Better control of my client area(WIN32) - c++

Question

I am trying to make a 3 x 3 grid with a black pen on a window. however I want it to be centered, for example that my grid is inside a white space,
10% of top, right, left and bottom. and my grid will fit in the remaining 80% even when we resize the window.
Now I could make the grid but after several attempts to create the 10% area, got frustrated.
case WM_SIZE:
//get the 10% range.
cxInvalid = LOWORD(lParam) * 0.1;
cyInvalid = HIWORD(lParam) * 0.1;
//get the grid, DIVISIONS = 3
cxBlock = LOWORD(lParam) / DIVISIONS;
cyBlock = HIWORD(lParam) / DIVISIONS;
return 0;
Thanks in advaced :)
case WM_PAINT:
hdc = BeginPaint(hwnd, &ps);
for (x = 0; x < DIVISIONS; x++)
for (y = 0; y < DIVISIONS; y++)
{
Rectangle(hdc, x * cxBlock, y * cyBlock,
(x + 1) * cxBlock, (y + 1) * cyBlock);
}
EndPaint(hwnd, &ps);
return 0;

Answer 1

This is exactly the sort of problem for which Windows mapping modes are intended to be used. For the moment I'm going to assume that you want your grid to remain square, regardless of the shape of the window it's in.
One way to do that is to switch from the default MM_TEXT mapping mode to the MM_ISOTROPIC mapping mode (but if we want the grid to change shape with the surrounding window, we'd use MM_ANISOTRCOPIC instead).
Using that, we can set our window as a virtual grid of, say, 1200 x 1200 cells, and then draw our 3x3 grid on that. I've chosen 1200 x 1200 so the part we care about will be a nice, convenient 1000 x 1000 grid.
// set up the mapping mode:
RECT rect;
GetClientRect(hWnd, &rect);
SetMapMode(hDC, MM_ISOTROPIC);
SetViewportExt(rect.x, rect.y);
// The virtual width/height for our window:
static const int width = 1200;
static const int height = 1200;
SetWindowExt(width, height);
SetWindowOrg(-100, -100); // Set the virtual 0 point ~10% of the way into the window.
// And then draw the grid. We always draw in a 1000 x 1000 grid, and Windows
// scales that to the actual window size for us.
//
static const int grid_size = 1000;
static const int step = grid_size / 3;
for (int i = step; i < grid_size-1; i += step) {
MoveTo(hDC, i, 0);
LineTo(hDC, i, grid_size);
MoveTo(hDC, 0, i);
LineTo(hDC, grid_size, i);
}
To reiterate the difference between MM_ISOTROPIC and MM_ANISOTROPIC, here are screen shots of the grid. First as it's drawn with MM_ISOTROPIC:
...and then as it's drawn with MM_ANISOTROPIC: