Pointy: A tool for editing matched points on images

Pointy is a simple viewing and editing system for sets of matched points on pairs of images. It allows the viewing and editing of lists of matched x, y points, lists of sift points, and lists of affine point pairs. Pointy also allows visualization of the transformation defined by a homography matrix, and the epipolar lines implied by a fundamental matrix.

Pointy is somewhat limited, and it would be ideal to rewrite it in QT, or Java, or just about anything else besides TCL, but I never find the time. Despite its limitations it has proved pretty useful, and I hope you find it useful too. Please send bug reports, or ideally bug fixes, comments, etc to rupert at rupertbrooks dot ca.

Obtaining Pointy

Download a zip archive from here, or check it out from the svn repository. http://svn.assembla.com/svn/pointy/ (Rather than browsing this link, put this url in your subversion client.)


Pointy is so simple, it almost seems pointless to license it. Nevertheless, Pointy is licensed under the Mozilla Public License (http://www.mozilla.org/MPL/) Briefly, this means that:
  • You may freely modify, use and distribute the software, but
  • such modifications must be open sourced, and clearly identified.
  • However, you may combine the software with other work, in which case only this software must be kept open source, but the other work can be licensed however you wish.
  • There is no warranty.

Installation and Usage

Pointy is implemented as a single Tcl/Tk script. Thus, on any system supporting Tcl/Tk, it can be started in the normal way for such a script. Examples follow, but your system may be different
  1. Linux:
  2. The shebang line at the top of the script should enable your shell to associate the Tcl/Tk interpreter (wish) with pointy. Then pointy.tcl should be placed in your path, and may be started at the command line by typing


  3. Windows:
  4. If a system such as Active Tcl is installed, files with the .tcl extension may be associated with the TclTk interpreter, and may be made executable by adding .tcl to the PATHEXT environment variable. Then pointy.tcl may be placed in your path and started from the command line as above.
  5. Cygwin and Active TCL:
  6. Only try this approach if the previous two approaches don't work for you. On my own system, I have both cygwin and activeTCL installed, and I prefer to run pointy from the cygwin bash prompt with the activeTCL interpreter rather than the cygwin one. This can only be achieved by taking the cygwin paths OUT of the system path before starting pointy. I achieve this using a small bash script called pointy which contains the following.

          SCRIPTPATHWIN=$(cygpath -m "$(which pointy.tcl)" )
          export PATH=`stripout.pl PATH '^/usr' '^/bin'`
          #echo "$SCRIPTPATHWIN"
          wish "$SCRIPTPATHWIN" $*

    stripout.pl is a perl script containing the following.

          for($i=1;$i<=$#ARGV;$i++) {
              for($j=0;$j<$#vararray;$j++) {
                 if($vararray[$j]=~/$ARGV[$i]/) {
                    # drop this item from the list
                    #print "Dropping $vararray[$j]\n";
                    #print "$j now $vararray[$j]\n";
          print join(":",@vararray),"\n";

    This type of approach may work for you if you have multiple installations of things that interfere with each other.

  7. Last resort:
  8. You can also always call the TclTk interpreter and pass the pointy script as an argument. For example

          wish [path/to/pointy/pointy.tcl] {arguments....}
However you start it, pointy takes the names of two images on the command line

      pointy leftimage rightimage

Starting pointy in this way brings up the interface with the left image on the left, and the right image on the right. Which brings us to...

Using Pointy


Pointy has three modes which can be selected in the selection bar on the left of the Pointy window. In "Add points" matched pairs of points can be added to the current set. In "Show H" the correspondence between the images implied by a homography matrix can be viewed interactively. In "Show F", the epipolar lines in one image corresponding to a point in the other can be viewed interactively. A point consists of a centroid, and a surrounding, oriented ellipse. Both the center position and surround characteristics can be changed interactively. Points are added by left clicking, and manipulated by moving the mouse over a point to select it, and then left, right, or middle clicking and dragging.
  • Add a point. Left Click
  • Move a point. Left Click and drag
  • Change size and orientation of surround. Right Click and drag.
  • Change eccentricity of ellipse. Center-click and drag.
  • Delete a point. While mouse is over it, press "delete".


The use of Pointy will be demonstrated via a number of examples. These are contained in the Examples subdirectory of the distribution. Since this manual is in the same directory, presumably you found it. Throughout these examples, it is assumed that you are working in the Examples directory.

Example 1: Hong Kong: Adding simple point matches.

First, let us load up two images with the following command

      C:\home\development\pointy\Examples>pointy hongkong/hk1a.png hongkong/hk2a.png

This should pop up the pointy window, with both images at least partly shown.

Now go to the file menu, and select "Load Plain matches". Navigate to the file hongkong/hkpoints.dat. This will load up 4 matched point pairs. The hkpoints.dat file simply contains lines which have the x, y pixel positions in the left image, followed by the right image. (Note that x is the columns, and y is zero at the top of the image.)

Example 2: Hong Kong: Adding simple point matches.

Four point pairs is not really enough to do much image matching work. Lets add a few more. Make sure that Pointy is in "Add Points" mode, based on the radio buttons on the far left of the window. Now left-click in either image frame. This will create a new point pair - but note that the matched point in the other image is not in the right place. Move the mouse over that point, and it will turn green. Once green, you can left click and drag it where you want it. To remove a point move the mouse over it so it turns green, and press the delete key.

Once you have added more points to your satisfaction, on the file menu select "Save Plain matches" to save them to a text file.

Matched points frequently have more information on the matched region than just the center point. Pointy allows you to edit the oriented ellipse that surrounds each point. Right-clicking and dragging changes the size and orientation of the surrounding region. You can stretch the point into an ellipse using the middle button and dragging.. If you save the point as an affine point the ellipse information is kept. If you save it as a sift point, only the size and orientation (ie, an oriented circle) is kept. Finally if you save it as a plain point, only the centroid is kept.

Example 3: Hong Kong: Viewing a homography.

Pointy can also display the transformation implied by a homography image between two images. To see this, either continue from the above example, or load the hongkong images again. Under the file menu, select "Load H". On the left sidebar, select Show H. Now when you move the mouse over either of the images, a little pink cursor will follow it, and a similar cursor will appear in the other image. The position of the cursor in the other image is determined by the homography transformation. Try moving one the mouse around one of the large buildings. The cursor should closely follow the corresponding position in the other image.

Example 4: Shed: Sift keypoints

Pointy can also display scale invariant and affine invariant points. Such points have extra parameters in addition to the x and y coordinates of the point centers. These extra parameters indicate the corresponding region of the image around each point. In the case of sift points, a scale and angle are provided. Start pointy using the "shed" example

      pointy shed\img_1596.pgm shed\img_1597.pgm

Now go to the file menu, select "Load Sift points" and navigate to the shed/img_1596_1597.sift file. You should see many points with circles of different sizes and different orientations. Move your mouse over a particular circle to highlight the matched pair in each image.

Example 5: Shed: Viewing epipolar lines

A fundamental matrix defines the geometric relationship between two cameras. A point in one image maps to a line in the other image. This can be visualized using Pointy. With the shed images loaded as above, open the file menu, and select "Load FM". Select the file shed/img_1596_to_1597.F. On the selection bar at the left of Pointy, choose "Show F". Now, when you move your cursor in one image, the epipolar line implied by this fundamental matrix appears in the other image.

Known bugs and todo list

These are current limitations of this program.
  • Does not support magnification or scaling of the images. This is because the canvas widget does not (easily) support magification or scaling.
  • Currently does not allow loading images from the menu. They must be specified on the command line.
  • Currently saving affine points is not implemented

Rupert Brooks <rupe at rupert brooks dot ca>
Last modified: Sun Feb 1 12:38:42 EST 2009