BMP Bitmap Format: Complete Technical Guide to Windows Bitmap Images

BMP: Windows Bitmap as Foundation of Raster Graphics

The BMP (Bitmap) format is Windows' native uncompressed raster image format—arguably the simplest image format ever standardized, with near-zero processing overhead. While modern use is limited (JPEG, PNG vastly superior), BMP remains important for: embedded systems with minimal computational resources, legacy medical/industrial imaging hardware, and as a pedagogical reference for understanding raw raster structure.

BMP File Structure: File Header and DIB Header

Every BMP file contains two headers:

File Header (14 bytes):

[Signature: 2 bytes ('BM' = 0x42 0x4D)]
[File Size: 4 bytes (little-endian)]
[Reserved: 4 bytes (0x00000000)]
[Offset: 4 bytes (byte offset to pixel data)]

DIB Header (variable size, typically 40 or 108 bytes): Most common: BITMAPINFOHEADER (40 bytes):

[Header Size: 4 bytes (40 for this variant)]
[Width: 4 bytes (signed, pixels)]
[Height: 4 bytes (signed, pixels; negative = top-down)]
[Color Planes: 2 bytes (always 1)]
[Bits Per Pixel: 2 bytes (1, 4, 8, 16, 24, 32)]
[Compression: 4 bytes (0=uncompressed, 1=RLE-8, 2=RLE-4)]
[Image Size: 4 bytes (in bytes; can be 0 for uncompressed)]
[Horizontal Resolution: 4 bytes (pixels per meter)]
[Vertical Resolution: 4 bytes (pixels per meter)]
[Colors Used: 4 bytes (0 = default for bit depth)]
[Important Colors: 4 bytes (0 = all colors)]
[Optional Color Table: variable (palette entries if bit depth ≤ 8)]
[Pixel Data: variable]

Example: Minimal 1×1 8-bit BMP:

File Header:
42 4D        (BM signature)
3A 00 00 00  (file size = 58 bytes)
00 00 00 00  (reserved)
36 00 00 00  (offset to pixel data = 54 bytes after headers)

DIB Header (BITMAPINFOHEADER):
28 00 00 00  (header size = 40)
01 00 00 00  (width = 1 pixel)
01 00 00 00  (height = 1 pixel)
01 00        (planes = 1)
08 00        (bits per pixel = 8)
00 00 00 00  (compression = none)
00 00 00 00  (image size = calculated by decoder)
13 0B 00 00  (72 DPI horizontal = 2835 pixels/meter)
13 0B 00 00  (72 DPI vertical)
00 00 00 00  (colors used = default for 8-bit)
00 00 00 00  (important colors = all)

Color Palette (8-bit: 256 entries × 4 bytes each = 1024 bytes):
[R, G, B, Reserved] for each color...

Pixel Data (1 pixel, 8-bit indexed):
00  (pixel value = color table index 0)

Color Depths and Encoding: 1-Bit to 32-Bit

Pixel encoding note: BMP stores pixels in bottom-up order (left-to-right, bottom row first) unless height is negative.

Color Palette: Indexed Color for 1-, 4-, 8-Bit Images

For palettized depths (1-, 4-, 8-bit), BMP includes a color table immediately after the header:

8-bit: 256 entries × 4 bytes (R, G, B, Reserved) = 1024 bytes
4-bit: 16 entries × 4 bytes = 64 bytes
1-bit: 2 entries × 4 bytes = 8 bytes

Each pixel value is an index into this table. Example:

• Pixel value 42 in 8-bit image → color table[42] = (255, 128, 0) = orange
• Pixel value 5 in 4-bit image → color table[5] = (0, 0, 255) = blue

RLE Compression: Run-Length Encoding

BMP supports optional RLE (Run-Length Encoding) for 4- and 8-bit images:

Compressed: [Escape (0x00), Count (1 byte), Value (1 byte), ...]
Uncompressed: [0x00, 0x00] (end of line marker)

RLE is effective for synthetic images (diagrams, scans) with large color blocks; ineffective for photographic content.

Example 8-bit RLE (10 consecutive red pixels):

0A FF  (10 copies of color index 255 = red)

BMP Variants: BITMAPCOREHEADER, BITMAPV4HEADER, BITMAPV5HEADER

• BITMAPCOREHEADER (12 bytes): Oldest variant; width/height stored as 2-byte integers (max 65K)
• BITMAPINFOHEADER (40 bytes): Standard; width/height as 4-byte signed (supports negative height for top-down)
• BITMAPV4HEADER (108 bytes): Adds color space (sRGB, Adobe RGB, custom ICC profile) and alpha mask
• BITMAPV5HEADER (124 bytes): Adds rendering intent and HDR metadata (rarely used)

BMP Limitations and Modern Alternatives

BMP weaknesses:

• No compression (by default): 1 MB image → 3–4 MB BMP file (vs. 50–200 KB JPEG)
• Bottom-up scanlines: Non-standard; complicates streaming decoders
• Poor color management: DIB headers have no native ICC profile support (BITMAPV4/V5 partially address this)
• Limited metadata: No EXIF, IPTC, or XMP support
• No interlacing: Cannot display progressive scans while downloading

Modern replacements:

• PNG: Lossless compression, transparency, ICC profiles, interlacing, superior compression
• JPEG: Lossy compression, excellent for photographs, 10–50× smaller than uncompressed BMP
• WebP: Modern compression (lossless and lossy), smaller than PNG/JPEG, good browser support

Processing BMP: Python, C Libraries, ImageMagick

Python (PIL/Pillow):

from PIL import Image

# Load BMP
img = Image.open('photo.bmp')
print(f"Size: {img.size}, Mode: {img.mode}")  # Mode: 'RGB', 'RGBA', 'L', 'P', etc.

# Convert to PNG (compress)
img.save('photo.png')  # ~90% size reduction typical

# Convert palette to RGB
if img.mode == 'P':
    img = img.convert('RGB')
    img.save('converted.bmp')

ImageMagick:

# BMP → PNG (compress)
convert photo.bmp photo.png

# BMP → JPEG (lossy, smaller)
convert photo.bmp -quality 85 photo.jpg

# Batch convert directory
mogrify -format png *.bmp

C library (libbmp via FFI):

#include <stdio.h>
// Platform-specific BMP read/write (minimal API, deprecated in favor of libpng/libjpeg)

Comparison: BMP vs PNG, JPEG, TIFF, WebP

When to Choose BMP

• Embedded systems: Minimal CPU overhead; decodable in firmware
• Legacy hardware: Medical imaging, industrial control systems still expect BMP
• Pedagogical purposes: Learning raster structure; simplest image format
• Avoid BMP if: Internet delivery (use PNG/JPEG/WebP), compression needed, metadata required, modern software (prefers PNG)

BMP is deprecated for nearly all modern use cases but remains a fundamental reference for understanding raw raster encoding—pixel-perfect but inefficient.

Related conversions

Frequent conversions across the catalogue:

• JPG to PNG
• MP4 to MP3
• PDF to DOCX
• PNG to JPG
• MP3 to WAV
• DOCX to PDF