Selection renderers are special Heidi renderers for selecting geometry in 2D or 3D.  They can be used for determining the exact items that were selected from the draw and for hit testing; for example, when a user is selecting geometry, closest to the cursor on the screen, to be translated or moved to a new location.

There are currently five variations:

• A proximity selection renderer — wherein, all geometry within a certain radius of the picked point is selected
• An area selection renderer — a 2D selection region described by a rectangular box specified by two points
• A volume selection renderer — described by a 3D bounding box, with a Z coordinate
• A polygonal selection renderer — wherein, any geometry that is completely inside the polygon is selected
• A selection renderer combining the proximity, area and polygonal renderers.


Setup
Like any Heidi renderer, a selection renderer first needs to be dynamically loaded using the HD_Load_Dynamic_Routine with a proper DLL name and the string "HD_Selector" as the first and second parameters respectively.  Then, the application typically sets the desired selection region or volume via corresponding renderer options.

For example, to set up the area selection renderer, the application should perform the following steps:

HT_Create_Selector generator = (HT_Create_Selector) HD_Load_Dynamic_Routine ("selarea7.hdi", "HD_Selector");

    m_selector = generator ();

    m_selector.set_configure ("Selection Area", HT_Option_Value (50, 250, 50, 250));

    m_selector.establish ();

The Selection Process
Once the selection region is setup, the application starts the selection process by issuing the normal Heidi graphics call.

For example, the following code fragment initializes the selection renderer by issuing a begin_picture and sends a DC polygon to the selection renderer.  The selection process is finished by issuing an update and an end_picture. The selection information is retrieved by calling show_selection_count  which returns the number of primitives that have been selected.  If the count comes back  > 0, then the application iterates through the HT_Selector_Item list via show_next_selection_item().
 

    stack       HT_DC_Point         sprite[4];

    sprite[0] = HT_DC_Point ((float)right, (float)top,    m_sprite_depth)
    sprite[1] = HT_DC_Point ((float)left,  (float)top,   m_sprite_depth);
    sprite[2] = HT_DC_Point ((float)left,   (float)bottom, m_sprite_depth);
    sprite[3] = HT_DC_Point ((float)right,  (float)bottom, m_sprite_depth);
    m_selector.begin_picture ();
    m_selector.draw_dc_polygon (rendition, 4, sprite);
    m_selector.update ();
    m_selector.end_picture ();
    m_selected = m_selector.show_selection_count();

    stack HT_Selection_Item  selector_item;
    stack long my_key;
    stack int  my_sequnece;
    stack float my_distance;

   for (I = 0; I < m_selected; I++) {
           if (!m_selector. show_next_selection_item (&selection_item)
                    break;
           my_key = selector_item.key();
           my_sequence = selector_item.sequence();
           my_distance = selector_item.distance();

  }


HT_Selector
Besides the app-defined key, you can also get the sequence number ¾ a count of how many draw_... calls you've made with the current key.  HT_Selector_Item also provides information for the distance between the selection area and the drawables.  The selected items are sorted by the distance from near to far.

The selection class now enables you to set a specific key to be selected as opposed to just any hit. When you draw something, you can determine if it was selected, since in most cases, the key can  identify the pointer to the object.  For example, you could determine which 3 particular items were actually selected from a draw that was stored in a data base ¾ such as polygons number 35, 36 and 37.

Selection also allows multiple keys to be assigned and a combined selection renderer is available. For example, the selection renderer could include a window, nearest and perpendicular for applications such as AutoCAD.

For a detailed description, see class HT_Selector, in Appendix A, "Heidi Class Reference."
 

Selection of wide lines
The HT_Selector method, set_thin_line_limit, allows for speed enhancement during the selection of wide lines. For example, in applications that do a lot of picking, wherein the pick window is known to be a few pixels out on each side, it would save time to avoid tessellating these lines. Thus, if the lines are not very thick, using set_thin_line_limit, they can be treated as 0 weight lines to enhance picking speed.

The default setting is one, in which case any line weight larger than one will be broken down into triangles and go through a slower yet more accurate selection path.