Using Coordinate Spaces and Transformations
This section contains an example that demonstrates the following tasks:
- Drawing graphics with predefined units.
- Centering graphics in the application's client area.
- Scaling graphics output to half its original size.
- Translating graphics output 3/4 of an inch to the right.
- Rotating graphics 30 degrees.
- Shearing graphics output along the x-axis.
- Reflecting graphics output about an imaginary horizontal axis drawn through
its midpoint.
The following example was used to create the illustrations that appear earlier
in this topic.
- oid TransformAndDraw(int iTransform, HWND hWnd)
{
HDC hDC;
XFORM xForm;
RECT rect;
/* Retrieve a DC handle for the application's window. */
hDC = GetDC(hWnd);
/*
* Set the mapping mode to LOENGLISH. This moves the
* client area origin from the upper left corner of the
* window to the lower left corner (this also reorients
* the y-axis so that drawing operations occur in a true
* Cartesian space). It guarantees portability so that
* the object drawn retains its dimensions on any
* display running Windows.
*/
SetMapMode(hDC, MM_LOENGLISH);
/*
* Set the appropriate world transformation (based on the
* user's menu selection).
*/
switch (iTransform) {
case SCALE: /* Scale to 1/2 of the original size. */
xForm.eM11 = (FLOAT) 0.5;
xForm.eM12 = (FLOAT) 0.0;
xForm.eM21 = (FLOAT) 0.0;
xForm.eM22 = (FLOAT) 0.5;
xForm.eDx = (FLOAT) 0.0;
xForm.eDy = (FLOAT) 0.0;
SetWorldTransform(hDC, &xForm);
break;
case TRANSLATE: /* Translate right by 3/4 inch. */
xForm.eM11 = (FLOAT) 1.0;
xForm.eM12 = (FLOAT) 0.0;
xForm.eM21 = (FLOAT) 0.0;
xForm.eM22 = (FLOAT) 1.0;
xForm.eDx = (FLOAT) 75.0;
xForm.eDy = (FLOAT) 0.0;
SetWorldTransform(hDC, &xForm);
break;
case ROTATE: /* Rotate 30 degrees counterclockwise. */
xForm.eM11 = (FLOAT) 0.8660;
xForm.eM12 = (FLOAT) 0.5000;
xForm.eM21 = (FLOAT) -0.5000;
xForm.eM22 = (FLOAT) 0.8660;
xForm.eDx = (FLOAT) 0.0;
xForm.eDy = (FLOAT) 0.0;
SetWorldTransform(hDC, &xForm);
break;
case SHEAR: /* Shear along the x-axis with a */
/* proportionality constant of 1.0. */
xForm.eM11 = (FLOAT) 1.0;
xForm.eM12 = (FLOAT) 1.0;
xForm.eM21 = (FLOAT) 0.0;
xForm.eM22 = (FLOAT) 1.0;
xForm.eDx = (FLOAT) 0.0;
xForm.eDy = (FLOAT) 0.0;
SetWorldTransform(hDC, &xForm);
break;
case REFLECT: /* Reflect about a horizontal axis. */
xForm.eM11 = (FLOAT) 1.0;
xForm.eM12 = (FLOAT) 0.0;
xForm.eM21 = (FLOAT) 0.0;
xForm.eM22 = (FLOAT) -1.0;
xForm.eDx = (FLOAT) 0.0;
xForm.eDy = (FLOAT) 0.0;
SetWorldTransform(hDC, &xForm);
break;
case NORMAL: /* Set the unity transformation. */
xForm.eM11 = (FLOAT) 1.0;
xForm.eM12 = (FLOAT) 0.0;
xForm.eM21 = (FLOAT) 0.0;
xForm.eM22 = (FLOAT) 1.0;
xForm.eDx = (FLOAT) 0.0;
xForm.eDy = (FLOAT) 0.0;
SetWorldTransform(hDC, &xForm);
break;
}
/* Find the midpoint of the client area. */
GetClientRect(hWnd, (LPRECT) &rect);
DPtoLP(hDC, (LPPOINT) &rect, 2);
/* Select a hollow brush. */
SelectObject(hDC, GetStockObject(HOLLOW_BRUSH));
/* Draw the exterior circle. */
Ellipse(hDC, (rect.right / 2 - 100), (rect.bottom / 2 + 100),
(rect.right / 2 + 100), (rect.bottom / 2 - 100));
/* Draw the interior circle. */
Ellipse(hDC, (rect.right / 2 -94), (rect.bottom / 2 + 94),
(rect.right / 2 + 94), (rect.bottom / 2 - 94));
/* Draw the key. */
Rectangle(hDC, (rect.right / 2 - 13), (rect.bottom / 2 + 113),
(rect.right / 2 + 13), (rect.bottom / 2 + 50));
Rectangle(hDC, (rect.right / 2 - 13), (rect.bottom / 2 + 96),
(rect.right / 2 + 13), (rect.bottom / 2 + 50));
/* Draw the horizontal lines. */
MoveToEx(hDC, (rect.right / 2 - 150), (rect.bottom / 2 + 0), NULL);
LineTo(hDC, (rect.right / 2 - 16), (rect.bottom / 2 + 0));
MoveToEx(hDC, (rect.right / 2 - 13), (rect.bottom / 2 + 0), NULL);
LineTo(hDC, (rect.right / 2 + 13), (rect.bottom / 2 + 0));
MoveToEx(hDC, (rect.right / 2 + 16), (rect.bottom / 2 + 0), NULL);
LineTo(hDC, (rect.right / 2 + 150), (rect.bottom / 2 + 0));
/* Draw the vertical lines. */
MoveToEx(hDC, (rect.right / 2 + 0), (rect.bottom / 2 - 150), NULL);
LineTo(hDC, (rect.right / 2 + 0), (rect.bottom / 2 - 16));
MoveToEx(hDC, (rect.right / 2 + 0), (rect.bottom / 2 - 13), NULL);
LineTo(hDC, (rect.right / 2 + 0), (rect.bottom / 2 + 13));
MoveToEx(hDC, (rect.right / 2 + 0), (rect.bottom / 2 + 16), NULL);
LineTo(hDC, (rect.right / 2 + 0), (rect.bottom / 2 + 150));
ReleaseDC(hWnd, hDC);
}
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