BMP

BMP is one of the oldest image file formats, but people continue to widely use it. What makes it remain competitive with more modern image formats? Maybe it has something to do with its structure or support by image editing software, or has some good compression algorithms? Let's find out the answers to these questions by inspecting all the ins and outs of this image format.

Origin of BMP

The BMP image format was created by Microsoft in the 1980s as part of their Windows operating system. BMP means Bitmap, you can meet this format also under the names Bitmap image format or Microsoft Bitmap format. Additionally, BMP sometimes can be also called Device Independent Bitmap format or simply DIB. To be exact, BMP actually is a specific implementation of DIB. In other words, every BMP file is a DIB file, but not every DIB file is a BMP file. Anyway, sometimes they are used interchangeably. BMP is a raster format, and you can recognize it by the extension .bmp.

Structure of BMP

Every BMP bitmap file consists of a File header and a block with bitmap data. Between them, there is a DIB header, which is also called a Bitmap information header. Some files may also have a Color Table.

Let's examine the purpose of each part:

• The Bitmap File header stores general information about the image file. Basically, it is used to identify the file as a BMP for a typical image application.
• The DIB header stores detailed information about the image. This information is used for displaying the image on the screen.
• The Color table stores a list of colors used in the image.
• The Bitmap data consists of concrete pixel values of the image.

There are several types of BMP files, which differ in the number of bits used to describe one pixel in the bitmap data. For example, images with 1 bit per pixel are usually monochrome, but they don't necessarily consist only of black and white colors. The type with 4 bits per pixel can store up to 16 colors in the Color Table. The 8-bit format BMP can store up to 256 colors. There can also be a 16-bit format BMP. The 24-bit format represents truecolor and can have up to 2^24 colors. It's worth noting that the Color Table is actually empty in these types of BMP files since it is not needed. In the bitmap data, every three-byte sequence represents the relative intensities of the Blue, Green, and Red colors. The last type is the 32-bit per pixel format, which, like the 24-bit format, can have a maximum of 2^24 colors. The Color Table is also empty in this case, and every three-byte sequence in the bitmap data encodes the relative intensities of the Blue, Green, and Red colors, along with one unused byte. However, BMP files of the 16-bit and 32-bit types are quite rare.

BMP files are mostly stored uncompressed, but to reduce size, they can use the Run-Length Encoding (RLE) compression scheme. Let's take a closer look at it.

Run-length encoding

RLE compression is one of the simplest compression schemes which can be used with 8-bit and 4-bit BMP files. Also, RLE is a lossless algorithm, so it saves images with no loss of detail or color accuracy.

In essence, in RLE, repeated values in the data are replaced by a count of repeated values and the value itself. For instance, if there are 10 orange pixels in a row, they can be encoded as "10 orange pixels". The scheme below shows the principle of Run-Length encoding.

Note, that RLE is beneficial mostly for images that have many repeated values, for example, cartoon drawings. However, for certain types of images, RLE can produce even larger file sizes.

Advantages

What are the advantages of using BMP?

• BMP stores uncompressed image data or uses a lossless compression algorithm, which means there is no loss of data in the image. This makes it a good choice for storing high-quality images.
• It supports various color depths, ranging from monochrome to true color, making it suitable for a wide range of applications.
• BMP was designed to be easily read and written by Windows applications. It continues to be widely used not only by Windows but also by applications on other platforms.
• The structure of a BMP file is simple, making it easy to edit. It even allows for editing individual pixels, which makes it a popular choice among graphic designers and artists.
• The BMP format is supported by most image editors.

Now let's explore why this image format may not be the best choice for you.

Downsides

So what are the weaknesses of the BMP format?

• First, it is limited to RGB only.
• Second, BMP files can be quite large due to their lack of compression, which makes them difficult to store and share. The relatively simple RLE compression scheme does not effectively reduce file size for complex images.
• Third, the BMP format does not support an alpha channel, meaning transparent areas in an image must be represented by a specific color.

Conclusion

The BMP format has a long history but remains useful for many applications today. For instance, it is ideal for high-quality image printing and software applications that handle uncompressed images. Additionally, you may opt for BMP because of its simple and straightforward structure compared to other formats. Remember that BMP consists of a File Header, Bitmap Information Header, Bitmap Data, and often, but not always, a Color Table. However, keep in mind the limitations of this format, such as large file sizes resulting from storing images uncompressed or using simple RLE compression. If your needs require a more complex image format, consider exploring other formats. In general, when working with graphics files in the Windows environment, BMP is often the best choice and is likely to be implemented without any issues.