In the MainWndProc function, add code to process WM_LBUTTONUP messages. This code defines the source and destination rectangles for painting, and forces a repaint. The code should appear as follows:
case WM_LBUTTONUP:
EndGDIX = LOWORD(lParam);
EndGDIY = HIWORD(lParam);
/* Use the mouse position's percentage offsets in the image rectangle
to determine the pixel offsets in the bitmap.
Using percentages allows for the possibility that the image is zoomed. */
EndPixelX = MulDiv(BITMAPWIDTH(&LeadBitmap), EndGDIX - rLeadDest.left, DisplayWidth);
EndPixelY = MulDiv(BITMAPHEIGHT(&LeadBitmap), EndGDIY - rLeadDest.top, DisplayHeight);
/* Do nothing if no area is defined */
if ((EndPixelX == StartPixelX) || (EndPixelY == StartPixelY))
return (0);
/* Get the source rectangle coordinates for painting.
Allow for different mouse drag directions */
if (StartGDIX < EndGDIX)
{
rLeadSource.left = StartPixelX;
rLeadSource.right = EndPixelX;
}
else
{
rLeadSource.left = EndPixelX;
rLeadSource.right = StartPixelX;
}
if (StartGDIY < EndGDIY)
{
rLeadSource.top = StartPixelY;
rLeadSource.bottom = EndPixelY;
}
else
{
rLeadSource.top = EndPixelY;
rLeadSource.bottom = StartPixelY;
}
/* Use this local function (described in Fitting an Image to a Window) to
center the cropped display in the client area. */
CalcDisplay (rClientSize.right, /* Width allowed */
rClientSize.bottom, /* Height allowed */
rLeadSource.right - rLeadSource.left, /* Width factor, for aspect ratio */
rLeadSource.bottom - rLeadSource.top, /* Height factor, for aspect ratio */
&DisplayTop, /* Resulting top value, for centering */
&DisplayLeft, /* Resulting left value, for centering */
&DisplayWidth, /* Resulting width value */
&DisplayHeight); /* Resulting height value */
/* Set the destination rectangle for painting */
SetRect(&rLeadDest, DisplayLeft, DisplayTop,
DisplayLeft + DisplayWidth, DisplayTop + DisplayHeight);
/* If necessary, translate the source rectangle to the bitmap's view perspective */
if (LeadBitmap.ViewPerspective != TOP_LEFT)
L_RectToBitmap(&LeadBitmap, TOP_LEFT, &rLeadSource);
/* Repaint the whole client area */
InvalidateRect(hWnd,NULL,TRUE);
return (0);
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