Bitmap Stride And 4 bytes Relation?

Question

Whats does this sentence mean:

The Stride property, holds the width of one row in bytes. The size of a row however may not be an exact multiple of the pixel size because for efficiency, the system ensures that the data is packed into rows that begin on a four byte boundary and are padded out to a multiple of four bytes.

Answer 1

Stride is padded. That means that it gets rounded up to the nearest multiple of 4. (assuming 8 bit gray, or 8 bits per pixel):

Width | stride
--------------
1     | 4
2     | 4
3     | 4
4     | 4
5     | 8
6     | 8
7     | 8
8     | 8
9     | 12
10    | 12
11    | 12
12    | 12

etc.

In C#, you might implement this like this:

static int PaddedRowWidth(int bitsPerPixel, int w, int padToNBytes) 
{
    if (padToNBytes == 0)
        throw new ArgumentOutOfRangeException("padToNBytes", "pad value must be greater than 0.");
    int padBits = 8* padToNBytes;
    return ((w * bitsPerPixel + (padBits-1)) / padBits) * padToNBytes;
}

static int RowStride(int bitsPerPixel, int width) { return PaddedRowWidth(bitsPerPixel, width, 4); }

Answer 2

That means if your image width is 17 pixels and with 3 bytes for color, you get 51 bytes. So your image width in bytes is 51 bytes, then the stride is 52 bytes, which is the image width in bytes rounded up to the next 4-byte boundary.

Answer 3

Let me give you an example:

This means that if the width is 160, stride will be 160. But if width is 161, then stride will be 164.