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Getting off the ground

Once we have a floor grid for our picture containing a plan for our objects to fix their location, we can mark the principal vanishing points to fix the perspective. Pick major lines on the plan in each direction (typically two) and extend them until they cross the horizon line. For a 45 degree angle you need only join up the diagonals of the grid and extend that. Finally create a new layer for the picture itself - that way we can make the grid and markers invisible whenever we like. I place the picture layers below the floor grid layer.

Ways of determining height.

But how to know what height the object should be to allow for distance? If we know the full height in terms of grid squares (place a copy on your plan grid), there are many approaches. There may be an adjacent object against which to align the height; alternatively:

1 Alberti took the point in the grid where the object is to be standing and measured the length of the equivalent number of grid squares sideways, transferring that to the vertical. In Xara we can draw a circle from the standing position outwards ( Ctrl Shift as we draw); when it covers the number of squares sideways, the top of the circle marks the height of the object.

2 We could place the object at the front of the picture (along the same grid line) and draw lines to the central vanishing point before resizing.

3 If you are making a lot of measurements in a particular plane you could make a plan or grid and mount this on the plan grid. We've seen how to erect a side grid and you could drag a horizontal guide down to convey the height to any point at the same depth on the floor grid. Some people erect grids to form a tunnel or structure. You can place a grid or elevation just as you would any vertical surface.

4 To create objects with a flat top and lots of sides, use vertical guides and a ceiling grid of known height. Mark the base on the plan grid. Mark the corners of the plan grid with vertical guides. Take the floor grid and markings, clone them and in select mode, nudge up to the correct height using a temporary copy of the plan grid to locate the front. Now stretch or squash using the vertical handles only until the vanishing point returns to coincide with that of the floor grid. If you go above the horizon line, your grid will flip in the process. Because you did not make any horizontal changes, the corners should still line up with the vertical guides, but do check. (Use this method generally to raise horizontal objects off the floor).

Adding vertical rectangles

Remember, with the perspective tool we are dealing with planes. The simplest and by far the most common is the vertical rectangle. As two edges will always be vertical we have only one vanishing point, to right or to left. The front edge is the most important, and once fixed the perspective's vanishing point can be dragged around without affecting it.

The procedure is similar to the one for setting plans into the plan grid. To install the red rectangle along the end of the object on the grid, left, do the following:

1 Select it and apply the right perspective preset with the Mould Tool.

2 In Select mode, put the front edge of the mould in place.

3 Stretch with the vertical handles to the required height (shown here using Alberti's method, having measured the height with the plan grid and found to be 3.5 squares).

4 Set foreshortening by compressing the mould with the horizontal handles until the width of the mould lines up horizontally with the end of the plan line.

5 Drag the vp handle to line up with the picture's left vanishing point. Repeat this final stretching and dragging for a more precise fit.

Adding tilted rectangles.

If the rectangle is tilted, you will have two vanishing points to satisfy, but you can cheat by putting a rectangular bounding box around your rectangle and fitting that into your picture. You can elevate the rectangle from floor level by marking the four corners of the square with vertical guides and use method 4 above for solving the height.

A rotated, tilted cube, using plan and elevation.

1 Construct the plan and floor grids (page 3); I put the horizon near the top to get a higher perspective.

2 Take the outline of a cube face, tilt it and draw in a bounding box - you now have a front elevation.

3 Project the horizontal positions upwards (using vertical grids) to create a plan view of the tilted cube using the length of one of the cube sides as the depth.

4 Put a rotated plan view on the plan grid.

5 Bring the grid with its plan down to form the floor grid.

6 Project the main lines of the plan to locate the vanishing points at the intersections with the horizon.

7 Place the front elevation of the tilted cube on the picture as in 'adding vertical rectangles above. If necessary reduce the height with Alberti's method, using the plan grid to get your sizes.

8 Connect the top corners of the front elevation bounding box to the left vanishing point (black lines).

9 Place another copy of the front elevation to form the back of the cube, adjusting dimensions until its corners line up with the black lines.

10 The horizontal edges of the cube all converge at the left vanishing point and complete the definition of the cube (orange lines)

11 (Shown offset so as not to obscure the remainder of the illustration) Take each side of the cube, apply the default perspective and drag the corners into place, starting with the corners that form the front edges. To add shading, copy the mould shape to the clipboard with the mould tool and paste over the original mould. Fill it black and give it an appropriate transparency. Now for some tricky bits.