Warping the Perspective of the Picture
Once everything is positioned and aligned correctly, you're ready to warp the perspective of the image. Click the Warp mode button in the Options Bar. This will change the appearance of the combined quads and enable three auto-straightening options (one for verticals, one for horizontals, and one that does both). However, you may not need to use more than one of these.
Here, I moved the cursor near the vertical edge that "connected" the quads, then held down the Shift key. When you do this, the edge should turn yellow (Figure 9). Click it and the line should straighten a bit. You can do this with any of the grid lines, but most commonly you'll want to see if you can straighten a common vertical edge that joined your two quads on the corner of the structure. Here that had no effect (the verticals were already straight), so I instead flatted out the grid line under the front of the hall (the most obvious area of distortion in the shot) using the same technique.
Figure 9 You can Shift-click the central vertical edge or one of the bottom edges to straighten out the structure a bit before warping the perspective.
If at any time you don't like a change you've made, click the reset button in the Options Bar (with the rotating arrow icon, third from the right). This will reset the warp but not the layout that you've created.
From this point, it's time to change the perspective. First, make sure the common vertical edge of your quads is selected (i.e., the edge on the corner of the structure). Next, click on either of the two pins that are connected to that edge, and drag left or right to warp the perspective. Don't push it too far; if you compress the details too much to one side, you will end up with a badly distorted shot. Photoshop is not a miracle-worker; it can only (realistically) reproduce the new perspective if you move a few degrees in either direction.
Once you have the perspective you like, check all the corners and tweak those as necessary to remove any slightly awkward-looking lines that the warping may have produced. For example, in this shot I had to push the right corner of the roofline down a bit in order to maintain a more realistic appearance (Figure 10).
You can see not only did Perspective Warp straighten the lines a bit (especially on the right corner of the hall), but the view of the structure has shifted to the right, shortening the composition on the left side of the structure. Once you like the look, click the button with the check mark in the Options Bar to apply your settings.