git.sesse.net Git - nageru/blob - jpeg_frame_view.cpp

   1 #include "jpeg_frame_view.h"
   2
   3 #include <stdint.h>
   4
   5 #include <atomic>
   6 #include <condition_variable>
   7 #include <deque>
   8 #include <mutex>
   9 #include <thread>
  10 #include <utility>
  11 #include <QGraphicsPixmapItem>
  12 #include <QPixmap>
  13
  14 #include "defs.h"
  15 #include "post_to_main_thread.h"
  16
  17 using namespace std;
  18
  19 string filename_for_frame(unsigned stream_idx, int64_t pts);
  20
  21 struct JPEGID {
  22         unsigned stream_idx;
  23         int64_t pts;
  24 };
  25 bool operator< (const JPEGID &a, const JPEGID &b) {
  26         return make_pair(a.stream_idx, a.pts) < make_pair(b.stream_idx, b.pts);
  27 }
  28
  29 struct LRUPixmap {
  30         shared_ptr<QPixmap> pixmap;
  31         size_t last_used;
  32 };
  33
  34 mutex cache_mu;
  35 map<JPEGID, LRUPixmap> cache;  // Under cache_mu.
  36 condition_variable any_pending_decodes;
  37 deque<pair<JPEGID, JPEGFrameView *>> pending_decodes;  // Under cache_mu.
  38 atomic<size_t> event_counter{0};
  39
  40 void prune_cache()
  41 {
  42         // Assumes cache_mu is held.
  43         vector<size_t> lru_timestamps;
  44         for (const auto &key_and_value : cache) {
  45                 lru_timestamps.push_back(key_and_value.second.last_used);
  46         }
  47
  48         size_t cutoff_point = CACHE_SIZE / 10;  // Prune away the 10% oldest ones.
  49         nth_element(lru_timestamps.begin(), lru_timestamps.begin() + cutoff_point, lru_timestamps.end());
  50         size_t must_be_used_after = lru_timestamps[cutoff_point];
  51         for (auto it = cache.begin(); it != cache.end(); ) {
  52                 if (it->second.last_used < must_be_used_after) {
  53                         it = cache.erase(it);
  54                 } else {
  55                         ++it;
  56                 }
  57         }
  58 }
  59
  60 void jpeg_decoder_thread()
  61 {
  62         size_t num_decoded = 0, num_dropped = 0;
  63
  64         pthread_setname_np(pthread_self(), "JPEGDecoder");
  65         for ( ;; ) {
  66                 JPEGID id;
  67                 JPEGFrameView *dest;
  68                 shared_ptr<QPixmap> pixmap;
  69                 {
  70                         unique_lock<mutex> lock(cache_mu);
  71                         any_pending_decodes.wait(lock, [] {
  72                                 return !pending_decodes.empty();
  73                         });
  74                         id = pending_decodes.front().first;
  75                         dest = pending_decodes.front().second;
  76                         pending_decodes.pop_front();
  77
  78                         auto it = cache.find(id);
  79                         if (it != cache.end()) {
  80                                 pixmap = it->second.pixmap;
  81                                 it->second.last_used = event_counter++;
  82                         }
  83                 }
  84
  85                 if (pixmap == nullptr) {
  86                         // Not found in the cache, so we need to do a decode or drop the request.
  87                         // Prune the queue if there are too many pending for this destination.
  88                         // TODO: Could we get starvation here?
  89                         size_t num_pending = 0;
  90                         for (const pair<JPEGID, JPEGFrameView *> &decode : pending_decodes) {
  91                                 if (decode.second == dest) {
  92                                         ++num_pending;
  93                                 }
  94                         }
  95                         if (num_pending > 3) {
  96                                 ++num_dropped;
  97                                 continue;
  98                         }
  99
 100                         pixmap.reset(
 101                                 new QPixmap(QString::fromStdString(filename_for_frame(id.stream_idx, id.pts))));
 102
 103                         unique_lock<mutex> lock(cache_mu);
 104                         cache[id] = LRUPixmap{ pixmap, event_counter++ };
 105
 106                         if (cache.size() > CACHE_SIZE) {
 107                                 prune_cache();
 108                         }
 109                         ++num_decoded;
 110                         if (num_decoded % 1000 == 0) {
 111                                 fprintf(stderr, "Decoded %zu images, dropped %zu (%.2f%% dropped)\n",
 112                                         num_decoded, num_dropped, (100.0 * num_dropped) / (num_decoded + num_dropped));
 113                         }
 114                 }
 115
 116                 dest->setPixmap(pixmap);
 117         }
 118 }
 119
 120 JPEGFrameView::JPEGFrameView(QWidget *parent)
 121         : QGraphicsView(parent) {
 122         scene.addItem(&item);
 123         setScene(&scene);
 124         setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
 125         setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
 126
 127         static once_flag once;
 128         call_once(once, [] {
 129                 std::thread(&jpeg_decoder_thread).detach();
 130         });
 131 }
 132
 133 void JPEGFrameView::update_frame()
 134 {
 135         unique_lock<mutex> lock(cache_mu);
 136         pending_decodes.emplace_back(JPEGID{ stream_idx, pts }, this);
 137         any_pending_decodes.notify_all();
 138 }
 139
 140 void JPEGFrameView::resizeEvent(QResizeEvent *event)
 141 {
 142         fitInView(&item, Qt::KeepAspectRatio);
 143 }
 144
 145 void JPEGFrameView::setPixmap(std::shared_ptr<QPixmap> pixmap)
 146 {
 147         post_to_main_thread([this, pixmap] {
 148                 item.setPixmap(*pixmap);
 149                 fitInView(&item, Qt::KeepAspectRatio);
 150         });
 151 }