filter: improve none context

Author: wang-bin

src/QtAV/EncodeFilter.h

@@ -76,7 +76,7 @@ class Q_AV_EXPORT VideoEncodeFilter : public VideoFilter
     DPTR_DECLARE_PRIVATE(VideoEncodeFilter)
 public:
     VideoEncodeFilter(QObject* parent = 0);
-    bool isSupported(VideoFilterContext::Type) const Q_DECL_OVERRIDE { return true;}
+    bool isSupported(VideoFilterContext::Type t) const Q_DECL_OVERRIDE { return t == VideoFilterContext::None;}
     /*!
      * \brief createEncoder
      * Destroy old encoder and create a new one. Filter has the ownership.

src/QtAV/LibAVFilter.h

@@ -75,7 +75,7 @@ class Q_AV_EXPORT LibAVFilterVideo : public VideoFilter, public LibAVFilter
     Q_PROPERTY(QStringList filters READ filters)
 public:
     LibAVFilterVideo(QObject *parent = 0);
-    bool isSupported(VideoFilterContext::Type) const Q_DECL_OVERRIDE { return true;}
+    bool isSupported(VideoFilterContext::Type t) const Q_DECL_OVERRIDE { return t == VideoFilterContext::None;}
     QStringList filters() const; //the same as LibAVFilter::videoFilters
 Q_SIGNALS:
     void optionsChanged();

src/filter/Filter.cpp

@@ -145,7 +145,7 @@ bool VideoFilter::prepareContext(VideoFilterContext *&ctx, Statistics *statistic
     DPTR_D(VideoFilter);
     if (!ctx || !isSupported(ctx->type())) {
         //qDebug("no context: %p, or context type %d is not supported", ctx, ctx? ctx->type() : 0);
-        return false;
+        return isSupported(VideoFilterContext::None);
     }
     if (!d.context || d.context->type() != ctx->type()) {
         VideoFilterContext* c = VideoFilterContext::create(ctx->type());//each filter has it's own context instance, but share the common parameters