Rectify Lens Grid: Just fix it!

Rectify Grid is a new capability to "just fix" arbitrarily distorted lens grids, producing STmap image distortion maps. In addition to describing that, the tutorial discusses creating and shooting lens grids, no matter how or what will process them, in hopes of addressing the plague of disastrous lens grids that customers receive to be analyzed. Crucially, lens grids must reach the image's edges!

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Hi, this is Russ Andersson, here to introduce you to another new capability in SynthEyes 2204: Rectify Lens Grid. "Rectify" means "to put right", and that's what Rectify Grid is all about. It's your answer for people asking "Can't you just straighten it out?" You've got a lens grid, perhaps from a gnarly misaligned anamorphic lens, and you want a calibration that just makes the grid perfect, no matter what. A normal lens calibration finds the best match of a grid's distortion to a mathematical model, producing distortion coefficients and an RMS error. Rectify Grid does none of that, it "just fixes it," producing an image distortion map, aka STmap. That's both a strength and a weakness. Any errors in the incoming calibration data will be faithfully inserted into your live shots. Anywhere there's no grid in your calibration image, there will be NO PICTURE AT ALL in your live shots. And because there's no mathematical model, just a map, it's impossible to animate any changes in the map. Accordingly, Rectify Grid is NOT a replacement for Lens Master Calibration and its computed lens distortion parameters. Rectify Grid is a last resort. Conveniently, both Rectify Grid and Lens Master Calibration have the same input, a grid of trackers, so you can try them both. Let's talk about grids; there are so many terrible grids out there. Grids should have a single repeating pattern, not nested patterns. To shoot with the same focus distance, patterns need to be big. Use a large-format printer. There's a PDF grid generator on our website. Here are a few rules on shooting lens grids. It's time for a Lightning Round on lens grids! Let's take a look at Rectify Grid in action. Here's my grid, it's from a Panasonic GH4, Metabones Speed Booster, and SLR Magic 2x Anamorphic converter. We'll set the pixel aspect ratio to 2. Here's a tip: to set up the tracker grid automatically, you should resample the image to square pixels. I can adjust the contrast from the Levels Tab. Now I can create the lens grid trackers. I'm using Create Lens Grid, but there are some other ways to create trackers, especially for checkerboards. I'll leave that for a different tutorial. Let's lock up the trackers. You need to look carefully at the trackers now. Delete any extra trackers that aren't on the grid. If someone gave you a multiresolution grid, you need to delete all the trackers that are NOT on the spacing of the outermost grid, so that all the remaining trackers are on a uniform grid spacing. Don't "make up" any missing locations! Next, especially for Rectify Grid, be sure to ADD as many trackers as you can towards the edge. Rectify Grid won't go more than one grid past the last tracker, so if the grid ends, so does your picture. Once we're done, we'll lock them all up. If you want to stick with the original 2-to-1 pixel aspect ratio after calibration, you should turn off resampling in the image preprocessor, to go back to the original ratio. We'll do that now. Now we're ready to start Rectify Lens Grid. You have to select whether you want to use a One-Pass or Two Pass workflow, as you do for Lens Master Calibration and Lens Workflow. Check the Camera Calibration manual for the full description of the parameters. It will take a little while to generate all the maps, and update the image. The explanatory note recaps all the settings. Notice that there is no RMS error for this kind of calibration; the trackers are now located exactly on the grid. Once you've calibrated, you'll want to apply the calibration to other shots. Both Rectify Grid and Lens Master Calibration now produce small Sizzle scripts to help with that. Just open the live shot, and use Run Script to run the Sizzle script for the calibration. It sets up the image preprocessor accurately. You might also want to generate a projection screen, which uses texturing to quickly undistort images in downstream 3-D applications. Since the projection screen depends on the lens field of view, it makes sense to generate the projection screen after solving the shot. Alembic, Blender, Filmbox, Lightwave, and USD ASCII exporters can do that automatically. Hopefully, Rectify Lens Grid helps with the material you encounter. Please spread the word about how to shoot lens grids to anyone you know, and we can end this terrible plague. This is Russ Andersson; thanks for watching!