[Freeciv-Dev] Re: (PR#4648) how to do wrapping in map_to_canvas_pos?

["Gregory Berkolaiko" <Gregory.Berkolaiko@xxxxxxxxxxxx>]

On Wed, 23 Jul 2003, Jason Short wrote:

> Gregory Berkolaiko wrote:
> > On Tue, 22 Jul 2003, Jason Short wrote:
> > 
> > 
> >>Gregory Berkolaiko wrote:
> >>
> >>>I do not understand why map_center_[xy] should stay unnormalized?
> >>>Let them be normalized, it should work just as well.  It would save you 
> >>>from removing CHECK_MAP_POSs, although you can do that to save time.
> >>
> >>No, the whole point is that you are looking for an absolute (non-wrapped 
> >>or "unnormalized") position.
> >>
> >>First consider that the center tile might not even be real.  If we do 
> >>things right, though, this shouldn't happen...
> >>
> >>But consider an 80x50 map with standard wrapping, non-iso tileset.  Say 
> >>x0=70 and the mapview window is 22 tiles wide.  Now the center tile is 
> >>at x=81 or x=1 once it's been normalized.
> >>
> >>Now you want to "unnormalize" some coordinates.  Since this example is 
> >>very simple, it should be easy to see the "unnormalized" values should 
> >>have X values in the range [70,150).
> >>
> >>Now start with x=1.  It's distance vector has dx=0.  If we use the 
> >>normalized value of center_map_x this will give us an unnormalized value 
> >>x=1, giving canvas_x=-69*NORMAL_TILE_WIDTH - way off the screen.  The 
> >>correct value should be x=81 giving canvas_x=11*NORMAL_TILE_WIDTH.
> > 
> > 
> > Err.  I don't quite understand the paragraph above.  OUr goal is to get 
> > canvas positions, isn't it.  So we get distance verctor dx = 0, therefore
> >     canvas_x = center_canvas_x + dx*NORMAL_TILE_WIDTH
> > gives the correct answer.
> 
> It only gives the correct answer if center_canvas_[xy] is unwrapped.  If 

What do you mean center_canvas is unwrapped.  It's canvas!
center_canvas_x = mapview_canvas.width / 2;
center_canvas_y = mapview_canvas.height / 2;

> you normalize it then you'll get something completely different.  With 
> center_map_x=81 we'll get a position 11 tiles right of the origin, 
> whereas with center_map_x=1 we'll get a position 69 tiles left of the 
> origin (which is wrong).
> 
> This is because the calculation isn't done as
> 
>    canvas_x = center_canvas_x + dx * NORMAL_TILE_WIDTH
> 
> but as
> 
>    map_x = center_map_x + dx;
>    canvas_x = NORMAL_TILE_WIDTH * (map_x - map_view_x0);
> 
> the former method is possible but means more changes (particularly for 
> iso-view).

So use the former method, it is more transperent, I think.

> As a side note, (map_x, map_y) may be unreal and so map_distance_vector 
> has to deal with unreal positions in either case.

It should be, otherwise more than 50% of the window is black.

> > Another issue is that I am not 100% sure your approach will give the right 
> > answer for iso tiles.
> 
> I'm pretty certain it does.  You can try using iso-view with a 40x25 
> map.  If you enlarge the window sufficiently you will be able to see the 
> whole map, approximately centered around the center.

I'll think more about it.

G.