INF: Rotating a Bitmap by 90 Degrees

ID Number: Q77127

3.00

WINDOWS

Summary:

There are no Windows functions that directly rotate bitmaps. All

techniques for rotating bitmaps in Windows involve copying the rows

from a source bitmap into the columns of a destination bitmap. The

following contains code for rotating a bitmap using GetPixel() and

SetPixel(), and contains an outline of code for rotating device

independent bitmaps (DIB).

More Information:

A device dependent bitmap (DDB) can be rotated using the GetPixel()

and SetPixel() functions. To rotate the bitmap, use the following

code:

HBITMAP Rotate90(HDC hDC, HBITMAP hSourceBitmap)

{

HBITMAP hOldSourceBitmap, hOldDestBitmap, hDestBitmap;

HDC hMemSrc, hMemDest;

int height, width;

int i, j;

BITMAP iSrcBitmap;

// Step 1: Create a memory DC for the source and destination bitmaps

// compatible with the device used.

hMemSrc = CreateCompatibleDC(hDC);

hMemDest= CreateCompatibleDC(hDC);

// Step 2: Get the height and width of the source bitmap.

GetObject(hSourceBitmap, sizeof(BITMAP), (LPSTR)&SrcBitmap);

width = SrcBitmap.bmWidth;

height = SrcBitmap.bmHeight;

// Step 3: Select the source bitmap into the source DC. Create a

// destination bitmap, and select it into the destination DC.

hOldSourceBitmap = SelectObject(hMemSrc, hSourceBitmap);

hDestBitmap = CreateBitmap(height, width, SrcBitmap.bmPlanes,

SrcBitmap.bmBitsPixel, NULL);

if (!hDestBitmap)

return(hDestBitmap);

hOldDestBitmap = SelectObject(hMemDest, hDestBitmap);

// Step 4: Copy the pixels from the source to the destination.

for (i = 0; i < width; ++i)

for (j = 0; j < height; ++j)

SetPixel(hMemDest, j, width - 1 - i,

GetPixel(hMemSrc, i, j));

// Step 5: Destroy the DCs.

SelectObject(hMemSrc, hOldSourceBitmap);

SelectObject(hMemDest, hOldDestBitmap);

DeleteDC(hMemDest);

DeleteDC(hMemSrc);

// Step 6: Return the rotated bitmap.

return(hDestBitmap);

}

If the bitmap is larger, using GetPixel() and SetPixel() may be too

slow. If this is the case, there are two options:

1. If the contents of the bitmap do not change, create two versions of

the bitmap, the normal version and one that is rotated by 90

degrees. Load the appropriate bitmap as required.

-or-

2. Find some way to manipulate the bits of the bitmap that is faster

than using SetPixel() and GetPixel(). The best way to do this is to

convert the bitmap to a device independent bitmap. The following

four steps detail how to create the DIB and to perform the

rotation:

a. Use GetDIBits to convert the bitmap to a device independent

format. It is necessary to create a BITMAPINFO structure

appropriate for the bitmap. This will write the bitmap as a

series of scanlines. Each scanline is padded so that it is DWORD

aligned.

b. Allocate memory for the destination bitmap. This bitmap requires

as many scanlines as the width of the source bitmap. Each

scanline is as many pixels wide as the height of the source

bitmap. Also, the scanlines must be DWORD aligned.

c. For each scanline in the source bitmap, copy the pixels to the

appropriate column in the destination bitmap. Note: The format

for each scanline depends upon the number of bits per pixel. See

the BITMAPINFO documentation for a description of the possible

formats.

d. Use SetDIBits() to copy the device independent bits into a

device dependent bitmap. Another BITMAPINFO structure,

appropriate for the destination device is required for this

step.

The four steps of this method require much more work than is required

if GetPixel() and SetPixel() are used; however, this method may be

faster because it directly manipulates the bits in the bitmap.

Additional reference words: 3.00