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Transformation for perspective correction options may be selected using toolbar icons

The two main operations may also be selected from the Transform menu item.

Open two images or an image and a map for which corresponding control points have been entered.  Select the image to be transformed by clicking on its upper border:

Clicking here permits selection of an area within an image to be transformed.  If not selected, the whole image will be transformed. 


If the horizon is visible in an image, you must select an area which does not include it. Ignoring this will cause transformation to fail, since a  perspective transformation goes to a vanishing line at the horizon and the transformation constants are undefined. Left-Drag the mouse cursor to define a selected area. A red-white box shows the area which is set when the left mouse button is released. If you are not satisfied, simply repeat the mouse-down drag operation.

The selection box may be closed by clicking on this symbol.

This little camera symbol with a blue lens when clicked transforms the active image (click on the top border to activate an image) to a rectified image on an inactive image with corresponding control points. After transformation, the transformed image becomes active. 

This camera symbol with a white lens transforms the whole active image to fit a new image whose scale may be set between 50 and 200% of the size of the original image.  

The scale may be set using the track bar in the middle of the bottom status bar before this operation is carried out. The default is to use the resolution of the original active image (100%).

The result is a single new transformed image:

This button undoes a transformation. It can not undo a transformation using the white camera symbol, since source and original target images are closed before displaying the result.

Clicking this symbol places a temporary grid over an  image to aid in placing control points or for comparing with an original un-transformed image. The grid will be removed prior to transformation and is not written permanently to an output file.

Testing placement of the transformed area:

Before transformation of any image, you may wish to test the placement of the transformed area. Click on Debug on the  Control menu and transform. The result area will be shown in red:

A window will pop up during transformation to show how things are progressing. If you have more than one physical processor, or have a dual-core processor,  or if you have a Pentium 4 with hyper threading enabled in the BIOS, then the number of progress bars will reflect the number of physical or virtual processors. If there is more than one physical or virtual processor, the source image will treated in as many sections as there are processors, thus speeding up the transformation. This shows the progress window with a two-processor machine having hyper threading for both enabled:

If control point placement leads to an average reprojection error for all points which is greater than 1 % of the average of the side lengths of all images, a warning dialogue pops up and asks if you want to continue anyway. 

Click No if you want to try to improve placement, and then press shift-F9 on the image window. Look for the control point with the highest error, then Alt-Right click on it and tweak it to reduce the error. Repeat this for the next highest error point and so on as described in the topic Move Control Points a Pixel at a Time in the Control Points chapter of this help. Then try the transformation again.

Undo Notes:

Undoing a transformation permits choosing new parameters in Setup or modifying control points and re-transforming without re-loading everything. The undo function also turns off any area selection, grid and repaints the control points and the navigation palette of the original image.  It may also be used on rotated and resampled images.

When Clone and transform is selected, the empty cloned window is shown after undo, leaving any control points intact. This window can then be re-sized if desired and the normal transformation operation repeated.

Multiple Files:

If you have opened more than two files using the Open Multiple option on the File menu:

then you can select two files out of all of those displayed, one as a source and another as a target image for transformation.  The files serving as source and target must be selected by clicking on the Source/Target main menu item 

which becomes visible when three or more files have been loaded. Click on the caption of an image to give it the focus first.  Then click on the Source/Target menu item or press F3 to make it the source.  

Click then on the image which is to be the target for transformation, and there Click twice on the Source/Target menu item or press F3 twice to make it the target:

If you have made a mistake, you can toggle through the options of Source, Target or neither of these simply by repeated clicking on the menu item or pressing F3.  If you have mistakenly chosen two images as sources and or two as a target but none as a source, you will get an error message when you attempt a transformation.

Proceed as above for transformation.