Track Without Scale?


#1

When applying my tracking data to an object, the scale data of the object changes for some reason. The original object tracked never changes scale, but I don’t seem to have the option of tracking without scale. The object is obscured some at times. Not sure if that’s the issue.

What can I do?

Thanks.


#2

You can track translation only if the object isn’t changing in scale.


#3

I need translation and rotation in 2D. Those 2 don’t seem to be available without scale, which gets selected automatically.


#4

You could use Mocha’s Dope Sheet to delete the Scale key frames it is quite easy to delete the Scale key-frames inside a host like After Effects after exporting them.

But - if your object is not scaling, but the Mocha track is, then I would say there is something off about the shape /search area. If the obscured area is getting into your spline, the thing to to is set a 2nd layer above your track layer that masks this obstructing object.

Hope this helps.


#5

Will try that. Thanks. So does the size of the surface (the square outline) represent a scale, and if that surface changes size, the scale is changed? Just wondering the source of the scale. It is not the spline, right? That only dictates where to track from?

Just got rid of the scale key frames. Had only done it in AE null track where I pasted the tracking data and didn’t work, but did as you said and that makes it work. Thanks.


#6

Yes - the best way to evaluate your track is the Surface. The spline layer represents the search area and is linked to the Surface. I suspect that is some other motion throwing your track off, causing scale. There are a lot of useful tips to use the multiple spline layers (for objects that get obscured) OR using multiple shapes on one spline layer (Using the X+ tool).

This is a great tutorial that Mary created to visually show some of these techniques:
https://borisfx.com/videos/mocha-tips--shape-based-tracking-part-2/


#7

Thanks. But what directly determines scale? Is it points within the surface that move with respect to each orther? Is it changes in the size of the surface?


#8

Mocha’s tracks an object’s translation, rotation and scaling data based on the movement of a user-defined plane. The Spline defines the search area and you are telling Mocha to assume that this texture of pixels is all moving on the same plane. When there is movement that is opposite (reflections, shadows, other layers or non planar areas) it is useful for the user to avoid these areas with some of the methods described.


#9

I get how obstructions can be matted out so they are ignored for best tracking. But when the object is greatly obstructed, do I leave the surface tool size/shape the same (and thus kind of guessing about how far into the obstruction it is and thus where the surface would likely be), or do I shrink the surface so it is only on the remaining part tracking object that is still visible?

Note - not sure I’m using the right term - the surface is the rectangle that you use to replace the object?


#10

The surface tool is the Blue Rectangle that represents your tracking data. Typically the workflow is to use the spline search area to handle obstructions and occlusions.

Changing the surface position or scale will not affect the tracking data that you are capturing while tracking. For example, you could track an object, display the Surface, move or scale the surface and retrack. You will not get a better track. The quality of the tracking will be affected by what you feed into the tracking algorithm and this is defined by the spline shapes and by the tracking properties page.

So if an object is greatly or totally obstructed, generally the technique is go back to the last frame of “good” tracking, then adjust your shapes. If you can find another object that is moving on the same relative plane, you can move your shape there, and continue tracking. If the shape is totally obscured, you may need to use “manual tracking”. This should be turned on as a last resort and will display ALL the surface key frames. You may find these 2 videos useful:
https://borisfx.com/videos/?tags=feature:Manual%20Track&search=

Hope this helps.


#11

Thanks. I think I get most of all of it except size/scale tracking data and how that is calculated.

You seem to say the size of the splines and surface tool can be changed during a track with an obstruction, on an object that is of constant size.

So if you have an obstruction, do you have to accurately guess where the surface tool would be and keep it always the same size? If the surface tool (or spline) is smaller in some places, doesn’t that affect the scale tracking data that will be used on a replaced object?

I’m just wondering how accurate I need to be size-wise with the surface tool in relation to the object I am tracking and replacing.

Thanks again.


#12

Find points you can use again when you use manual track and then realign the surface tool once the occlusion is past, the hand animation should scale between the objects. If you can’t find points for the surface tool to find again, yeah, you just have to use what visually looks right and then guess.

You can also use the non-manual Mocha tracking to adjust keyframes and ease them into the manual track and delete any problematic ones in the dope sheet. Total occlusions require a bit of massage to work well.

Be as accurate as possible, guess where that isn’t possible.

Cheers,
Mary


#13

Thanks.

I am still left with 1 fundamental question - how does Mocha compute changing scale? Is it change between points within the surface tool? The spline? Does the size of the surface tool or spline have any effect on scale computation?

Thanks.


#14

No, just the track scales as the texture it is tracking scales, that’s it. There’s no surface size or shape size that does it, it’s the track of the texture itself. Like all the tracking data in Mocha.