A complete description of GTK can be found at the
GTK Website. A few points are
- GTK+ stands for "The GIMP Toolkit", and was created as part of the GIMP
("GNU Image Manipulation Program") implementation. It is used for
creating graphical user interfaces.
- It is written in c, and in version 1.2.x (the version that OpenEV
uses), it is composed of three main libraries:
- glib- provides definitions and routines for lists, trees, memory
allocation,basic data types (points, lines), etc.
- gdk ("The GIMP Drawing Kit")- provides functions to trap keyboard and
mouse events, and also to create a drawable window and draw basic
objects such as lines, points, rectangles etc. The code in gdk is
highly platform-dependent: event trapping, window drawing are
very different between X-windows systems such as Linux, and Windows.
gdk wraps these functions into a common API so that the gtk code can
be less platform dependent. Events include button press/click/releases,
key pressing, cursor motion, etc.
- gtk - provides more advanced widgets such as buttons, toolbars,
menus, tree views, file selection dialogs. For instance, the gtk
button (gtkbutton.c) contains functions that know how to draw a button
in a window, and functions to execute when gdk events are trapped
(eg. what to do when the button is pressed, how to show it is
raised/lowered, etc). These are based heavily on gdk functions.
- All of OpenEV's windows (the main viewing window, dialogs, etc.) build
upon the GtkWindow class.
- There is a helpful online tutorial describing GTK's python bindings
An example of OpenGL commands follows:
- OpenGL defines a hardware-independent software interface to graphics
hardware. It specifies how various drawing-related functions should
be defined (names, parameters, etc.). The advantage of this is that
code using this API can work with any hardware that supplies an OpenGL
- Mesa implements this API in software- if /lib/Mesa
is added to your LD_LIBRARY_PATH variable on Linux/Unix, Mesa (libGL.so,
libGLU.so) will be used. Hardware-accelerated platforms provide the
interface without the need for Mesa, and are faster (though sometimes
flakey). Mesa relies on having
the "glx" extension on X-Windows systems and the "wgl" routines on
Windows. These extend the window systems enough so that the basic OpenGL
commands can be defined in an efficient manner. For instance, on X
servers, the glx extension allows OpenGL to bypass the X server's
involvement and render graphics directly in places where it is necessary for
performance. For more on Mesa, see this site.
- The actual OpenGL commands in OpenEV are all at the c-level.
gvgeocoord x, y;
x+0, y+0 );
x+1, y+1 );
x+2, y+2 );
x+3, y+3 );
glColor3f(1.0, 0.5, 0.0);
This is the piece of code that draws the little orange rectangle when
you left-click and drag to zoom in on a region. It takes the corner
coordinates of the area you dragged out, maps them into georeferenced
coordinates (the gv_view_area_map_pointer calls), sets the drawing
colour to orange (the glColor3f command), specifies that it is beginning
to draw a closed loop (the glBegin call), draws the vertices, then
ends the loop with glEnd. Mesa or your hardware driver provide the
definitions of the OpenGL commands (glColor3f, glBegin, etc.). For
more, see the gv_view_area_expose function in gvviewarea.c, or the draw
functions in the layer c-code (gvrasterlayer.c, gvshapeslayer.c).
GTK defines gtk_drawing_area as a base class for widgets that need a box
to draw into. gtkglarea extends this class to allow it to accept OpenGL
commands for drawing. OpenEV's gvviewarea adds the concept of layers
and tools, and adds the OpenGL commands necessary for georeferenced
display, 3D, etc. For more information, see
Proj is used by GDAL to do projection transformations, for instance
between lat/longs and utm. OpenEV will function without proj, but in certain
cases the georeferenced display will not be correct. For instance, when a
georeferenced MFF2 file is read in,
corner coordinates are specified in lat/longs even if the image is
projected in utm coordinates. In the utm case, GDAL must convert the corner
coordinates from lat/longs to utm prior to display so that the image does
not appear incorrectly warped. Proj is used in this conversion. For more
information see the Proj website.
Provides plotting functionality. This is used in OpenEV through the
gvplot.py wrapper. For more information, see the
Developer Course Outline