That said, it's probably going to take a rather zen-like philosophical outlook to get through the next phase. The short tutorial on the LOC website doesn't call it "the most difficult step" for nothing. If you're doing web work and not interested in printing, you can bail now.

To begin, we'll take a hint from the LOC tutorial and select reference points near the corners of the image. Angles are best, because they define a point at their apex. Right angles are even better, because it's easier to line guides up at right angles.

The points should be as close to the far corners as possible because that's where the distortion will be worst. Unfortunately the exact corners generally don't have a visible reference point, so we just get as close to them as we can. The references, obviously, need to be at places that are visible on all three channels.

You're going to want to set up the points on the Green channel, because that's going to be the target to which we adjust the other two. Here I've chosen the angle of the picture frame in the top left, the angle of the picture matting on the top right, the intersection of the arm of the bench with the bottom of the horizontal slat at the bottom right, and the intersection of the arm with the top of the slat at the bottom left.

Why use the Green channel as the target?

We know that Prokudin-Gorskii used a single-lens camera, we've seen it. We know that he thought of "RGB" in much the same way we do today, by the way he ordered the colors, putting green second (or in the center). Logically, he would have lined the center of the plate behind his lens for a direct shot, and fired the other two colors to the opposite ends of the plate.

Therefore since the center image of each plate is the green channel, that channel likely was the straight-on image. That would make it the truest representation of the subject (and might also hint at why it is generally also the largest). The ones top and bottom would therefore have been done (I assume) with mirrors, and are screwed up. So we'll adjust those two, using the green channel as our target.

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