[Freeciv-Dev] (PR#8703) design for an error-free mapview

["Jason Short" <jdorje@xxxxxxxxxxxxxxxxxxxxx>]

<URL: http://rt.freeciv.org/Ticket/Display.html?id=8703 >

With the recent changes to update_map_canvas I think we are not too far 
from having a mapview that is both error-free and reasonably fast.

It's pretty obvious what error-free means.  The current mapview is not 
error-free.  Aside from some miscellaneous bugs (occasional flickering 
city names or sprites) there is the problem that city descriptions may 
be drawn on top of each other.  This isn't correct if the text is 
anti-aliased.

Reasonably fast means faster than the current mapview.  The current 
mapview can easily be made faster; see PR#8603.

A while ago I made a diagram showing the difference between "partial 
tile" and "tricky buffered" drawing.  Up until now we've done partial 
tile drawing only.  But this won't work for city descriptions.  So to 
have things be fully correct we need tricky buffered drawing.  This may 
or may not invalidate the current need for partial tile drawing.

So my design is this:

- We need two backing stores.  One will be the main backing store, the 
other a temp store.  This allows us to manually enforce a "clip rect" 
(which is not portable between GUIs afaik).

- When drawing an area via update_map_canvas, you draw the area onto the 
temp buffer.  First draw terrain, the way it's done now.  Then call 
show_city_descriptions(...) to put the city descriptions on top of this 
terrain.  Finally copy the selected area of the buffer over to the main 
backing store.

- show_city_descriptions should therefore take a rectangle over which 
you are going to update the descriptions.  This is quite easy.

- When update_city_descriptions_visible() is called, you must redraw 
everything.  This is slow.

- There should be a new function update_city_description(...) which 
takes a pointer to a city.  Again everything in this area must be 
redrawn, but now it's a much smaller area.  This function should almost 
entirely replace update_city_descriptions_visible().

- When the map origin is changed, we often don't have to call a full 
update_map_canvas_visible.  Instead we can copy the area that remains in 
the mapview (having a second buffer helps here as well) then make one or 
two update_map_canvas() calls to redraw the rest of the area.  But for 
this to work properly update_map_canvas must obey the clip rect it's given.

I'd like to work toward this goal in steps.  PR#8603 may be the first 
such step.

jason