Say why a comparison is showing nothing

A comparison with no B file draws black, and so does a file compared
with itself, a file that could not be read, and two files that never
meet in time. Four issues have been reported for the same empty screen
-- #770, #774, #782, #795 -- each asking for the control to be taken
away, when what was missing was the picture saying what it was.

The viewport says which of the two it is that the file selection can
tell: no B file, or a B that is only the A file again. Both follow from
the selection rather than from frames arriving, so neither can appear
while a seek is still catching up. A file compared with itself alongside
others still has something to show, so it says nothing then.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: darbyjohnston

lib/djv/App/Viewport.cpp

@@ -76,6 +76,14 @@ namespace djv
             std::shared_ptr<ftk::Label> infoLabel;
             std::shared_ptr<ftk::Label> renderLabel;
             std::map<models::HUDItem, std::shared_ptr<ftk::IWidget> > hudWidgets;
+            //! What is being compared, and how many images arrived for it: a
+            //! comparison with no B file draws black, which four different
+            //! things all look like.
+            tl::Compare compare = tl::Compare::A;
+            std::shared_ptr<models::FilesModelItem> a;
+            std::vector<std::shared_ptr<models::FilesModelItem> > b;
+            std::shared_ptr<ftk::Label> compareLabel;
+
             bool toastActive = false;
             std::shared_ptr<ftk::Label> toastLabel;
             std::shared_ptr<ftk::Timer> toastTimer;
@@ -88,6 +96,8 @@ namespace djv
             std::shared_ptr<ftk::Observer<tl::PlayerCacheInfo> > cacheObserver;
             std::shared_ptr<ftk::Observer<double> > fpsObserver;
             std::shared_ptr<ftk::Observer<size_t> > droppedFramesObserver;
+            std::shared_ptr<ftk::Observer<std::shared_ptr<models::FilesModelItem> > > aObserver;
+            std::shared_ptr<ftk::ListObserver<std::shared_ptr<models::FilesModelItem> > > bObserver;
             std::shared_ptr<ftk::Observer<tl::CompareOptions> > compareOptionsObserver;
             std::shared_ptr<ftk::Observer<tl::OCIOOptions> > ocioOptionsObserver;
             std::shared_ptr<ftk::Observer<tl::LUTOptions> > lutOptionsObserver;
@@ -192,6 +202,11 @@ namespace djv
             p.hudLayouts[models::HUDPos::BottomLeft]->setVAlign(ftk::VAlign::Bottom);
             p.hudLayouts[models::HUDPos::BottomRight]->setVAlign(ftk::VAlign::Bottom);
 
+            p.compareLabel = ftk::Label::create(context);
+            p.compareLabel->setMarginRole(ftk::SizeRole::MarginSmall);
+            p.compareLabel->setBackgroundRole(ftk::ColorRole::Overlay);
+            p.compareLabel->setVisible(false);
+
             p.toastLabel = ftk::Label::create(context);
             p.toastLabel->setMarginRole(ftk::SizeRole::MarginSmall);
             p.toastLabel->setBackgroundRole(ftk::ColorRole::Overlay);
@@ -211,6 +226,19 @@ namespace djv
                 context, ftk::Orientation::Vertical, p.hudLayout);
             spacer->setStretch(ftk::Stretch::Expanding);
 
+            auto noBLayout = ftk::HorizontalLayout::create(context, p.hudLayout);
+            noBLayout->setVAlign(ftk::VAlign::Center);
+            spacer = ftk::Spacer::create(
+                context, ftk::Orientation::Horizontal, noBLayout);
+            spacer->setStretch(ftk::Stretch::Expanding);
+            p.compareLabel->setParent(noBLayout);
+            spacer = ftk::Spacer::create(
+                context, ftk::Orientation::Horizontal, noBLayout);
+            spacer->setStretch(ftk::Stretch::Expanding);
+            spacer = ftk::Spacer::create(
+                context, ftk::Orientation::Vertical, p.hudLayout);
+            spacer->setStretch(ftk::Stretch::Expanding);
+
             auto bottomLayout = ftk::VerticalLayout::create(context, p.hudLayout);
             bottomLayout->setSpacingRole(ftk::SizeRole::SpacingSmall);
             auto toastLayout = ftk::HorizontalLayout::create(context, bottomLayout);
@@ -248,11 +276,29 @@ namespace djv
                     _hudUpdate();
                 });
 
+            p.aObserver = ftk::Observer<std::shared_ptr<models::FilesModelItem> >::create(
+                app->getFilesModel()->observeA(),
+                [this](const std::shared_ptr<models::FilesModelItem>& value)
+                {
+                    _p->a = value;
+                    _compareUpdate();
+                });
+
+            p.bObserver = ftk::ListObserver<std::shared_ptr<models::FilesModelItem> >::create(
+                app->getFilesModel()->observeB(),
+                [this](const std::vector<std::shared_ptr<models::FilesModelItem> >& value)
+                {
+                    _p->b = value;
+                    _compareUpdate();
+                });
+
             p.compareOptionsObserver = ftk::Observer<tl::CompareOptions>::create(
                 app->getFilesModel()->observeCompareOptions(),
                 [this](const tl::CompareOptions& value)
                 {
+                    _p->compare = value.compare;
                     setCompareOptions(value);
+                    _compareUpdate();
                 });
 
             p.ocioOptionsObserver = ftk::Observer<tl::OCIOOptions>::create(
@@ -539,6 +585,7 @@ namespace djv
                     {
                         FTK_P();
                         p.videoFramesSize = value.size();
+                        _compareUpdate();
                         p.missing = false;
                         p.heldFrom.reset();
                         // The first source, which is the one the time in the
@@ -709,6 +756,43 @@ namespace djv
             setDisplayOptions(displayOptionsList);
         }
 
+        void Viewport::_compareUpdate()
+        {
+            FTK_P();
+            // Everything but A needs a B file that is not the A file. Neither
+            // draws anything, and an empty picture is the one thing a black
+            // frame, an unreadable file and a comparison out of sync all look
+            // like as well.
+            std::string s;
+            if (p.compare != tl::Compare::A)
+            {
+                if (p.videoFramesSize < 2)
+                {
+                    s = "No B file selected";
+                }
+                else if (p.a && !p.b.empty())
+                {
+                    // Only when there is nothing else in B: a file compared
+                    // with itself alongside others still has something to
+                    // show.
+                    const bool allA = std::all_of(
+                        p.b.begin(),
+                        p.b.end(),
+                        [this](const std::shared_ptr<models::FilesModelItem>& i)
+                        {
+                            return i == _p->a;
+                        });
+                    if (allA)
+                    {
+                        s = "A and B are the same file";
+                    }
+                }
+            }
+            p.compareLabel->setText(s);
+            p.compareLabel->setVisible(!s.empty());
+            ftk::setScreenshotTag(p.compareLabel, !s.empty() ? "View.Compare" : "");
+        }
+
         void Viewport::_hudUpdate()
         {
             FTK_P();

lib/djv/App/Viewport.h

@@ -62,6 +62,7 @@ namespace djv
             ftk::V2I _fromSourcePixel(const ftk::V2I&) const;
             void _videoUpdate();
             void _toastUpdate();
+            void _compareUpdate();
             void _hudUpdate();
             void _hudLayout();