Re: Error when drawing grid, grid lines a bit too short

From:

Eric Lilja <mindcoolerremoveme@gmail.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Mon, 02 Jul 2007 22:11:46 +0200

Message-ID:

<eIPy3UOvHHA.412@TK2MSFTNGP04.phx.gbl>

Ah, I forgot to take the x and y offsets into account. So I need add the
offset and subtract the width of the grid line. So the code changes into:
for vertical grid lines:
dc->LineTo(x, (grid_bar_width * (num_rows_ + 1) + num_rows_ *
cell_height) + initial_y_offset - grid_bar_width);
for horizontal grid lines:
dc->LineTo((grid_bar_width * (num_cols_ + 1) + num_cols_ * cell_width) +
initial_x_offset - grid_bar_width, y);

renamed grid_bar_thickness to grid_bar_width.

- Eric

Eric Lilja wrote:

Hello, as I've mentioned in some other post I'm making a simple "Game of
Life" with a small, fixed-sized grid. Anyway, the function I used for
drawing the actual grid was written ages ago when doing some pure Win32
programming and it was full of magic numbers, assuming a 16*16 grid. I
tried to rewrite it without magic numbers and to take the actual size
into consideration. Right now each cell in the grid is 20*20 and I have
horizontal and vertical bars that are four pixels wide. The x and y
offset is 20 pixels.

Hope I explained it properly, here's the code:

void MyView::DrawGrid(CDC *dc)
{
   static const int cell_width = 20;
   static const int cell_height = 20;

   static const int initial_x_offset = 20;
   static const int initial_y_offset = 20;

   static const int grid_bar_thickness = 4;
   static CPen pen(PS_SOLID, grid_bar_thickness, RGB(0x00, 0x00, 0xFF));

   dc->SelectObject(&pen);

   /* Drawing vertical lines. */
   int x = initial_x_offset;
   int y = initial_y_offset;

   for (int i = 0; i < (num_cols_ + 1); ++i)
   {
      dc->MoveTo(x, y);
      dc->LineTo(x, grid_bar_thickness * (num_rows_ + 1) + num_rows_ *
cell_height);

      x += cell_width + grid_bar_thickness;
   }

   /* Drawing horizontal lines. */
   x = initial_x_offset;
   y = initial_y_offset;

   for (int i = 0; i < (num_rows_ + 1); ++i)
   {
      dc->MoveTo(x, y);
      dc->LineTo(grid_bar_thickness * (num_cols_ + 1) + num_cols_ *
cell_width, y);

      y += cell_height + grid_bar_thickness;
   }
}

I'm testing a grid with 16 rows and 18 columns. I suspect I'm missing to
take something into account when calling LineTo() because the grid lines
are not as long as they should be, neither vertically nor horizontally.
Here's a screenshot of how it looks:
http://student.stunet.se/hivemind/grid.png

Where am I going wrong here? I've made sure the values of num_rows_ and
num_cols_ are correct using the debugger. Also, I'm getting the cells
coordinates correct I just have problems with the grid lines. :)

- Eric