[fpc-devel] Slight calculation error in Bounds() procedure in Classes unit.

[Aleksa Todorovic]

On Wed, Jun 9, 2010 at 12:22, Graeme Geldenhuys <graemeg.lists at gmail.com> wrote:
> Op 2010-06-09 12:08, Aleksa Todorovic het geskryf:
>> point of view regarding coordinates. You assume that integer
>> coordinates represent center of pixel, but they actually represent
>> top-left corner of pixel. In other words, think of coordinate 0 as
>> 0.0, not 0.5 and the math will work perfectly fine :-)
>
>
> An if you read the rest of my reply, you would have seen the reason why I
> use that coordinate system, instead of the intersecting lines (and rather
> like a spreadsheet or TStringGrid). The way the OS graphics system works
> (at least for GDI & X11) - confirms my thinking. If you have a 1x1 pixel at
> (0,0) on the screen, the mouse cursor can be anywhere inside the 1x1
> rectangle (rectangle with the x in image below), and the OS reports it as
> location (0,0). My coordinate system reflects this behaviour. If only the
> top/left corner (intersecting lines) was (0,0), then my mouse cursor would
> also have to be in the top/left corner before the OS reports (0,0) - which
> it doesn't.
>
>     0   1
>   +---+---+---
> 0  | x |   |
>   +-+-+---+---
> 1  |   |   |
>   +-+-+---+---
> 2  |   |   |
>   +-+-+---+---
>

Let's imagine (it doesn't have to be true) that OS calculates mouse
coordinates as floating point values. In that case your mouse
coordinates are in the range (0.0 - 0.999999), but OS reports it as
(0, 0), which means that OS internally calls Trunc. So, you can think
of coordinates as Trunc-ed (not Round-ed!) version of real
coordinates.

>
> Regards,
>  - Graeme -
>
> --
> fpGUI Toolkit - a cross-platform GUI toolkit using Free Pascal
> http://opensoft.homeip.net/fpgui/
>
> _______________________________________________
> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
> http://lists.freepascal.org/mailman/listinfo/fpc-devel
>



-- 
Aleksa Todorovic - Lead Programmer
Eipix Entertainment
www eipix com