Warp (Rubber Sheet/Calibrate)

Before After


Warp deforms a raster image by moving specified points on the image together with all the parts of the image attached to those points. It can be likened to sticking pins through a stretched rubber sheet and then moving the pins. As you move the pins, the rubber sheet is deformed.

You can use Warp to achieve multiple effects including stretch, rotate, expand, shrink and for georeferencing.


To warp an image:

2. If you want to warp the whole image make sure the whole image is displayed. You can display the whole image by clicking or pressing the Home key.

If you only want to warp an area of the image, choose the area you want to warp by windowing it or zooming into it.

3. Do the warp by:
Pressing Ctrl+W.
Selecting Raster Effects Menu > Warp > Warp Run.
Pressing Shift+D or selecting Raster Effects Menu > Warp > View/Edit Control Points and then clicking on the Run button.

The image is warped.

If Scan2CAD can’t resolve the control points you have entered a Warp Error message will appear. If a Warp Error message appears, try adding more control points. You will usually need a minimum of 5 control points for a successful warp.


Click to undo.


What happens during a warp

When Scan2CAD warps an image it tries to move all the source points you have defined to their corresponding destination points.

Scan2CAD is unlikely to be able to move every source point to its exact destination point because of contentions between the various source and destination points. Some might pull the image in one direction while others pull it in another direction.

Instead, Scan2CAD moves each source point to a position that is as close as possible to its destination point but that also maintains a minimum overall error across all the source-destination point pairs.


Viewing errors

After warping you can view the error between each target and actual destination point.

To do this, enter the Edit Warp Control Points dialog by pressing Shift+D or selecting Raster Effects Menu > Warp > View/Edit Control Points.

The errors in the X and Y directions are shown in the X err and Y err columns and are measured in pixels. The largest X and Y errors are marked with a *.

If you find that most control points are producing a small error but that one or two control points are producing large errors you can omit the control points with the large errors and try the warp run again.


Control point display is not persistent, meaning that control points will no longer be displayed on the screen if you redraw, zoom, pan etc. To display them, press Shift+C. If you change the control points the display will not be updated until you use Shift+C again.


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