Logo detection required a minimum number of pixels in a mask found by

examining several video frames to be greater than 50 to be considered a
valid logo mask. No consideration of the variety of video resolutions was
taken into consideration. And the 50 pixel minimum was not adjusted when
lowres decoding was implemented. The patch, mcf-logo-pixInMask.patch,
assumes the 50 pixel value was established for 1280x720 video and uses that
area to compute a minimum pixel value using this formula: 50 *
(width*height) / (1280*720 / 16). It improves accuracy to 75%.

Signed-off-by: John Poet <jpoet@mythtv.org>

mythtv/programs/mythcommflag/ClassicLogoDetector.cpp

@@ -100,12 +100,23 @@ bool ClassicLogoDetector::searchForLogo(MythPlayer* player)
 
     edgeCounts = new EdgeMaskEntry[width * height];
 
+    // Back in 2005, a threshold of 50 minimum pixelsInMask was established.
+    // I don't know whether that was tested against SD or HD resolutions.
+    // I do know that in 2010, mythcommflag was changed to use ffmpeg's
+    // lowres support, effectively dividing the video area by 16.
+    // But the 50 pixel minimum was not adjusted accordingly.
+    // I believe the minimum threshold should vary with the video's area.
+    // I am using 1280x720 (for 720p) video as the baseline.
+    // This should improve logo detection for SD video.
+    int minPixelsInMask = 50 * (width*height) / (1280*720 / 16);
+
     for (i = 0; edgeDiffs[i] != 0 && !logoInfoAvailable; i++)
     {
         int pixelsInMask = 0;
 
-        LOG(VB_COMMFLAG, LOG_INFO, QString("Trying with edgeDiff == %1")
-                .arg(edgeDiffs[i]));
+        LOG(VB_COMMFLAG, LOG_INFO,
+            QString("Trying with edgeDiff == %1, minPixelsInMask=%2")
+            .arg(edgeDiffs[i]).arg(minPixelsInMask));
 
         memset(edgeCounts, 0, sizeof(EdgeMaskEntry) * width * height);
         memset(edgeMask, 0, sizeof(EdgeMaskEntry) * width * height);
@@ -237,7 +248,7 @@ bool ClassicLogoDetector::searchForLogo(MythPlayer* player)
 #endif
         if (((logoMaxX - logoMinX) < (width / 4)) &&
             ((logoMaxY - logoMinY) < (height / 4)) &&
-            (pixelsInMask > 50))
+            (pixelsInMask > minPixelsInMask))
         {
             logoInfoAvailable = true;
             logoEdgeDiff = edgeDiffs[i];