The preview of .mp3, .avi, .rmvb, .wmv, etc. media files, is available during the downloading process. (However, since version 1.23, the preview program can open any files from task list regardless of the file type granted certain conditions are fulfilled. Read further details on that, here.)
With Bitcomet (version 0.60 or above) you can preview the videos only minutes after downloading has begun, instead of waiting even dozens of hours to finish the download. At present, most of the video files on the Internet have huge sizes, some files going even as big as several Gigabytes. And there are some torrents containing fake files which may waste lots of your time to download.
With this new function, you can check the content by previewing files and deciding whether to continue the download or remove the task and its files from your computer.
There is also another function built in BitComet which once enabled, attempts to make the whole file available for preview, while downloading (see “How to enable preview of the whole file” down below).
When the “Optimize download strategy for preview” option is enabled on the Options-->Task page, BitComet downloads first, up to 10% of the task's pieces (usually 1%-2.5% for a 700MB-1400MB torrent), in the normal “rarest first” mode. After that, it attempts to download a number of sequential pieces from the beginning and the end of the torrent in high-priority mode, in order to enable the preview of the beginning of video and audio files.
Once that is accomplished, it switches back to normal download mode and proceeds further with downloading the rest of the task. Since it only downloads a few adjacent pieces from the very beginning and the end of the torrent, only when they are available, this doesn't bear any impact on the downloading speed for that task.
Right click a task and select “Preview”. If there is a single video file in the torrent, BitComet will automatically pass it to the default assigned player for BitComet which will attempt to play it. If the task contains multiple video files then a dialog box will appear, containing all the video files detected for that task; tick the checkbox before the file you want and click on “Preview” to play.
Alternatively you can click on the “Preview” button in the Preview column of the Task List or in the Files tab of the Detailed Info Pane.
However, this will only allow you to preview, roughly, the first minute of the file (since it only downloads a few adjacent pieces at the beginning and the end of the file).
Starting with BitComet v.1.23 a new advanced option was introduced (ui.preview_any_file), which, when enabled, will allow the user to “play” any type of file from inside the Files tab of the Detailed Info Pane, not just video/audio files. The “Play File…” command, which is accessible from the right-click context menu of any selected file in the Files tab will be enabled for any file type, once this advanced option is enabled and it will pass the selected file to the player assigned for BitComet (in Advanced Options–> ui.preview_program_path), which will attempt to play the file.
If your designated player is one capable of playing video content from inside RAR archives (see the end of this subsection for details on that), then you will be able to play the video file without having to extract the contents of the archive first.
This option is also really useful for previewing the video contained inside multi-volume RAR archives (i.e. for checking if the torrent is a fake or a valid one) shortly after starting the download.
In order to accomplish that, you will have to select the Files tab and in there you will have to select the first and last volumes of the archive and set the Highest download priority for them (so that they are downloaded first).
Once they're 100% downloaded, you will be able to preview the content, just as you would preview any normal video file. This should be enough to let you tell if the content is what you seek or its garbage.
Note: For multi-volume RAR archives you may encounter 2 cases:
Attention: For many of the players capable of playing video from inside RAR archives, the “Play File…” command has to be issued on the first volume of the archive (if it's a multi-volume RAR archive). That would be the file with the .part001.rar extension (for RAR 3.0 type archives) or the file with the .rar extension, for RAR archives of the older type.
For more detailed info on playing video from inside RAR archives and on players capable of doing that, read this guide.
In order to enable the preview of the whole file while downloading, you will have to also enable Preview Download Mode for that task. You can do that by right-clicking the task in the tasklist and putting a check-mark before that option in the context menu.
This option, when enabled, makes BitComet attempt to download all the pieces for that task, from the beginning towards the end, in a sequential order (it executes first the actions implied by “Optimize download strategy for preview”, as explained above, and then attempts to download the rest of the remaining pieces in a sequential order).
This should let you play the media file while it's being downloaded; that is, if the downloading speed and piece availability allows BitComet to stay ahead of the playing bitrate.
However, be advised that this will have, most probably, a negative impact on the downloading speed for that task. This option needs “Optimize download strategy for preview” to be enabled, in order to work.