1) Pop in your DVD and open KMPlayer (this should be obvious :P). Note: Even though I am talking about DVD's in this tutorial, the screencapping process will also work for AVI's, MKV's, etc.!
2) THIS IS A VERY IMPORTANT STEP. Don't skip this step or you might get the error "Impossible to read at Superspeed mode or there is no KMP transform filter." Sometimes you have to close out of the player and restart the program before these settings take place! Okay, so before I get into the actual how to screencap part, you need to make sure you have the following settings. So, first open up the Preferences window (Right click > Options > Preferences or just press F2).
Then, go to 'Video Processing' on the side menu (which should take you to the 'General' tab) and underneath 'KMP Video Transform Filter', where it says 'Condition', change it to say 'Always Use (Strongly Recommended)'
Now, under the side menu, click 'DVD settings' and go the 'Navigator' tab. Under 'DVD Navigator', there will be a check box with the label 'Disable KMP Video Transform Filter' Make sure this box is checked! Okay, that's it for Preferences settings! :) Now, onto screencapping!
3) Make sure your DVD is playing (click the very top left corner of the player and select 'Open DVD' or Ctrl +D) and right click > Options > Advanced Menu.
This will now show a 'Capture'option in the menu (right click > Capture > Frame: Extract or Ctrl +G). Click and you should get a window that looks like this:
4) Now, this is where you can pretty much use whatever settings you want.
The extraction folder is where all your screencaps are gonna end up, so I'd make a brand new folder designated for screencaps somewhere on your harddrive and select that to be your extract-to folder.
The Prefix is pretty self-explanatory. It's gonna be your files' well, prefix. So, since I have a Veronica Mars DVD playing, I'm gonna make my Prefix "VM-" Now every screencap that's taken will have that prefix and a different 4 (or how ever many) digit number after it.
Image Format: If you want a decent-looking screencap at a fraction of the file size, I'd select JPG as the image format. If you want super quality captures at a higher file size, go with PNG. :)
Image Quality: Adjust as necessary. Note the higher the quality, the higher the file size!
Numbers to Extract: If you want to take multiple screencaptures, make sure the 'Continuously' bubble is selected!
IMPORTANT EDIT: Apparently you might still get the error message if you don't have 'Continuously' bubbled in, so make sure you do! Sorry if the screencap above confuses you!
Frames to Extract: This determines how often a screencapture is taken. I personally like to select the bubble 'In 1sec [blank] frame(s)' and I usually fill in that [blank] with 1, so that 1 screencap is taken every second.
Size to Extract: I stick with capping it at the original size, but if you want it a little smaller you can use a preset size. :) This is what my window looks like now.
5) Click 'Start' and whatever is playing in the window will now be continuously screencapped! You can move the Frame Extraction window out of the way during this if you would also like to watch what you are screencapping. To stop the capture process, just click 'Close' in the Frame Extraction window and confirm that you'd like to stop recording. That's it. You're now done screencapping and can check them out in whatever you chose as your extraction folder! =]
EDIT: If you do these steps and you still get an error message, again, make sure you have tried cloising out of the program and restarting it. Sometimes also unchecking settings and then rechecking them works, too. IDK, it's weird. :P And also make sure that you're not trying to cap the previews or the DVD menu, the actual content has to be playing or paused in the background for the screencapping to work! Or, some people have told me it has worked for them when they don't check the 'Disable KMP Video Transform Filter' box in the preference. Otherwise, if none of this makes it work still, I don't know what to tell you, sorry! =/
I hope this was a helpful tutorial! I'm not the best at explaining things, so if you have any more questions, just leave a comment!