hud: Modify histogram to use buckets and report p50/p90/p99

This makes the histogram option behave like a histogram instead of
simply rendering the same data as frame_timing without the histogram
option.

This is mostly for reference and personal use as is and does not match
some of the repo's coding standards.

src/hud_elements.cpp

@@ -537,8 +537,31 @@ void HudElements::frame_timing(){
         double min_time = 0.0f;
         double max_time = 50.0f;
         if (HUDElements.params->enabled[OVERLAY_PARAM_ENABLED_histogram]){
-            ImGui::PlotHistogram(hash, get_time_stat, HUDElements.sw_stats,
-                                ARRAY_SIZE(HUDElements.sw_stats->frames_stats), 0,
+			uint32_t p50=0, p90=0, p99=0;
+			// one frame latency between this and update_hud_info_with_frametime.
+			// That makes p99 only kick in after 100 frames.
+			uint32_t frame_count =  std::min(HUDElements.sw_stats->n_frames, ARRAY_SIZE(HUDElements.sw_stats->frames_stats));
+			uint32_t frames_seen = 0;
+			for(uint32_t i = 0; i < ARRAY_SIZE(HUDElements.sw_stats->frame_times); i++) {
+				auto ft = HUDElements.sw_stats->frame_times;
+				frames_seen += ft[i];
+				if(p50 == 0 && frames_seen >= frame_count/2) {
+					p50 = i;
+				}
+				if(p90 == 0 && frames_seen >= frame_count - (frame_count/10)) {
+					p90 = i;
+				}
+				if(p99 == 0 && frames_seen >= frame_count - (frame_count/100)) {
+					p99 = i;
+				}
+			}
+            ImGui::PushFont(HUDElements.sw_stats->font1);
+			ImGui::TextColored(HUDElements.colors.text, "p50/p90/p99");
+			ImGui::TextColored(HUDElements.colors.text, "%3d/%3d/%3d ms", p50, p90, p99);
+			ImGui::TextColored(HUDElements.colors.text, "s:%d,c:%d", frames_seen, frame_count);
+            ImGui::PopFont();
+            ImGui::PlotHistogram(hash, HUDElements.sw_stats->frame_times,
+                                ARRAY_SIZE(HUDElements.sw_stats->frame_times), 0,
                                 NULL, min_time, max_time,
                                 ImVec2(ImGui::GetContentRegionAvailWidth() * HUDElements.params->table_columns, 50));
         } else {

src/overlay.cpp

@@ -170,6 +170,17 @@ void update_hud_info_with_frametime(struct swapchain_stats& sw_stats, struct ove
    if (sw_stats.last_present_time) {
         sw_stats.frames_stats[f_idx].stats[OVERLAY_PLOTS_frame_timing] =
             frametime_ns;
+		uint32_t frametime_ms = frametime_ns/1000000;
+		if (frametime_ms < 1000) {
+			sw_stats.frame_times[frametime_ms] += 1.0;
+		}
+		// keep the histogram for the same window as the normal frame graph.
+		int32_t fp_idx = sw_stats.n_frames - ARRAY_SIZE(sw_stats.frames_stats);
+		if (fp_idx > 0) {
+			fp_idx = (fp_idx + 1) % ARRAY_SIZE(sw_stats.frames_stats);
+			uint32_t oldest_frametime_ms = sw_stats.frames_stats[fp_idx].stats[OVERLAY_PLOTS_frame_timing]/1000000;
+			sw_stats.frame_times[oldest_frametime_ms] -= 1.0;
+		}
    }
 
    frametime = frametime_ns / 1000;

src/overlay.h

@@ -51,6 +51,7 @@ struct swapchain_stats {
    double time_dividor;
    struct frame_stat stats_min, stats_max;
    struct frame_stat frames_stats[200];
+   float frame_times[1000]; // histogram buckets.
 
    ImFont* font1 = nullptr;
    ImFont* font_text = nullptr;
@@ -167,4 +168,4 @@ extern void process_control_socket(struct instance_data *instance_data);
 void render_mpris_metadata(overlay_params& params, mutexed_metadata& meta, uint64_t frame_timing);
 #endif
 
-#endif //MANGOHUD_OVERLAY_H
+#endif //MANGOHUD_OVERLAY_H