EddieOffermann dot com

Just updated my blogroll, the little list of links to other people's blogs, specifically related to visual effects. I'd started with a couple of off-livejournal blogs of friends - and realized that'll have to be when I do the other blog that's just me and my personal stuff. (Unless of course, those friends start posting about visual effects)

I may add an additional "also of interest but not vfx" category.

Anyway, if you're one of my not-vfx-industry readers but you've been interested in some of what I've been writing about, check out the other visual effects blogs under "Links" on the right. My ramblings recently have been especially technical, but they won't always be so: and many of these are a little more down-to-earth and a good way to find out a little more about the world I work in.

Also, if you're a vfx artist with a vfx-centric blog, ping me and I'll put you on there as well.

_{(And if you're just finding this blog randomly, check out my shake scripting , MEL scripts, and SynthEyes sizzle scripting info, tutorials, and tools.) Sorry about tooting my horn so much there. Has to be done. Really.}

ReDistort - Optically accurate for typical lens distortion.I received an email today from Miles Lauridsen over at Pac Title. He was comparing the results of my ReDistort tool for Shake with the output from SynthEyes applying what (should be) the same algorithm. His report, however, was that the results he was getting were somewhat different using the same distortion value.

I've been comparing your ReDistort shake script to the images produced by outputting a re-distorted image from Synth Eyes and noticed some discrepancies. Namely, the shake script does not distort enough using the same value as in SE. Have you noticed the difference in the image distortions, and if so, have you found a workaround for this?

He's totally right of course, and it's something I'd meant to mention originally. While all the math is the same, there are some noticeable discrepancies in the execution of it in the Shake environment that's always made it a little tricky. (No trickier, of course, than dealing with lens distortion without the tool - it's always a sticky business!) Now, this isn't the first time I've experienced the problem - and truth be told, having it brought up by someone else makes me think I ought to investigate it a little more closely and make the tool work dead-on right away, but I do have a workaround and I figured I'd post my response here. A video walkthrough of this process (and a few other tricks) should be coming along shortly.

My reply to Miles:
Interesting - it's using the same function as SE... or at least was when it was created... Yes, when I've compared them there were some differences but it was pretty subtle, in ways I've always attributed to the way Shake pushes and pulls the pixels in the displaceX.  I wonder if the function has changed.

The math isn't purely reversible so you'll get different results from positive and negative distortion.  I've never been curious enough about it to do more than just make it work, but I'm sure positive vs. negative distortion values should be handled slightly differently.  (I did write a version that adjusts for that, but that's not written into the tool I distributed because it's pretty arbitrary what I did in it)  It's because the node processes the distortion from the viewable image plane, something which actually doesn't work in both directions with the same value (since either one direction or the other is starting with data across a smaller area)

I've always avoided processing the plates through SE since it seems the wrong place for that (though very small studios obviously have reasons to disagree with me - notably having nowhere else to process them)

In cases of strong plate distortion where I suspect the discrepancy might cause slipping, I'd usually pull a plate in through SE, calibrate it in SE so its distortion is compensated for using SE's internals, write out an undistorted plate, then build a shake script with the original plate and the undistorted plate passing into an iSubA.  View the iSubA, add a ReDistort under the undistorted plate from SE and adjust until it's warped back out to the original plate.  You'll see the relief-map appearance at the iSubA reduce to subtle shades instead of harsh lines as it gets close.

That gets you the reverse value that you'll need to use to process a render.  Then you can pass along that value to the compositors or whoever in your pipeline is handling undistortion of the render passes.

Hope that helps.  It's part of what I was hoping to cover when I record the walkthrough tutorial for that tool.  It's not perfect... but it's handy - and we used it @ Asylum for that Propel Stress Monster commercial that we won a bunch of awards for last year, so I feel good about the results.  (It's been used on a lot of stuff, here and elsewhere - it's pretty battletested)

The ReDistort lens distortion tool can be downloaded from here.