| Windows Bitmap |
Article Index for Windows |
Website Links For Windows |
Information AboutWindows Bitmap |
BITMAP FILE OVERVIEW .BMP or '''.DIB''' ( Device -independent '''bitmap''') is a Bitmapped Graphics Format used internally by the Microsoft Windows graphics Subsystem ( GDI ), and used commonly as a simple Graphics File Format on that Platform . Images are generally stored with a Color Depth of 2 (1-bit), 16 (4-bit), 256 (8-bit), 65,536 (16-bit), or 16.7 Million (24-bit) colors(the bits represent the bits per pixel). 8-bit images can also be greyscale instead of indexed color. An Alpha Channel (for Transparency ) may be stored in a separate file, where it is similar to a Greyscale image. A 32-bit version with integrated alpha channel has been introduced with Windows XP and is used within its logon and theme system; it has yet to gain wide support in image editing software. STORAGE ALGORITHM BMP files are usually not Compressed , so they are typically much larger than compressed image file formats for the same image. For example, an 800×600 image will occupy almost 1.4 Megabyte s. As such they are generally unsuitable for transferring images on the Internet or other slow or capacity-limited Media . Depending on the color depth, a pixel in the picture will be stored using one or more bytes, which is determined by n/8 (1 byte=8 bits). The color of the pixel will be calculated (by the picture viewer, for example) based on the ASCII code of the bytes and the corresponding values read from the color palette. As such, the approximate size for a n-bit (2n colors) bitmap in Byte s can be calculated as: size of BMP file , where height and width are given in Pixels . It should be noted that C in the above formula is the size of the Header of the bitmap file, which contains information about the file size, height, width, color depth, and more importantly, the color Palette . As the contents of the color palette depend on the bit depth of the image, C also varies accordingly. However, C is usually very small compared with the size of the image, and as such, it can be ignored if we just want to have a rough estimation of the file size. Due to storage algorithm, the calculated size will be slightly difference from the actual file size, depending on several image parameters. MISCELLANEOUS Despite the huge file size, the simplicity of BMP and its widespread familiarity in MS Windows and elsewhere, as well as the fact that this format is well- Documented and free of Patent s, makes it a very common format that image processing programs from many operating systems can read and write. The X Window System uses a similar .XBM format for true single- Bit Black And White images, and '''.XPM''' ('''pixelmap''') for Color images. There is also a '''.RAW''' format, which saves raw Data with no other Information . The Portable Pixmap File Format ('''.PPM''') and Truevision TGA ('''.TGA''') formats also exist, but are rarely used - or only for special purposes. Yet other formats store as "bitmaps" (as opposed to Vector Graphics ), but use compression or color indexes, and thus are not strictly considered true bitmaps. Most BMP files compress very well with Lossless Data Compression algorithms such as ZIP because they contain redundant data. SEE ALSO EXTERNAL LINKS
|
|
|