2-D Warping

Clicking on ``2-D warp'' creates a new copy of the image with a square grid of blue lines. The grid vertices are the control points. Move the control points as desired to distort the image in 2 dimensions. When finished, pressing Esc or clicking the main Cancel button will erase the grid lines and return to normal mode.

There are two ways to move the control points:

1. Click near the control point with the mouse and drag the control point in the desired direction.
2. Click near the control point to select it and release the mouse button. Then use the arrow keys on your keyboard to move the control point. This has the advantage of moving the point by a fixed number of pixels each time.

Pixels/grid point: selects the spacing between grid points.

Cursor movement: selects the distance a grid point is moved when arrow key is pressed.

Commands available while warping is active:

• c Makes a copy of the warped image. Warping then continues, making it easy to create multi-frame images with different amounts of warping (see example below).
• g Toggles the grid lines on/off. Warping is still possible.
• r Causes the grid to revert back to its original square shape.
• Esc Press Esc or click the main cancel button when finished.

Example of several frames created by pressing `c' during 2D warping, showing increasing degrees of distortion to the original image.

Tips on 2D warping

1. Grid lines should not cross each other. This can cause discontinuities in the warped image. If a large displacement is needed, move the adjoining grid points as well to prevent this.
2. It is okay if the control points are off the edge of the image.
3. Speed can be increased by turning grid lines off. Even though the grid lines are not visible, warping is still active. You should make sure the individual displacements are small enough to prevent the control points from crossing. Also, extreme displacements can cause an unnaturally smooth appearance due to replication of adjacent source pixels.
4. Individual grids should be kept as rectangular as possible to preserve spatial relationships within features.
5. Images should not be deleted during warping. If the original image or warped image are deleted during warping, an error message is displayed.
6. Future versions of imal will be able to move sets of grid points simultaneously instead of one at a time.
7. As with all functions in imal, only the active color planes will be warped. For example, you can warp the red component of your image by clicking ``Config..configure'' and un-checking the Green and Blue Color Planes checkboxes. This may be useful in removing chromatic aberration (see also Chromatic aberration in Sec.8.8).