8 Replies Latest reply on Sep 25, 2009 9:06 AM by Mylenium

    Motion tracking with cropped footage

    AntonyM6-sPksqC Level 1

      I am familiar with motion tracking in AE CS4.

       

      But this time there is a twist. The footage which I need to track originates as HD, but the comp (and final output) is standard def PAL.

       

      So effectively, the comp always shows a part of the entire frame. i.e. the Scale property is set to 100%.

       

      So how do I do motion tracking when the object to track moves out of the cropped frame?

       

      Do I have to Scale to 50%, do the motion tracking like that, then increase the Scale to 100% again afterwards?

        • 1. Re: Motion tracking with cropped footage
          Mylenium Most Valuable Participant

          I'm not sure what you are getting at. Tracking is a per-layer process, not tied to the comp. So whatever you do, you will be tracking the original HD item, assuming it is not already pre-composed. What you do with the data thereafter is entirely up to you...

           

          Mylenium

          • 2. Re: Motion tracking with cropped footage
            AntonyM6-sPksqC Level 1

            Maybe I was not clear. I will drawn a diagram to help illustrate...

             

            I have a standard def PAL sized comp (720x576px).

            In the comp is footage from a Sony Z1, shot at 1080i (1440x1080, pixel aspect 1.33).

             

            So with the Scale left at 100%, what you can see in the comp is effectively a "centre-crop" of the footage.

            Which is what I want, because this allows me to "zoom in" for tighter framing than the original.

             

            However, I now need to motion track an object.

             

            But because I am "zoomed in", sometimes the object goes out of the "zoomed in" comp view, a;though it is still in the original footage and the motion tracking does not cope with this.

             

            What I was hoping is that the motion tracking would move the footage position in the comp, so that the tracked object stays in the view.

            • 3. Re: Motion tracking with cropped footage
              Mylenium Most Valuable Participant

              Then you would stabilize. I still don't see your problem, though. You can simply use the tracking data and invert it to compensate for any offsets.  The easiest way would be to use an expression:

               

              [value[0]*-1,value[1]*-1]
              

               

              If you want to go all fancy, you could add custom offsets and multipliers, but this should be enough for a start. Simply copy&paste your tracking data to the position, apply the expression. Use the anchor point as an offset to define the exact region that is visible...

               

              Mylenium

              • 4. Re: Motion tracking with cropped footage
                AntonyM6-sPksqC Level 1

                Hi Mylenium,

                 

                I know how to stabilise after I have tracked the motion, That is not the problem.

                 

                The problem is that as soon as the object being tracked moves out of the COMP view (like the dotted line in the GIF), the tracker stops following it.

                Even though the object is still in the actual footage.

                 

                Is that any clearer?

                • 5. Re: Motion tracking with cropped footage
                  Mylenium Most Valuable Participant

                  I think we are not understanding each other. I really don't see your problem - what prevents you from offsetting the layer after the track? I don't see anything in your setup that could not be solved by pre-composing or feeding the result into an expression. So what exactly are you trying to do?

                   

                  Mylenium

                  • 6. Re: Motion tracking with cropped footage
                    AntonyM6-sPksqC Level 1

                    Yes, there is a misunderstanding.

                    You are trying to explain what can be done AFTER the tracking.

                     

                    But the problem I am describing is that AE doesn't seem able to do the track itself, when the point being tracked moves out of the comp view (even though it is still in the footage frame). See the dotted purple line in the GIF a few posts above.

                     

                    So what I am having to do manually is:

                    - track for a few seconds

                    - manually re-centre the footage so that the object is centred in the comp

                    - track for a few more seconds

                    - repeat....

                     

                    OR

                     

                    - Scale layer to 50% so that it is all visible in the comp

                    - track

                    - Increase Scale back to 100%

                     

                    The problem with this appoach is that the accuracy of the tracking is reduced because it is being done on a 50% scaled footage.

                    • 7. Re: Motion tracking with cropped footage
                      Lost In My Mind Level 1

                      Hi Antony,

                      I think what you want to do is called 'offset tracking' which is where you have to make more than one motion track if you don't have a single object that stays in view for the entire event.  For your first track, pick and follow a stationary object until it is almost out of screen, then stop. Pick a new object and a new track motion and do the same as many times as necessary. Create your null objects, link them to the Track Motion data. Then all you have to do is go to the last keyframe in your first track motion (which should overlap the first frame of your second track motion) and parent it with the second, third, etc. and it will give you a continuous set of tracking data for the entire shot. Let us know if this helps. Cin

                      • 8. Re: Motion tracking with cropped footage
                        Mylenium Most Valuable Participant

                        But the problem I am describing is that AE doesn't seem able to do the track itself, when the point being tracked moves out of the comp view (even though it is still in the footage frame).

                         

                        I really do not follow you at all. Yes, of course when you try to directly apply it to another layer in the comp, this is not possible, but otherwise it's a simple matter of using an expression point control. You are simply employing the wrong method to get your tracking data. Any other objects can be made to follow your track using a simple expression:

                         

                        tRef=thisComp.layer("Tracker");
                        tCen=tRef.effect("Circle")("Center");
                        
                        X=tCen[0]+(tRef.position[0]-tRef.anchorPoint[0]);
                        Y=tCen[1]+tRef.position[1]-tRef.anchorPoint[1];
                        
                        [X,Y]
                        

                         

                        Mylenium