"Gone Nuts 3D" Takeaway #3: Move Over Roll Angst, Meet Zoom Disparity

3. Move Over Roll Angst -- Meet Zoom Disparity

The Day Zoom Disparity Became My Top Nemesis

During the introductory film production labs I teach, I have seen the gamut of problems beginning videographers face: forgetting to white balance, neglecting to use neutral density filters, using the wrong neutral density filter, and my favorite, the infamous camera-bag-and-tripod-in-the-shot.  (Granted, you did a great job not leaving your gear unattended!)

Keetin in Booth 3: Home of the S3D editor

Stereoscopic 3D introduces a whole new field for mistakes and disparities that arise between the left and right clips.  In editing the clips together, I performed vertical image translation (VIT) to change the vertical positioning of one clip in order to align the clips.  After VIT, the two images still did not match.  So, I changed the rotation of one the clips.  Still, something was off.  After minutely adjusting the VIT and rotation, I was no closer to adjusting the problem.  Then I recalled what the stereoscopic editor from the first project had mentioned during their project post-mortem: for some shots, he discovered the cameras were zoomed to different distances.  I opened up the Sony XDCam Browser to examine the metadata for the clips and see if this, too, was my problem.  Sure enough, for a handful of clips, the camera zooms were set differently. 

To fix the problem, I scaled one of the images up and proceeded to adjust the VIT and correct the roll.  Voilà!  The disparities were fixed and the clips were for the most part aligned.  

S3D requires an even more watchful eye on the part of the stereographer who now has double the cameras to monitor and watch for small problems and mistakes.  Thankfully, zoom disparity is a detectable problem and easy to fix during production and is fixable during post-production.

 Now whenever I am editing and cannot align the two clips, I instantly suspect a zoom disparity and check the metadata.  The sneaky thing about detecting a small zoom disparity is that the size difference is very, very subtle (especially in grass). So, when in doubt, check for zoom disparity. 

LESSONS LEARNED

1.  When importing your clips, never delete the metadata information.  This may prove to be very useful to you later, especially if your footage log did not include this specific information.

2.  When the two clips do not align, double check to see if there is a problem aside from roll or vertical disparity.

3.  Faithfully stick to the left camera to not only watch for the matte box but more importantly, to check that both cameras are on the same zoom.

4.  Always check with those who have performed your job before.  They hold a rich trove of experiential knowledge.

5.  A 1 mm zoom difference does not correlate to a 1% scale difference.  So, experiment to see which up-scaling percentage minimizes disparities.
6.  Editing angst is bonding.  The editors from the first round of projects enjoyed commiserating and after each project, another editor developed a distaste for zoom disparity.  The first round editors also watched the left camera like a hawk during the second round of projects because we understood the headache slight differences between the two cameras made.
7.  All stereographers should edit S3D sequences. Through the process of editing and aligning the left and right clips, I saw and grasped the different technical challenges in stereoscopic 3D production that I was aware of before because of our reading but did not understand.  I approached our subsequent projects with a changed eye, knowing what had caused me consternation in the editing booth and looked out to catch these problems during production.

"Gone Nuts 3D" Takeaway #4: Roll, Baby, Roll

4. Roll, Baby, Roll

The Day the Alignment Grid Became My New BFF

Grass is fun to roll in, but grass is not fun to roll to properly align the left and right clips of your stereosequence sequences.  Gone Nuts 3D was shot entirely outdoors, affording us a beautiful autumn film shoot.  This also meant that grass was in the background of almost every shot.  

Grass,

and more grass

    

       grass, 

I soon learned that when adjusting the degree of rotation of a video clip to align two homologous points, it helped if the video clips included strong lines and patterns.  These strong lines clearly display when two points are or are not aligned.  In contrast, grass is not a symmetrical and I could not anticipate which direction a given blade of grass should point.  This made it tough to know whether the left side and right sides of both video clips matched when one clip screened the other. 

After this project, our instructor Chris Eller introduced the alignment grid to our filming procedure.  The alignment grid is held within sight of both cameras for a few seconds after the slate.  This alignment grid provides the strong lines to align the homologous points of the left and right clips.  Example A below shows the left and right video clips screened in their stereosequence before any other alignment adjustments were made.  

Example A:
Alignment grid before adjustments

Example B:
Alignment grid after adjustments

The alignment grid introduced an easier way to align the images.  My strategy to fix roll is to adjust the rotation of one of the clips so that the diagonal lines of the alignment grid are parallel to each other.  (See Example B)  The diagonal lines of the two images may line up perfectly depending upon how you adjust the horizontal image translation (HIT).  The alignment grid is useful fixing more than just roll, but the alignment grid significantly speeds the alignment process.  Now whenever I am on a shoot, I monitor to ensure we always begin with the slate and alignment grid.  I am thankful for editing without the alignment grid because I more deeply understand and appreciate its handiness.   

LESSONS LEARNED

1.  Use an alignment chart to speed up manual stereoscopic adjustments like roll, VIT, etc.

2.  Stay current on tips from other stereographers 

3.  Take your instructors' advice.  Groups not consistently using the alignment grid regretted it!