Scaling Bitmaps

The memory device context also comes to the rescue when we need to scale fonts to a different display resolution or aspect ratio. I created the four bitmaps used in GRAFMENU to be the correct size for a display that has a system font height of 8 pixels and width of 4 pixels. For other system font dimensions, the bitmap has to be stretched. This is done in GRAFMENU's StretchBitmap function.

The first step is to get the device context for the screen, obtain the text metrics for the system font, and create two memory device contexts:

hdc = CreateIC ("DISPLAY", NULL, NULL, NULL) ;

GetTextMetrics (hdc, &tm) ;

hdcMem1 = CreateCompatibleDC (hdc) ;

hdcMem2 = CreateCompatibleDC (hdc) ;

DeleteDC (hdc) ;

The bitmap handle passed to the function is hBitmap1. The program can obtain the dimensions of this bitmap using GetObject:

GetObject (hBitmap1, sizeof (BITMAP), (LPSTR) &bm1) ;

This copies the dimensions into a structure bm1 of type BITMAP. The structure bm2 is set equal to bm1, and then certain fields are modified based on the system font dimensions:

bm2 = bm1 ;

bm2.bmWidth = (tm.tmAveCharWidth * bm2.bmWidth) / 4 ;

bm2.bmHeight = (tm.tmHeight * bm2.bmHeight) / 8 ;

bm2.bmWidthBytes = ((bm2.bmWidth + 15) / 16) * 2 ;

Then a new bitmap with handle hBitmap2 can be created based on the altered dimensions:

hBitmap2 = CreateBitmapIndirect (&bm2) ;

You can then select these two bitmaps into the two memory display contexts:

SelectObject (hdcMem1, hBitmap1) ;

SelectObject (hdcMem2, hBitmap2) ;

We want to copy the first bitmap to the second bitmap and stretch it in the process. This involves the StretchBlt call:

StretchBlt (hdcMem2, 0, 0, bm2.bmWidth, bm2.bmHeight,

hdcMem1, 0, 0, bm1.bmWidth, bm1.bmHeight, SRCCOPY) ;

Now the second bitmap has the properly scaled bitmap. We'll use that one in the menu. Cleanup is simple:

DeleteDC (hdcMem1) ;

DeleteDC (hdcMem2) ;

DeleteObject (hBitmap1) ;