CODEWITHSUNDEEP
c++bitmapmsdnbitmapfactory
JosiahDaniels
JosiahDaniels

Reputation: 2521

Are bitmaps supposed to be 2-byte or 4-byte aligned?

MSDN documentation seems to contradict itself:

Here it says:

For uncompressed RGB formats, the minimum stride is always the image width in bytes, rounded up to the nearest DWORD.

While here it says:

The number of bytes in each scan line. This value must be divisible by 2, because the system assumes that the bit values of a bitmap form an array that is word aligned.

So sometimes MSDN wants a 4-byte aligned stride and sometimes it wants a 2-byte aligned stride. Which is right?

To be more specific, when saving a bitmap file should I use a 4-byte stride or a 2-byte stride?

Upvotes: 3

Views: 2318

Answers (3)

DJm00n
DJm00n

Reputation: 1411

CreateBitmap/CreateBitmapIndirect/BITMAP struct - are all pre-Windows 3.0 APIs that was supposed to be used on 16-bit processors. Thats why they are using this 16-bit aligned stride.

All newer APIs are using 32bit stride aligment (sizeof(DWORD)).

You can use "newer" APIs (post-Windows 3.0) like CreateDIBitmap or CreateCompatibleBitmap/SetDiBits if your buffer have 32-bit aligned strides.

As for files - they are using BITMAPINFO/BITMAPINFOHEADER structure and implies 32bit stride aligment.

Upvotes: 0

Hans Passant
Hans Passant

Reputation: 941218

The first quote is accurate. The second dates back to the 16-bit version of Windows and did not get edited as it should have. Not entirely unusual, GDI32 docs have had a fair amount of mistakes.

Do note that the up voted answer is not accurate. Monochrome bitmaps still have a stride that's a multiple of 4, there is no special rule that makes it 2. A bit of .NET code to demonstrate this:

var bmp = new Bitmap(1, 1, System.Drawing.Imaging.PixelFormat.Format1bppIndexed);
var bdata = bmp.LockBits(new Rectangle(0, 0, 1, 1), System.Drawing.Imaging.ImageLockMode.ReadWrite, bmp.PixelFormat);
Console.WriteLine(bdata.Stride);

Output: 4

Upvotes: 6

MuertoExcobito
MuertoExcobito

Reputation: 10039

For uncompressed RGB formats, the minimum stride is always the image width in bytes, rounded up to the nearest DWORD.

Bitmaps are not necessarily always uncompressed RGB, they might be monochrome. In the BITMAP structure, the member bmBitsPixel specifies the number of bits per pixel, so it is valid for it to be 1. So, you should save RGB bitmaps with a byte stride that is a multiple of 4, and save monochrome bitmap with a stride that is a multiple of 2.

Upvotes: 0

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