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         pthread_setname_np(pthread_self(), "JPEGDecoder");
  63         for ( ;; ) {
  64                 JPEGID id;
  65                 JPEGFrameView *dest;
  66                 shared_ptr<QPixmap> pixmap;
  67                 {
  68                         unique_lock<mutex> lock(cache_mu);
  69                         any_pending_decodes.wait(lock, [] {
  70                                 return !pending_decodes.empty();
  71                         });
  72                         id = pending_decodes.front().first;
  73                         dest = pending_decodes.front().second;
  74                         pending_decodes.pop_front();
  75
  76                         auto it = cache.find(id);
  77                         if (it != cache.end()) {
  78                                 pixmap = it->second.pixmap;
  79                                 it->second.last_used = event_counter++;
  80                         }
  81                 }
  82
  83                 if (pixmap == nullptr) {
  84                         // Not found in the cache, so we need to do a decode or drop the request.
  85                         // Prune the queue if there are too many pending for this destination.
  86                         // TODO: Could we get starvation here?
  87                         size_t num_pending = 0;
  88                         for (const pair<JPEGID, JPEGFrameView *> &decode : pending_decodes) {
  89                                 if (decode.second == dest) {
  90                                         ++num_pending;
  91                                 }
  92                         }
  93                         if (num_pending > 3) {
  94                                 continue;
  95                         }
  96
  97                         pixmap.reset(
  98                                 new QPixmap(QString::fromStdString(filename_for_frame(id.stream_idx, id.pts))));
  99
 100                         unique_lock<mutex> lock(cache_mu);
 101                         cache[id] = LRUPixmap{ pixmap, event_counter++ };
 102
 103                         if (cache.size() > CACHE_SIZE) {
 104                                 prune_cache();
 105                         }
 106                 }
 107
 108                 dest->setPixmap(pixmap);
 109         }
 110 }
 111
 112 JPEGFrameView::JPEGFrameView(QWidget *parent)
 113         : QGraphicsView(parent) {
 114         scene.addItem(&item);
 115         setScene(&scene);
 116         setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
 117         setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
 118
 119         static once_flag once;
 120         call_once(once, [] {
 121                 std::thread(&jpeg_decoder_thread).detach();
 122         });
 123 }
 124
 125 void JPEGFrameView::update_frame()
 126 {
 127         unique_lock<mutex> lock(cache_mu);
 128         pending_decodes.emplace_back(JPEGID{ stream_idx, pts }, this);
 129         any_pending_decodes.notify_all();
 130 }
 131
 132 void JPEGFrameView::resizeEvent(QResizeEvent *event)
 133 {
 134         fitInView(&item, Qt::KeepAspectRatio);
 135 }
 136
 137 void JPEGFrameView::setPixmap(std::shared_ptr<QPixmap> pixmap)
 138 {
 139         post_to_main_thread([this, pixmap] {
 140                 item.setPixmap(*pixmap);
 141                 fitInView(&item, Qt::KeepAspectRatio);
 142         });
 143 }