Hi,
Are you sure it would be that hard to do in OpenGL?
In my GtkGlExt based animation app (kudu.sourceforge.net) I have to draw
a selection rectangle based on a mouse drag, which is essentially a 2D
drawing over a 3D scene (including rotations and zooming), I simply
switched to a orthographic viewing volume and then draw my rectangle,
like this:
int viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);
glPushMatrix();
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
gluOrtho2D(0.0, viewport[2], 0.0, viewport[3]);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
/* Perform simple 2D drawing here */
/* sx, sy, ex, ey being (in my case) the four points of the rectangle */
glBegin(GL_LINE_LOOP);
glVertex2i(sx, viewport[3] - sy);
glVertex2i(sx, viewport[3] - ey);
glVertex2i(ex, viewport[3] - ey);
glVertex2i(ex, viewport[3] - sy);
glEnd();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glPopMatrix();
Unless I've misunderstood your question, I don't see why that wouldn't
work...
Apologies to all for the somewhat off-topic code pasting.. ;-)
Daniel Pekelharing
On Thu, 2006-02-09 at 08:40 -0600, Douglas Vechinski wrote:
> I'm writing a program using Gtk and GtkGLExt. I have a drawing area
> which is using OpenGL to display a scene. I receive mouse motion events
> to rotate, translate, zoom, on this scene. In the display window, I
> would also like to draw some primitives on top of the scene rendered by
> OpenGL. For example, I would like to draw a small coordinate axis in
> one corner of the window that is oriented in the same manner as the
> scene. (While this particular example might be done via OpenGL, I
> believe a lot of extra work would need to be done to keep in in a
> particular location on the screen as and a consistent size as rotation,
> zooming, and panning are performed. Knowing the current orientation and
> view location, I can calculate the appropriate transform and draw three
> lines of different colors.)
>
> At the moment, I'm just drawing some lines on the screen using
> gdk_draw_lines in the Expose event callback function which is
> responsible for redrawing the OpenGL scene. I've tried placing the
> gdk_draw_lines before and after swapping the front and back buffers. I
> see the lines, but they flicker (when mouse dragging is used to orient,
> zoom, pan the scene) and typically do not show up in the last update
> when the mouse stops moving.
>
> The coordinate axes are not the only thing I wish to draw, I may also
> wish to draw other lines, circles, etc. So in general my question is
> what is the proper way of performing 2D drawing (which may not have
> anything to do with the underlying 3D OpenGL scene) on top of an OpenGL
> drawing area and have it show up with out flicker.
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