CG Jam

Blender tracking- single point track

Need to stick something to a wall? don’t waste time tracking the entire scene, just track what you need!

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Single point tracking in Blender.

Single point tracking is a 2D track method. we are simply analysing the location of a pixel in our video clip and transfering that data to the 3D scene. we did a very similar thing in our whats it for video about the track position node.  but in that case we were transfering the data directly to the compositor. In this case we will be using 3d objects instead of  images.

so lets first open up our motion tracking workspace. and most of these windows can be ignored for this lesson

We dont need to worry about the dope sheet or the graph editor, i will cover these features in another video.

so first we need something to track, so open your clip. now the secret of one point tracking is that you dont want any depth or rotation movement in your clip so no forward or backwards dollying and no crazy camera spinnig tecniques! a simple panning motion or a fixed tripod pan is great or you can place the marker on a moving object like a persons hand as the talk to the camera, otherwise things go wrong as you can see in this clip

as you can see  even though the pointer and the text is pointed at the seagull, because the bird is moving around into the distance and back, the words and the pointer  get misaligned. had the bird been just walking back and forth across the screen this problem wouldn’t occur.

Setting up.

ok before we start our track we need to do a couple of things first. We need to first set the scene frames to match the length of the clip and cache our clip into the RAM of our PC which we can do easily by pressing the set scene frames and the prefetch buttons in our tool bar.

Then we just need to adjust the timeline to incorporate just the frames we want to track, in this example I want to start at frame 342 so i move the timeline marker to that point and press the S on my keyboard to trim the clip.

Now we can get on with the tracking. before we add our first track we need to look at the tracking settings

the presets will automatically adjust the settings appropriately to the type of footage you are using. whether that’s fast moving, where your track point moves a great distance between frames, blurry footage, whether that’s motion blur or a defocused clip. but for today we are going to leave it at default.

As the tracker looks at contrast sometimes it is useful to isolate a colour channel to get the best possible contrast, which is what the RGB  buttons are for.

the pattern and search boxes define the size of the tracker in pixels and the size of the area that the tracker will look for the correct image within your footage.

the motion menu allows you to define what kind of keyframes you are looking for in your clip  whether that’s location, rotation, scale or a combination of them. there is also Affine and perspective options which are more advanced track type and we will cover them separately.

The match options allow you to choose between referencing the pattern image from the last keyframe or the previous frame.

Prepass kind of forces blender to recognise the track point, somehow and normalise makes allowances for light changes in the footage.

and we’ll cover the extra settings in another video, dont worry about them today!

So on to it, press the add track button and then click on a spot in your clip that has a nice balance of contrast. then scale up the tracker to encompass the whole of the point you want to can see the pattern you are tracking in the track window on the right of the screen

you dont want it to be on something that has a lot of edges because that can cause the track to slide around in your clip. try to find isolated points that can hold their pattern. in my example i’m tracking a small hole in the wall. if you don’t have a point to track in the area that you want to place your object find a point on the same plane that you want to track. if you use a point in a different plane, for example in my scene if i want to track the wall i need to not choose anything on the shelf to track as they are forward in space and will alter the perspective of the tracking point giving us  bad results as seen in the duck video.

ideally it is best to plan your shot first and place something on the wall that could be tracked like a piece of blu tack.

Now we can track. just press the play forward button in the track tools to start the tracking process.

there may come a point where your track hits the edge of the screen. if this happens before the end of your clip the tracker will stop and there will be no data after this point so any thing connected to this point will simply stop and look bad on your finished clip, so wee need to make the tracker carry on after the track point has left the screen. we can do this by selecting the track and double tapping G and moving the track to anothe part of the scene. again the new point must be on the same level as the original track or you will get perspective errors. this is called an offset track.

we can now continue the track and as you can see the point tracks off the screen for those last few frames.

ok thats it, the track is done and we can move on.

all we need to do now is  open the reconstruction options and choose link empty to track


now we can go back to our 3D view and start using the info.

Setting up the scene.

once into the 3D view enter camera view and set your film clip as the background image.  then orient the camera so it is facing the from view and press alt-A and you should see your empty following the marker that you just tracked. you#ll notice if you look at the grid floor that the image in the camera is not aligned to the grid properly. simply rotate the camera slightly to align the grid floor to the floor of your clip. you could use a plane object as a helper item for this.

now all we have to do is parent our object to the empty and we should be done! ( you can delete the plane now if you want  Or use it as a shadow catcher, whatever you need.)

its important that the thing you are parenting to the empty is in the same location as the empty.  so select the empty and  press shift s and choose cursor to selected from the menu. this will ensure any object you now add will be placed in the exact spot as the empty. Select your object, then  Select the empty and with ctrl P, set parent to object.

now your object will follow the path of the empty

now all you need to do is composite the layers together and render it out! check out our render layers tutorial for more about that.




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