BMP file format
By Microsoft
BMP (Bitmap Image File) is a raster image format from Microsoft that stores pixel color data in an uncompressed or minimally compressed form, along with a simple header describing image dimensions, color depth, and pixel layout. Its…
Definition
BMP (Bitmap Image File) is a raster image format from Microsoft that stores pixel color data in an uncompressed or minimally compressed form, along with a simple header describing image dimensions, color depth, and pixel layout. Its straightforward, well-documented structure made it an early standard for storing and exchanging images on Windows systems, though its large file sizes have led most modern applications to prefer compressed formats instead.
Overview
BMP was introduced as the native bitmap image format for the Microsoft Windows operating system, at a time when a simple, predictable way to store raw pixel data was needed for GUI elements, icons, and basic image handling within Windows applications. Its design prioritized simplicity and speed of decoding over file size, since storage and bandwidth were treated as secondary concerns compared to how easily an application could read pixel values directly into memory. A BMP file consists of a file header identifying the format and overall file size, a bitmap information header describing image width, height, color depth, and compression method, an optional color palette for indexed-color images, and finally the raw pixel data itself, typically stored bottom-to-top and padded to four-byte row boundaries. Because pixel data is usually stored with no compression, or only simple run-length encoding in some variants, decoding a BMP requires little more than reading the header and copying bytes, which is part of why it was easy for early software to support. BMP differs sharply from formats like JPEG, PNG, or WebP, which apply lossy or lossless compression algorithms to shrink file size substantially at the cost of more complex encoding and decoding logic. Where PNG achieves lossless compression through filtering and DEFLATE, and JPEG achieves large size reductions through lossy frequency-domain compression, BMP's near-absence of compression makes files far larger for the same image content, often by an order of magnitude or more. In practice, BMP still appears in specific niches: as an intermediate format for image-processing pipelines or scientific tools that prioritize simplicity and lossless fidelity over size, as an output format for certain legacy Windows utilities and screenshot tools, and in embedded or resource-constrained systems where decoding simplicity outweighs storage cost. It is rarely used for web delivery or general photo storage today. The format's principal limitation is file size: an uncompressed BMP of a modest photograph can be many times larger than an equivalent JPEG or PNG, making it impractical for web use, mobile applications, or anywhere bandwidth or storage is constrained. It also lacks native support for modern features like transparency gradients (alpha channels are supported inconsistently across BMP variants) or embedded metadata standards common in newer formats, which is why BMP has largely been superseded outside of niche technical use cases. Even in image-processing pipelines that favor BMP for its simplicity, files are typically converted to a compressed format before long-term storage or distribution, treating BMP as a transient working format rather than a final deliverable.
Key Concepts
- Simple header structure describing dimensions and color depth
- Pixel data usually stored uncompressed for fast, simple decoding
- Supports indexed color via an optional embedded color palette
- Rows padded to four-byte boundaries in the on-disk layout
- Native bitmap format historically tied to Microsoft Windows
- Large file sizes compared to compressed image formats
- Inconsistent alpha channel support across format variants
- Straightforward to decode without specialized compression libraries
Use Cases
Frequently Asked Questions
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