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

Depth	Type	Colors	Packing	File Size Example (1 MP)
1-bit	Bilevel	2 (B&W)	8 px/byte	~125 KB
4-bit	Indexed palette	16	2 px/byte	~250 KB
8-bit	Indexed palette	256	1 px/byte	~1 MB
16-bit	RGB555 or RGB565	32K or 64K	2 bytes/pixel (packed)	~2 MB
24-bit	True color RGB	16.7M	3 bytes/pixel (B, G, R order)	~3 MB
32-bit	RGBA (with alpha)	16.7M + transparency	4 bytes/pixel	~4 MB

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

Aspect	BMP	PNG	JPEG	TIFF	WebP
Compression	None (or RLE)	Lossless (Deflate)	Lossy (DCT)	Optional (LZW, JPEG, none)	Lossless/lossy
File size	Largest (~1 MB per MP)	Small (~200 KB per MP)	Very small (~50 KB per MP)	Medium (300 KB–1 MB)	Small (~100 KB per MP)
Transparency	32-bit alpha	Yes (8-bit alpha)	No	Yes (via clipping)	Yes (8-bit alpha)
Metadata	Minimal (DPI only)	Chunks (EXIF via chunk)	Markers (EXIF, IPTC)	Extensive (IFD tags)	Minimal
Color management	DIB color space (BITMAPV4+)	ICC profile (chunk)	Limited markers	Extensive ICC	Yes (VP8 CICP)
Interlacing	No	Yes (Adam7)	No (baseline progressive)	No (pyramidal TIFF)	Yes (VP8 preview)
Browser support	Universal	Universal	Universal	Poor (requires plugin)	Good (modern)
Archival rating	Poor (uncompressed)	Excellent	Fair (lossy artifacts)	Excellent	Unknown

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.