[wx-dev] mysterious wxImage::Paste() logic

[Tim Kosse]

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Vadim Zeitlin wrote:
>  Does anyone know what does wxImage::Paste() do when its x/y parameters are
> negative? I don't understand this code at all and can't make any sense of
> it nor document it (which I'd like to do).

> int xx = 0;
> int yy = 0;
> int width = image.GetWidth();
> int height = image.GetHeight();
> 
> if (x < 0)
> {
> 	xx = -x;
> 	width += x;
> }
> if (y < 0)
> {
> 	yy = -y;
> 	height += y;
> }

xx and yy are offsets into the source image.
If x and y are >=0, we can start copying from the source image at the
topleft corner, in other words xx and yy are 0.
If x and y are less than zero, we have to clip the source, starting with
the rectangle starting at position xx = -x and yy = -y, extending to the
lower-right.

Note I omit the cases (postive, negative) and (negative, positive here,
they are analogous.

> if ((x+xx)+width > M_IMGDATA->m_width)
> width = M_IMGDATA->m_width - (x+xx);
> if ((y+yy)+height > M_IMGDATA->m_height)
> height = M_IMGDATA->m_height - (y+yy);

case 1: x >= 0:
  if (x + width > M_IMGDATA->m_width) // equals width >
M_IMGDATA->m_width - x
    width = M_IMGDATA->m_width - x

case 2: x < 0
  if (width > M_IMGDATA->m_width)
    width = M_IMGDATA->m_width

With that information, the loops later in wxImage::Paste are easy to
understand:

It takes the data starting at coordindates xx and yy and pastes it into
the target image at coordinates (x, y) if >=0 or (0, 0) otherwise.

Tim
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