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.ft title Sample Session\n
\n
This example demonstrates how
.ft sdout mpeg_bits
might be used to monitor and modify the encoding of an mpeg stream.
\n
\n
Get the mpeg stream entitled \"waterski.mpg\" from the following ftp
site:
\n
.ft sdout mm-ftp.cs.berkeley.edu:/pub/multimedia/mpeg/movies.
\n
Suppose we have just produced this stream from a set of yuv files using
.ft sdout mpeg_encode
with parameter file \"waterski.param\", and we wish to observe and perhaps
modify the results of the encoding using
.ft sdout mpeg_bits.
\n
\n
Using \"Open Video\" from the
.ht .filHelp File
menu, open the stream \"waterski.mpeg\". The
first frame appears on the screen, along with associated information. Hit the
\"step forward\" button on the
.ht .ovrHelp control window
several times; the video steps
forward several frames. Hit the \"play\" button on the
.ht .ovrHelp control window;
the video will play until you hit the stop button. Notice that the
.ht .ovrHelp information window
changes with each frame to reflect the current frame's size and type. Click and
hold the middle mouse over a block in the
.ht .ovrHelp bit field.
Note that the block's
size appears in the upper right of the
.ht .ovrHelp information window.
Click and hold the
middle mouse over any of the frames represented in the
.ht .ovrHelp relative frame sizes field.
Note that the frame's size appears in the upper right of the
.ht .ovrHelp information window.
\n
\n
Use the \"step-back\" button in the
.ht .ovrHelp control window
to step back to the first frame.
\n
\n
Suppose, having viewed the stream, we decide that it needs some sprucing up. In
particular, we would like for the woman running into the water and the cooler
beneath her at the center of the display to be encoded more clearly.
Using the left mouse button,
drop and drag over the region in the
.ht .ovrHelp bit field
corresponding the region where
the woman is in the
.ht .ovrHelp display window.
Or, drop and drag with the left mouse button over the image itself in the
.ht .ovrHelp display window.
The selected region will darken. You may
select more regions at this point with dropping and dragging, or deselect and
start over by clicking the right mouse button.
\n
\n
Now select \"Mark First Frame\" from the
.ht .ediHelp Edit
menu. The Frame Selection dialog
will appear. Use the buttons in the control window to advance to the last
frame to be included in the edit, say frame 6. Stop at frame 6, and click
the \"Mark Last Frame\" button.
\n
\n
The Re-encode Frames dialog will appear. Since we want the woman and cooler
to be clearer, we want to decrease the Q-scale for the blocks that display
their images. Set the Q-scale adjustment to -3, and click \"Confirm Edit\". The
dialog will disappear, the shaded regions will clear up, and the edit is
confirmed.
\n
\n
To view a list of confirmed edits, select \"View Edit List\" from the
.ht .ediHelp Edit menu.
The Edit List window appears, listing edits with the frames affected and the
adjustment to be made. In this case we see that we have one edit affecting
frame 0 to frame 6, adjusting the Q-scale by -3. To see the macroblocks
affected by the change, select the edit with the left mouse button, then click
\"View Macroblocks\". A mini version of the bit field will appear, indicating by
shading which blocks are included in the edit.
\n
\n
We can now generate a parameter file to support the actual encoding of all
confirmed edits. Select \"Save\" from the
.ht .filHelp File
menu. A dialog appears prompting
for a file name. Enter \"waterski.param.spec\" and hit Return.
\n
\n
Once the file is generated, you may exit from
.ft sdout mpeg_bits
by selecting Quit from the
.ht .filHelp File
menu.
\n
\n
To include these edits in subsequent encodings of waterski.mpg, create a line
in waterski.param reading:
\n
.ft sdout SPECIFICS_FILE waterski.param.spec
\n
\n
Now, suppose we re-encode, view the results using
.ft sdout mpeg_play
, and decide
that we'd like to de-emphasize the people in the water around the woman, while
making the woman and the cooler even more clear over a wider range of frames.
\n
\n
Again, we load waterski.mpg from the
.ht .filHelp File
menu (we might want to use the
version we just generated from the re-encoding to see how the re-encoding
affected bit distribution). To de-emphasize the people in the water, we proceed
as described above: position the stream to the first frame in the edit (say 2),
select the macroblocks to change, select \"Mark First Frame\", move the end of
the range of frames we want included in the edit (say 8), select \"Mark
Last Frame\", enter a Q scale adjustment (say 2), and click \"Confirm Edit\".
\n
\n
To re-encode the woman and the cooler, we can re-use the product of the
previous session. Select \"Open Edit List\" from the
.ht .filHelp File
menu, and enter
\"waterski.param.spec\" in the file dialog provided. The edits used to generate
that file will be loaded into mpeg_bits. Select \"View Edit List\" from the
.ht .ediHelp Edit
menu, and note that both the edit you just made and the edit from
the previous session are confirmed.
To change the re-encoding of the woman, select the edit in the Edit List and
click \"Adjust\". The Adjust Edit dialog appears. Change the final frame to
12 and the Q-scale adjustment to -5. Click \"Confirm Adjustment\". The
adjusted edit now appears in the Edit List.
Note that unless we were extremely careful in selecting the macroblocks in
each edit, there are some frames in which we have set a particluar macroblock
to have both a higher Q-scale, because it is near the swimmers we want to
de-emphasize, and also to have a lower Q-scale, because it is near the woman.
In such cases,
.ft sdout mpeg_bits
will treat the macroblock as part of the topmost edit
in the edit list. Here, suppose we decide that we want the re-encoding of the
woman to take priority. To raise the priority of that edit over the other
one, select the bottom edit and click \"Raise Priority\". The edit will
switch places with the one above it.
\n
\n
At this point, we can save again to \"waterski.param.spec\", which will overwrite
the previous version of this file.
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