Graphic driver development

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Writing a new graphic driver

The graphic driver tells NavIt how to render the map using a specific library. The same graphic driver can be used in several GUIs. For example, you can render the map in OpenGL with the GTK GUI and the SDL GUI.

When the GUI handles the user interaction, the graphic driver is called internally by NavIt when necessary.

Here is what you need to write your own graphic driver:

To register the graphics_driver, call plugin_register_graphics_type("your_type", graphics_new_func). "your_type" is what you will need to put in navit instance, in navit.xml.

  • graphics_new_func(struct graphics_methods *meth) should fill meth and return a private pointer representing the graphics for the driver

See graphics.h for the graphics_methods

  • graphics_destroy: Forget the private data, graphics no longer needed
    • draw_mode: Is either called with draw_mode_begin (the display list will be drawn), draw_mode_end (the display list is finished) or draw_mode_cursor (the cursor is drawn)
  • draw_lines: Draw a polyline, parameters are the graphics context, the point count and the points
  • draw_polygon: Like draw_lines, but draw a filled polygon instead
  • draw_rectangle, draw_circle: Obvious
  • draw_text: Render text. dx and dy is the direction for the text, fg and bg Foreground and Background graphic contexts, font a driver private font structure
  • draw_image draws an image which has been created before
  • draw_image_warp is similar, but can adjust the image on coordinates, used for bitmap maps
  • draw_restore restores the damaged area from the cursor
  • font_new: Creates a new font, used for draw_text. Parameters are font_methods (which is just a destroy function) and the size of the font
  • gc_new: Creates a new graphics context (Not usefull for every renderers, imagine it as the pen you are drawing with) Methods are:
    • gc_destroy,
    • gc_set_linewidth,
    • gc_set_dashes (for dotted lines etc.),
    • gc_set_foreground,
    • gc_set_background (don't know if this one is used)
  • background_gc: Sets the background graphics context (Background of the map). This one will be removed soon
  • overlay_new: Creates a new overlay. Currently unused
  • image_new: Creates a new image for use with draw_image or draw_image_warp. Methods is just a destroy function
  • get_data: Returns a private pointer for the gui, representing an object specified in type, or NULL if the graphics driver is unable to provide such an object
  • register_resize_callback: Registers a function which should be called if the graphics area changes its size
  • register_button_callback: Registers a function which should be called when a mouse button is pressed or released
  • register_motion_callback: Registers a function which should be called when the mouse is moved


So, now the stuff that you (probably) really need:

  • draw_mode: Useful for example to swap the opengl buffers when called with draw_mode_end
  • draw_lines, draw_polygon to draw the stuff
  • draw_text: Implement aftger draw_lines and draw_polygon works
  • draw_image: Maybe later, only used for POIs
  • draw_image_warp: A very optional feature
  • font_new: When draw_text is implemented
  • gc_new: Required, you should at least support gc_set_linewidth and gc_set_foreground
  • image_new: Needed when draw_image will be implemented
  • get_data: Required! At least a dummy.
  • register_*_callback: Optional, you can also handle it in your GUI
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