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Bitmap (.bmp) – hardware-independent Windows bitmap image — supported by any Windows-compatible software. The BMP file structure is used by Windows to store bitmap images. For example, background images, thumbnails, and other Windows bitmaps are stored in this format. The format minimizes the likelihood of errors or misinterpretation of raster data.
The main disadvantage of this format is that only 4 and 8-bit color versions are compressible. Therefore, 24-bit BMP files will be very large. In addition, the use of BMP files is limited to Windows and OS / 2 platforms. All this makes it impossible to use this format on the web. At the same time, this format is used when creating graphic files in graphic editors and when scanning images. In this regard, knowledge of the features of this format of graphic files to the developer of web pages is advisable.
Each BMP file contains a file header, image header, raster data, and a color map (except the image in 24-bit color).
The header of the BMP file contains information about the type and size of the file, as well as the location of the data in it. The file header is followed by a structure that specifies information about the size, color and image compression.
Compression. The biCompression field in BITMAPINFO determines what type of compression is used in the file. If the field value is BI_RGB, then the picture is not compressed. If the field value is BI_RLE4 or BI_RLE8, then the picture uses the group encoding compression method for images, respectively, with 4-bit / pixel or 8 bits / pixel.
Map of colors. The value of the biCirUsed field determines whether indexed colors will be used by the raster image. If the drawing consists of 1, 4, 8-bit pixels, then a color map should be used. If the biCirUsed field value is zero, then either a full map or a 24-bit color is applied. If the field is not equal to zero, then its value is equal to the actual number of colors that will be available to the program-generator graphics or device drivers. The biClrimportant field determines how many colors in the indexed map are considered important for the image image. If its value is zero, then all colors will be important.
Raster data. The color map is followed by raster data. They are stored as 1-, 4-, or 8-bit colormap indexes or as letter 24-bit data of the RGB system. Pixels in the raster are recorded starting from the lower left corner of the image, and are read from left to right and from bottom to top.