关于BMP文件格式中RowSize与PixelArraySize含义及相关操作的技术咨询

Hey fellow dev! I totally get the confusion around RowSize and PixelArraySize in BMP files—this alignment rule trips up almost everyone when writing their first BMP reader. Let's break this down step by step.

RowSize isn't just the "logical" size of a row (i.e., image_width * bytes_per_pixel). BMP has a mandatory 4-byte alignment requirement for each row of pixel data, which dates back to older display hardware that could read memory more efficiently in 4-byte chunks.

To calculate RowSize correctly, use this formula:

RowSize = ((BitsPerPixel * ImageWidth + 31) // 32) * 4

Let's unpack this with an example: say you have a 10-pixel-wide image using 24-bit color (3 bytes per pixel).

• Logical row size: 10 * 3 = 30 bytes
• Applying the formula: (24*10 +31) = 271 → 271 //32 =8 → 8*4=32 bytes
  That extra 2 bytes is padding added to make the row fit the 4-byte boundary.

PixelArraySize is the total number of bytes occupied by the entire pixel array in the BMP file. Since each row uses RowSize bytes (including padding), you calculate it by multiplying RowSize by the image's height:

PixelArraySize = RowSize * ImageHeight

Important: Don't try to calculate this as (ImageWidth * ImageHeight * BitsPerPixel) //8—that ignores the padding bytes and will give you a wrong total, leading to corrupted image data when reading.

For the same 10x5, 24-bit image:

• PixelArraySize = 32 *5 =160 bytes (instead of 10*5*3=150 bytes, which misses the 10 total padding bytes)

Yes! Both calculations exist explicitly to handle the 4-byte row alignment requirement. The RowSize formula ensures each row is padded to a multiple of 4, and PixelArraySize uses that padded row size to get the full pixel data length.

Here's a quick code snippet (Python) to illustrate these calculations in practice:

def get_bmp_row_size(width, bits_per_pixel):
    # Calculate aligned row size (multiple of 4 bytes)
    return ((bits_per_pixel * width + 31) // 32) * 4

def get_bmp_pixel_array_size(width, height, bits_per_pixel):
    row_size = get_bmp_row_size(width, bits_per_pixel)
    # Use absolute height since BMP can store height as negative (top-down pixel order)
    return row_size * abs(height)

# Test with our example
width, height, bpp = 10, 5, 24
print(f"Row Size: {get_bmp_row_size(width, bpp)} bytes")
print(f"Total Pixel Array Size: {get_bmp_pixel_array_size(width, height, bpp)} bytes")
# Output:
# Row Size: 32 bytes
# Total Pixel Array Size: 160 bytes

A quick note: This alignment rule applies to all BMP variants (1-bit, 4-bit, 8-bit, 24-bit, 32-bit). Even for 32-bit color (4 bytes per pixel), the logical row size is already a multiple of 4, so padding isn't needed—but the formula still works, since (32*width +31)//32 *4 simplifies to width*4.

内容的提问来源于stack exchange，提问作者Alazar