Ported the dovtk-lasso example to gtk3

​ith a few years delay, I finally got around to porting the dovtk-lasso example to gtk3. It is available from:

​The included test-dovtk-lasso program draws some graphics and then allows creating and moving a semi-transparent ​caliper measuring tool on top of the graphics. It is completely flicker free even in full screen mode.

Here is the including readme file:

dovtk-lasso is a general framework for doing interactive overlay selections annotations in gtk.

See the file test-dovtk-lasso.c for an example of how to use. It draws a caliper like measurement device.

To use dovtk-lasso a callback function with the following signature has to be created:

    typedef void (*DovtkLassoDrawing)(cairo_t *cr,
                                  DovtkLassoContext Context,
                                  gpointer user_data);

The variable Context defines one of the three modes that this function is used for:

    DOVTK_LASSO_CONTEXT_PAINT - The function will be used for drawing the overlay graphics.
    DOVTK_LASSO_CONTEXT_MASK - The function will be used for creation of a low resolution mask. When in this mode the color shouldn't be changed, but has already been set up by gtk lasso.
    DOVTK_LASSO_CONTEXT_LABEL - Used to drawing label areas that may be used for picking and moving. E.g. a caliper may have a left, a right and a center area.

To start using a lasso, first create a lasso instance on any gtk widget and pass the lasso draw function as a variable:

    lasso = dovtk_lasso_create(w_draw,

Create button-press, button-release, and motion_event callbacks an use these to create and interact with an existing overlay.

In the button-press callback the label of the pixel may be queried to get information of what area should be moved:

    pixel_label  = dovtk_lasso_get_label_for_pixel(lasso, event->x, event->y);

If the graphics of the overlay should be changed e.g. by the button_press or the motion_event callbacks, dovtk_lasso should be informed by doing:



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