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

[Graeme Geldenhuys]

Op 2010-06-09 11:02, Florian Klaempfl het geskryf:
> interpretation of bounds, the current behaviour is perfectly valid for
> any other uses.

Not as I see it, and described in the bug report. Think of the pixel
screen/grid like the grid of a spreadsheet (as as a magnified look of the
top corner of your screen)

     0   1
   +---+---+---
0  | x |   |
   +-+-+---+---
1  |   |   |
   +-+-+---+---
2  |   |   |
   +-+-+---+---


x is at (0,0). A pixel is always 1x1, you don't get smaller (we are not
using sub-pixels like on LCD screens, because they don't apply to all
screens).

So with that example, Width = 1 pixel and Height = 1 pixel. In the case of
Classes.Bounds() the Right and Bottom values are (1,1) which looking at the
above image will be wrong, the pixel only covers one block, so in a
screen/pixel environment, Right, Bottom is also (0,0), the same as Top/Left.


     0   1
   +---+---+---
0  | x | x |
   +-+-+---+---
1  | x | x |
   +-+-+---+---
2  |   |   |
   +-+-+---+---

In this example we have a rectangle. Top/Left = (0,0) and Bottom/Right =
(1,1). This means Width = 2 pixels and Height = 2 pixels. Clearly visible
in the image above.

But if we call Classes.Bounds(0,0,2,2) it reports Bottom/Right as (2,2)
which is wrong (again looking at the image above as reference). This is my
point. Bounds() cannot be used in this case in a graphical environment.


The reason I don't use a grid layout as below (as Mattias hinted at), is
because if the mouse is over the x, the OS (graphics system) reports its
position as (0,0). You don't have to have the mouse pointer over the
cross-section lines (0,0) as indicated in the image below. The whole square
containing the x is referenced as (0,0), hence my grid layout as show in
the images above. The same grid layout as used in TStringGrid or a
spreadsheet etc.

   0   1
0  +---+---+---
   | x |   |
1  +-+-+---+---
   |   |   |
2  +-+-+---+---
   |   |   |
3  +-+-+---+---



Regards,
  - Graeme -

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