(std::max(0.0f, sensitivity), 2.0f);

src/libprojectM/ProjectM.hpp

@@ -103,6 +103,22 @@ class PROJECTM_EXPORT ProjectM
 
     void RenderFrame(uint32_t targetFramebufferObject = 0);
 
+    /**
+     * @brief Sets a user-specified time for rendering the next frame
+     * Negative values will make projectM use the system clock instead.
+     * @param secondsSinceStart Fractional seconds since rendering the first frame.
+     */
+    void SetFrameTime(double secondsSinceStart);
+
+    /**
+     * @brief Gets the time of the last frame rendered.
+     * @note This will not return the value set with SetFrameTime, but the actual time used to render the last frame.
+     *       If a user-specified frame time was set, this value is returned. Otherwise, the frame time measured via the
+     *       system clock will be returned.
+     * @return Seconds elapsed rendering the last frame since starting projectM.
+     */
+    double GetFrameTime();
+
     void SetBeatSensitivity(float sensitivity);
 
     auto GetBeatSensitivity() const -> float;

src/libprojectM/ProjectMCWrapper.cpp

@@ -7,6 +7,7 @@
 #include <cstring>
 #include <sstream>
 #include <projectM-4/render_opengl.h>
+#include <projectM-4/parameters.h>
 
 
 namespace libprojectM {
@@ -179,6 +180,18 @@ void projectm_opengl_render_frame_fbo(projectm_handle instance, uint32_t framebu
     projectMInstance->RenderFrame(framebuffer_object_id);
 }
 
+void projectm_set_frame_time(projectm_handle instance, double seconds_since_first_frame)
+{
+    auto projectMInstance = handle_to_instance(instance);
+    projectMInstance->SetFrameTime(seconds_since_first_frame);
+}
+
+double projectm_get_last_frame_time(projectm_handle instance)
+{
+    auto projectMInstance = handle_to_instance(instance);
+    return projectMInstance->GetFrameTime();
+}
+
 void projectm_set_beat_sensitivity(projectm_handle instance, float sensitivity)
 {
     auto projectMInstance = handle_to_instance(instance);

src/libprojectM/TimeKeeper.cpp

@@ -14,11 +14,26 @@ TimeKeeper::TimeKeeper(double presetDuration, double smoothDuration, double hard
     UpdateTimers();
 }
 
+void TimeKeeper::SetFrameTime(double secondsSinceStart)
+{
+    m_userSpecifiedTime = secondsSinceStart;
+}
+
+double TimeKeeper::GetFrameTime() const
+{
+    return m_currentTime;
+}
+
 void TimeKeeper::UpdateTimers()
 {
-    auto currentTime = std::chrono::high_resolution_clock::now();
+    double currentFrameTime{m_userSpecifiedTime};
+
+    if (m_userSpecifiedTime < 0.0)
+    {
+        auto currentTime = std::chrono::high_resolution_clock::now();
+        currentFrameTime = std::chrono::duration<double>(currentTime - m_startTime).count();
+    }
 
-    double currentFrameTime = std::chrono::duration<double>(currentTime - m_startTime).count();
     m_secondsSinceLastFrame = currentFrameTime - m_currentTime;
     m_currentTime = currentFrameTime;
     m_presetFrameA++;

src/libprojectM/TimeKeeper.hpp

@@ -11,6 +11,25 @@ class TimeKeeper
 public:
     TimeKeeper(double presetDuration, double smoothDuration, double hardcutDuration, double easterEgg);
 
+    /**
+     * @brief Sets a custom time value to use instead of the system time.
+     * If less than zero, the system time will be used instead.
+     * @param secondsSinceStart Fractional seconds since rendering the first frame.
+     */
+    void SetFrameTime(double secondsSinceStart);
+
+    /**
+     * @brief Gets the time of the last frame rendered.
+     * @note This will not return the value set with SetFrameTime, but the actual time used to render the last frame.
+     *       If a user-specified frame time was set, this value is returned. Otherwise, the frame time measured via the
+     *       system clock will be returned.
+     * @return Seconds elapsed rendering the last frame since starting projectM.
+     */
+    double GetFrameTime() const;
+
+    /**
+     * @brief Updates internal timers with either the system clock or a user-specified time value.
+     */
     void UpdateTimers();
 
     void StartPreset();
@@ -93,6 +112,8 @@ class TimeKeeper
     std::random_device m_randomDevice{};
     std::mt19937 m_randomGenerator{m_randomDevice()};
 
+    double m_userSpecifiedTime{-1.0}; //!< User-specifed run time. If set to a value >= 0.0, this time is used instead of the system clock.
+
     double m_secondsSinceLastFrame{};
 
     double m_easterEgg{};