[KPhotoAlbum] speeding up video thumbnail generation

[Andreas Schleth]

Hi,

I was trying to use KPA for managing a collection of longish video files 
and found thumbnail generation extremely slow (with ffmpeg).

However, there is an easy fix: move the -ss parameter in front of the -i 
parameter:

change Lines 69ff in ImageManager/ExtractOneVideoFrame.cpp to:

         arguments << STR("-ss") << QString::number(offset,'f',4) << 
STR("-i") << fileName.absolute()
                   << STR("-sn") << STR("-an")
                   << STR("-vframes") << STR("20")
                   << m_workingDirectory + STR("/000000%02d.png");

Even for videos of 2h runtime this is speedy.

The difference between the -ss up front and the -ss after -i is:

With the option up front ffmpeg searches the file to the approximate 
position and then starts decoding. Searching (jumping to a position) a 
file is fast.

With the option in the back the file gets decoded up to this position 
and then the thumbs are generated. Thus, the whole file would be decoded 
5 times for 10 thumbnail frames.

This change would make the positions less precise, but in this case 
precsion would be overrated.

Best regards, Andreas

PS: this is the output of git diff (is this of any use?):

diff --git a/ImageManager/ExtractOneVideoFrame.cpp 
b/ImageManager/ExtractOneVideoFrame.cpp
index 3ce0d78c..a5a4242e 100644
--- a/ImageManager/ExtractOneVideoFrame.cpp
+++ b/ImageManager/ExtractOneVideoFrame.cpp
@@ -66,8 +66,8 @@ ExtractOneVideoFrame::ExtractOneVideoFrame(const 
DB::FileName &fileName, double
m_process->start(MainWindow::FeatureDialog::mplayerBinary(), arguments);
      } else {
          QStringList arguments;
-        arguments << STR("-i") << fileName.absolute()
-                  << STR("-sn") << STR("-an") << STR("-ss") << 
QString::number(offset,'f',4)
+        arguments << STR("-ss") << QString::number(offset,'f',4) << 
STR("-i") << fileName.absolute()
+                  << STR("-sn") << STR("-an")
                    << STR("-vframes") << STR("20")
                    << m_workingDirectory + STR("/000000%02d.png");
          //qDebug( "%s %s", 
qPrintable(MainWindow::FeatureDialog::ffmpegBinary()), 
qPrintable(arguments.join(QString::fromLatin1(" "))));