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| On Dec 31, 2011, at 11:13 AM, Dave Bittner wrote: > I've got some footage from a Photron high speed camera that's presented an interesting challenge to me. The camera records a sequence of images that have a 48 bit depth, which is a large dynamic range. On export to QuickTime, however, you are forced choose a bracketed range of 8 bits. After multiple exports, I'm left with a movie that's underexposed, one that's properly exposed, and one that's overexposed. I want to combine then, HDR-style, to show the details in the highlights. (The footage is a series of explosions, so there's a lot going in the highlights.) This is interesting. So when you say bracketed range, do you mean that you're only able to choose 1/6 of the histogram, or that you set your white point and it crams everything from black to that point into an 8-bit file? The latter is essentially what you get from doing different exposures in a camera, the former would be unusual. Anyway, it is possible to do an HDR merge in After Effects. If I weren't so lazy, I would have put up a video tutorial by now. I'll try to describe how to do it photo-style here. If you have the 1/6 histogram situation, it would be a little different. 1. Set your project to a 32-bit linear space. 2. Import your bracketed photos, drag them into a comp. AE will have converted them from sRGB to linear. Many HDR programs calculate the actual response curve for the camera, obviously AE is just using the sRGB curve. (Do the Photron people tell you if they're applying a gamma?) 3. Pick one photo to be your base, and leave it be. Then take the next darker image (captured at -1 stop) and use the Exposure filter to brighten back up by +1 stop. Hide the other photos for now. If your camera truly had a perfect sRGB response and the photos were in perfect alignment, the brighter pixels from the base image would now match the mid-range pixels from the -1 stop image. You can tweak the exposure controls and re-align the image to try to get them to match as closely as possible. The darker pixels from the -1 stop image will have a lot of noise in them, and the brighter pixels will add detail into the 1.0-2.0 range. 4. Comp the useful parts of the brightened -1 stop image onto the base image. Do this in AE by dragging the -1 stop image into AE again to be used as a luma matte for the exposure-adjusted version. Apply shift channels to set RGB to Luma. Set this layer as the other layer's luma matte. Then apply Levels and set input white to 0.5 and input black to something like 0.25. This will keep all the 1.0-2.0 overbrights and blend between the two pictures in the 0.5-1.0 range. We're just using Normal transfer mode here. 5. Repeat for all the other photos, increasing your -2 stop exposure by +2 stops, -3 to +3, etc. The Levels settings will be the same because you apply those to the un-adjusted images. Sorry if that's hard to follow. I realize a video tutorial would be better. Brendan |
