BMP格式说明书
Also Known As:
BMP, DIB, Windows BMP, Windows DIB, Compatible Bitmap
|
Type 类型 |
Bitmap |
|
Colors 颜色 |
1-, 4-, 8-, 16-, 24-, and 32-bits |
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Compression 压缩 |
RLE, uncompressed |
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Maximum Image Size 最大图像尺寸 |
32Kx32K and 2Gx2G pixels |
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Multiple Images Per File 每个文件有多个图像 |
No |
|
Numerical Format 数值格式 |
Little-endian 一种存放二元化资料的格式（所有数字的低位组放在最前面）； |
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Originator 发起人 |
Microsoft Corporation |
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Platform 平台 |
Intel machines running Microsoft Windows, Windows NT, Windows 95, OS/2, and MS-DOS |
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Supporting Applications 支持的应用 |
Too numerous to list |
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See Also 另外参看 |
Usage
用法
Used as the standard bitmap storage format in the Microsoft Windows
environment. Although it is based on Windows internal bitmap data structures,
it is supported by many non-Windows and non-PC applications.
微软windows环境中使用的标准位图存储格式。尽管它是基于windows的内部位图数据结构，但它仍然被非windows和非PC应用支持。
Comments
评论
A well-defined format for programmers having access to the Microsoft
Developer's Network Knowledge Base and Software Development Kits (SDKs). Its
simple RLE compression scheme is rather inefficient for complex images. Its
many variations and differences from the OS/2 BMP format can be confusing.
BMP是一种为开发者明确定义的格式，可以让他们使用微软开发者网络知识库和软件开发工具箱（SDKs）。它的简单的游程编码压缩策略对于复杂图像的效率很差。它的很多变体以及不同于OS/2 BMP可能会引起混淆。
Vendor specifications are available for this format.
Code fragments are available for this format.
Sample images are available for this format.
The Microsoft Windows Bitmap (BMP) file format is one of several graphics file formats supported by the Microsoft Windows operating environment. BMP is the native bitmap format of Windows and is used to store virtually any type of bitmap data. Most graphics and imaging applications running under Microsoft Windows support the creation and display of BMP format files. BMP is also very popular in the MS-DOS operating system. It is also the native bitmap file format of OS/2.
BMP文件格式是被微软windows操作环境支持的多种图像格式中的一种。BMP是windows的原生图像格式，被用于存储几乎所有的位图数据类型。运行在微软windows操作系统下的大部分图像应用支持创建并展示BMP图像格式。BMP在MS-DOS操作系统中也非常的流行。它也是OS/2操作系统的原生位图文件格式。
Contents:
File
Organization
File Details
For Further
Information
The original bitmap format created for Windows 1.0 was very simple. It had a fixed color palette, did not support bitmap data compression, and was designed to support the most popular IBM PC graphics cards in use at the time (CGA, EGA, Hercules, and others). This defunct format is now often referred to as the original Windows device-dependent bitmap (DDB).
起初为windows1.0创作的位图格式是非常简单的。它有一个固定的颜色板，不支持位图数据压缩，在那时它被设计为支持最流行的IBM PC图形卡（彩色图形适配器、增强型图形适配器、大力神等）。这个已经过时的格式现在经常被称为原始windows设备依赖位图（DDB）。
As Windows 2.0 was being developed, support for a programmable color palette was added to both Windows and BMP, allowing user-definable color data to be stored along with the bitmap data that used it. When stored in memory, this collection of information is known as a Microsoft Device Independent Bitmap, or DIB. When this information is written out to a file, it is known as the Microsoft Bitmap Format, or BMP. When you hear references to the DIB file format, it is BMP that is actually being referred to.
在windows2.0被开发的时候，对可设计的颜色板的支持被加入到windows系统和BMP中，这让用户可定义的颜色数据同使用它的位图数据一同被存储起来。被存储在内存中的时候，采集到的信息被称为微软设备独立图像（DIB）。当这个信息被写入到一个文件中，它被称为微软位图格式（BMP）。当你听到引用DIB文件格式时，实际所指是BMP。
During the development of BMP, Microsoft shared responsibility with IBM for the development of early versions of IBM's OS/2 operating system. When Presentation Manager, the OS/2 graphical user interface, required a bitmap format, the Windows BMP format was used. Thus, the Windows 2.x and OS/2 1.x BMP formats are identical.
在BMP的开发中，微软和IBM共同承担了IBM的OS/2操作系统早期版本的开发。当OS/2图形用户界面的展示工具需要一个位图格式，使用的就是windows的BMP格式。因此，windows2.x和OS/2 1.x的BMP格式是一样的。
The BMP format modified for Windows 3.0 differs only slightly from the OS/2 Presentation Manager bitmap format that preceded it. Note that later revisions designed to support IBM OS/2 Presentation Manager 2.x have resulted in further divergence between the Microsoft Windows and IBM OS/2 BMP file formats. The current version of BMP for Windows 4.0 (Windows 95) contains all of the features and history of the Windows 2.x, 3.x, and Windows NT BMP formats.
为windows3.0修改的BMP格式同OS/2展示工具位图格式只有细微的差别。注意到为支持IBM OS/2展示工具2.x做的后期修改导致了微软windows和IBM OS/2 BMP文件格式的分歧。当前windows4.0（windows95）的BMP版本包括了windows2.x，3.x历史版本以及windows NT的BMP格式所有的特性。
The structure of BMP format files is closely tied to the API of both Windows and OS/2. In this regard, BMP was never meant to be a portable format or used for bitmap data interchange between different operating systems. As each of these operating system APIs has changed, the BMP format has changed along with it.
BMP文件格式的结构同windows和OS/2的API均有紧密的联系。就这一点而言，BMP从来都不是注定成为一个可移植的格式，或者在不同的操作系统中被用作位图数据交换。随着这些操作系统API的更换，BMP格式也随着改变。
There are currently three versions of BMP under Windows (2.x, 3.x, and 4.x [Windows 95]), two versions under OS/2 (1.x and 2.x, with six possible variations), and a single version for Windows NT. This article details the three versions used under Microsoft Windows, as well as the Windows NT version. The original Microsoft device-dependent bitmap format is also discussed. For a discussion of the OS/2 BMP format versions and variants, see the article about the OS/2 BMP format.
目前在windows（2.x，3.x和4.x[windows95]）中BMP有三种版本，两种版本在OS/2（1.x和2.x，有6中可能的变体），一种格式对于windows NT。这篇文章详细描述了windows系统下的三种格式，同时也描述了windows NT中的版本。原始的微软设备依赖的位图格式也被讨论了。对于OS/2 BMP格式版本和变体的讨论，请查看本文的OS/2 BMP 格式部分。
All of the BMP versions originated on Intel-based machines and thus share a common little-endian heritage. The current BMP format is otherwise hardware-independent and can accommodate images with up to 32-bit color. Its basic design makes it a good general purpose format that can be used for color or black-and-white image storage if file size is not a factor. Its main virtues are its simplicity and widespread support in the PC marketplace.
所有的BMP版本起源于基于Intel的机器，因此也共用同一个小端字节传承。当前BMP格式是硬件依赖的，并且可以使图片适应32位色。如果不考虑文件尺寸的话，它的基本设计使它是一个好的格式，可以被用作彩色或者黑白图片存储。它的主要的优点是它的间接性和对不同PC的广泛支持。
The compression method used is a type of run-length encoding (RLE), although most BMP files to date have been stored uncompressed. A notable exception is the Microsoft Windows 3.1 sign-on screen shipped with all copies of the product. Although the BMP RLE scheme is lossless and easily and quickly decompressed, it is not considered a superior compression method.
使用的压缩方法是游程编码的一种，尽管至今大部分的BMP文件都是没有压缩存储的。一个值得注意的例外是微软windows3.1 sign-on screen装载了这个产品的所有版本。尽管BMP的游程编码方案是无损的，容易病快速的压缩，但它并不是一个优秀的压缩策略。
Although the BMP format is well-defined, there is no actual format specification document published by Microsoft. Information about structure and data encoding methods is contained in a number of programmer's references, manuals, online help facilities, and include files associated with the Microsoft Windows Software Development Kits (SDKs) and Microsoft Developers Network Knowledge Base.
尽管BMP格式是定义明确的，但实际上微软没有发布相关的格式文件。关于结构和数据编码方法的信息包含在一些程序员的引用、手册、在线帮助，并且同微软的SDK和微软开发者网络知识库中包括的文件相联系。
File Organization
文件组织
Windows 1.x DDB files contain two sections: a file header and the bitmap data. There is no provision for a color palette or any other features that would make this format device-independent. Support for compression of the bitmap data is also lacking.
Windows1.x DDB文件包括两个部分：一个文件头和位图数据。没有规定颜色板和其它任何特点使得这个格式是设备独立的。不支持位图数据压缩。
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File Header |
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文件头 |
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Bitmap Data |
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位图数据 |
Windows 2.x, 3.x, and 4.x BMP files contain four sections: a file header, a bitmap information header, a color palette, and the bitmap data. Of these four sections, only the palette information may be optional, depending on the bit depth of the bitmap data. The BMP file header is 14 bytes in length and is nearly identical to the 1.x DDB header. The file header is followed by a second header (called the bitmap header), a variable-sized palette, and the bitmap data.
Windows 2.x，3.x和4.x BMP文件包括四个部分：文件头，位图信息头，颜色板，位图数据。这四个部分中，只有颜色板是可选的，这将依赖于位图数据的位深度。文件头之后是有一个文件头（称作位图头），一个可变尺寸的颜色板，位图数据。
|
File Header 文件头 |
|
Bitmap Header 位图头 |
|
Color Palette 颜色板 |
|
Bitmap Data 位图数据 |
File Details
文件细节
This section describes the original Windows DDB format and BMP format versions 2x, 3x, and 4x in greater detail.
这个部分描述了原始的windows 2.x，3.x和4.x版本中的DDB格式和BMP格式。
Version 1 Device-Dependent Bitmap (Microsoft Windows 1.x)
版本1 设备依赖位图（微软windows1.0）
DDB files contain only a file header followed by uncompressed bitmap data. The following shows the structure of the 10-byte DDB file header:
DDB文件只包括一个文件头，接着是一个未压缩的位图数据。下面展示了这个10字节大的文件头的数据结构。
typedef struct _Win1xHeader
{
WORD Type; /* File type identifier (always 0) */
WORD Width; /* Width of the bitmap in pixels */
WORD Height; /* Height of the bitmap in scan lines */
WORD ByteWidth; /* Width of bitmap in bytes */
BYTE Planes; /* Number of color planes */
BYTE BitsPerPixel; /* Number of bits per pixel */
} WIN1XHEADER;
Type indicates the file type; for v1.x headers, it is always 0.
Type表明了这个文件的类型；对于v1.x头，它通常是0
Width and Height represent the size of the bitmap in pixels and in scan lines, respectively.
Width and Height分别代表用像素和扫描线衡量的位图的尺寸。
ByteWidth shows the width of the bitmap in bytes. It is assumed that this value will include the size of any scan line padding that is present.
ByteWidth代表位图的宽度。人们认为这个值将会代表任何展示的扫描线padding。
Planes is the number of color planes used to store the bitmap. This value is always 1.
Planes是用于存储位图颜色位面的个数。这个值通常是1.
BitsPerPixel is the size of each pixel in bits. This value is typically 1, 4, or 8.
BitsPerPixel是每一个像素点的比特尺寸。这个值一般是1,4,8.
The image data immediately follows the header and is stored in an uncompressed format. Each pixel stores an index value into the fixed system colormap used by Windows 1.0. The presence of scan line padding may be determined by comparing the calculated width of a line in bytes with the actual width of the line in bytes stored as the value of the ByteWidth field.
图像数据紧跟着头并且被存储在一个没有被压缩的格式中。每一个像素存储一个索引值到windows1.0固定的系统颜色图中。 The presence of scan line padding may be determined by comparing the calculated width of a line in bytes with the actual width of the line in bytes stored as the value of the ByteWidth field.
BMP Version 2 (Microsoft Windows 2.x)
BMP 版本2（微软windows2.x）
All versions of BMP format files begin with the following 14-byte header:
BMP格式所有的版本都是以如下的14字节的头开始的：
typedef struct _WinBMPFileHeader
{
WORD FileType; /* File type, always 4D42h ("BM") */
DWORD FileSize; /* Size of the file in bytes */
WORD Reserved1; /* Always 0 */
WORD Reserved2; /* Always 0 */
DWORD BitmapOffset; /* Starting position of image data in bytes */
} WINBMPFILEHEADER;
FileType holds a 2-byte magic value used to identify the file type; it is always 4D42h or "BM" in ASCII. If your application reads Windows bitmap files, make sure to always check this field before attempting to use any of the data read from the file.
FileType 存储了一个2字节奇妙的值被用作分辨文件的类型；它的值通常是ASCII值的4D42h或者BM。如果你的应用读取windows位图文件，在尝试从文件中读取数据时，确保总是检查这个域。
FileSize is the total size of the BMP file in bytes and should agree with the file size reported by the filesystem. This field only stores a useful value when the bitmap data is compressed, and this value is usually zero in uncompressed BMP files.
FileSize 是BMP文件总大小，并应该同文件系统报告的文件尺寸相符。这个域仅仅当位图数据被压缩的时候存储一个有用的值， 这个值在没有压缩的BMP文件中通常是0。
Reserved1 and Reserved2 do not contain useful data and are usually set to zero in a BMP header written to disk. These fields are instead used by an application when the header is read into memory.
Reserved1 and Reserved2 没有包含有用的数据，并且在BMP头通常被设置为0。这些域通常当头被读到内存中时被用在一个程序中。
BitmapOffset is the starting offset of the bitmap data from the beginning of the file in bytes.
BitmapOffset 是位图数据从文件开始的起始偏移。
Following the file header in v2.x BMP files is a second header called the bitmap header. This header contains information specific to the bitmap data. This header is 12 bytes in length and has the following format:
在v2.0 BMP文件中接着文件头之后是第二个头，叫做位图头。这个头包含位图数据的特殊信息。这个头是12字节，并且有如下格式：
typedef struct _Win2xBitmapHeader
{
DWORD Size; /* Size of this header in bytes */
SHORT Width; /* Image width in pixels */
SHORT Height; /* Image height in pixels */
WORD Planes; /* Number of color planes */
WORD BitsPerPixel; /* Number of bits per pixel */
} WIN2XBITMAPHEADER;
Size is the size of the header in bytes. For Windows 2.x BMP files, this value is always 12.
Size是头的尺寸。对于windows 2.x BMP文件，这个值一般是12。
Width and Height are the width and height of the image in pixels, respectively. If Height is a positive number, then the image is a "bottom-up" bitmap with the origin in the lower-left corner. If Height is a negative number, then the image is a "top-down" bitmap with the origin in the upper-left corner. Width does not include any scan-line boundary padding.
Width和Height分别是图像像素的宽和高。如果Height是一个整数，那么图像就是一个“底-上”位图，它的起始点在左下角。如果Height是一个整数，那么图像是一个“顶-下”位图，它的起始点在左上角。Width不包括任何扫描线边界填充。
Planes is the number of color planes used to represent the bitmap data. BMP files contain only one color plane, so this value is always 1.
Planes是被用作代表位图数据的颜色板的数量。BMP文件包括仅仅一个颜色板，所以这个值就是1。
BitsPerPixel is the number of bits per pixel in each plane. This value will be in the range 1 to 24; the values 1, 4, 8, and 24 are the only values considered legal by the Windows 2.x API.
BitsPerPixel是每个plane中每像素的位数。这个值可以是1到24中的任意一个；windows 2.x API中采用的是值1,4,8,24中的一个。
The Windows 2.x bitmap header is identical to the OS/2 1.x bitmap header except that the Width and Height fields are signed values in Windows BMP files.
Windows 2.x位图头与OS/2 1.x位图头是相同的，除了Width和Height在windows BMP文件中是符号值。
Following the header is the color palette data. A color palette is always present in a BMP file if the bitmap data contains 1-, 4-, or 8-bit data. Twenty-four-bit bitmap data never uses a color palette (nor does it ever need to). Each element of the palette is three bytes in length and has the following structure:
接着这个头是颜色板数据。如果位图数据包括1-，4-，8-位数据，一个颜色板通常包含在BMP文件中。24位位图数据不使用颜色板。颜色板的每一个元素是三字节大小，并且有如下的结构：
typedef struct _Win2xPaletteElement
{
BYTE Blue; /* Blue component */
BYTE Green; /* Green component */
BYTE Red; /* Red component */
} WIN2XPALETTEELEMENT;
Blue, Green, and Red hold the color component values for a pixel; each is in the range 0 to 255.
Blue,Green,Red存储一个像素的色彩成分值；变化范围是0到255。
The size of the color palette is calculated from the BitsPerPixel value. The color palette has 2, 16, 256, or 0 entries for a BitsPerPixel of 1, 4, 8, and 24, respectively. The number of color palette entries is calculated as follows:
颜色板的尺寸是从BitsPerPixel计算过来的。颜色板有2,16,256，或者0条分别对应于BitsPerPixel的1,4,8,24。颜色板条目的数量通过如下方式计算：
NumberOfEntries = 1 << BitsPerPixel;
To detect the presence of a color palette in a BMP file (rather than just assuming that a color palette does exist), you can calculate the number of bytes between the bitmap header and the bitmap data and divide this number by the size of a single palette element. Assuming that your code is compiled using 1-byte structure element alignment, the calculation is:
为了检测BMP文件颜色板的存在（而不是仅仅假设存在颜色板），你可以计算位图头和位图数据之间的大小，并按照一个板元素分割开来：假设你的代码是使用1字节结构元素编译的，计算公式是：
NumberOfEntries = (BitmapOffset - sizeof(WINBMPFILEHEADER) -
sizeof(WIN2XBITMAPHEADER)) / sizeof(WIN2XPALETTEELEMENT);
If NumberOfEntries is zero, there is no palette data; otherwise, you have the number of elements in the color palette.
如果NumberOfEntries是0，那么就没有颜色板数据；否则你就得到颜色板中元素的数量。
BMP Version 3 (Microsoft Windows 3.x)
BMP 版本3（微软windows 3.x）
Version 3.x BMP files begin with the same 14-byte header as v2.x BMP files. The file header is also followed by a bitmap header, which is an expanded version of the v2.xbitmap header. It is 40 bytes in size and contains six additional fields:
版本3.x BMP文件以和v2.x相同的14字节头开始。文件头之后是位图头，它是v2.x的扩展。它是40字节并包括6个额外的域：
typedef struct _Win3xBitmapHeader
{
DWORD Size; /* Size of this header in bytes */
LONG Width; /* Image width in pixels */
LONG Height; /* Image height in pixels */
WORD Planes; /* Number of color planes */
WORD BitsPerPixel; /* Number of bits per pixel */
/* Fields added for Windows 3.x follow this line */
DWORD Compression; /* Compression methods used */
DWORD SizeOfBitmap; /* Size of bitmap in bytes */
LONG HorzResolution; /* Horizontal resolution in pixels per meter */
LONG VertResolution; /* Vertical resolution in pixels per meter */
DWORD ColorsUsed; /* Number of colors in the image */
DWORD ColorsImportant; /* Minimum number of important colors */
} WIN3XBITMAPHEADER;
Size is the size of the header in bytes. For Windows 3.x BMP files, this value is always 40.
Size是头的大小。对于windows 3.x BMP文件，这个值通常是40.
Width and Height are the width and height of the image in pixels, respectively. If Height is a positive number, then the image is a "bottom-up" bitmap with the origin in the lower-left corner. If Height is a negative number, then the image is a "top-down" bitmap with the origin in the upper-left corner. Width does not include any scan-line boundary padding.
Planes is the number of color planes used to represent the bitmap data. BMP files contain only one color plane, so this value is always 1.
BitsPerPixel is the number of bits in each pixel. This value is in the range 1 to 24; the values 1, 4, 8, and 24 are the only values considered legal by the Windows 3.x API.
Compression indicates the type of encoding method used to compress the bitmap data. 0 indicates that the data is uncompressed; 1 indicates that the 8-bit RLE algorithm was used; 2 indicates that the 4-bit RLE algorithm was used. (See the section called "Image Data and Compression" below for more information on BMP RLE encoding.)
SizeOfBitmap is the size of the stored bitmap in bytes. This value is typically zero when the bitmap data is uncompressed; in this case, the decoder computes the size from the image dimensions.
HorzResolution and VertResolution are the horizontal and vertical resolutions of the bitmap in pixels per meter. These values are used to help a BMP reader choose a proper resolution when printing or displaying a BMP file.
ColorsUsed is the number of colors present in the palette. If this value is zero, and the value of BitsPerPixel is less than 16, then the number of entries is equal to the maximum size possible for the colormap. BMP files with a BitsPerPixel value of 16 or greater will not have a color palette. This value is calculated by using the value of the BitsPerPixel field:
ColorsUsed = 1 << BitsPerPixel;
ColorsImportant is the number of significant colors in the palette, determined by their frequency of appearance in the bitmap data; the more frequent the occurrence of a color, the more important it is. This field is used to provide as accurate a display as possible when using graphics hardware supporting fewer colors than are defined in the image. For example, an 8-bit image with 142 colors might only have a dozen or so colors making up the bulk of the image. If these colors could be identified, a display adapter with only 16-color capability would be able to display the image more accurately using the 16 most frequently occurring colors in the image. The most important colors are always stored first in the palette; ColorsImportant is 0 if all of the colors in the palette are to be considered important.
The color palette that may follow the bitmap header is basically the same as the v2.x palette but adds an extra byte of padding to increase its size to four bytes. This allows palette entries to be read as 4-byte values, making these values more efficient to read in memory and easier to see in a hex dump or debugger.
typedef struct _Win3xPaletteElement
{
BYTE Blue; /* Blue component */
BYTE Green; /* Green component */
BYTE Red; /* Red component */
BYTE Reserved; /* Padding (always 0) */
} WIN3XPALETTEELEMENT;
Blue, Green, and Red hold the color component values for a pixel; each is in the range 0 to 255.
Reserved pads the structure to end on an even-byte boundary and is always zero.
BMP Version 3 (Microsoft Windows NT)
Windows NT uses a variation of the Windows 3.x BMP format to store 16- and 32-bit data in a BMP file. This variation adds three additional fields that follow the bitmap header in place of a color palette. The bitmap header is 40 bytes in length and has the following format:
typedef struct _WinNtBitmapHeader
{
DWORD Size; /* Size of this header in bytes */
LONG Width; /* Image width in pixels */
LONG Height; /* Image height in pixels */
WORD Planes; /* Number of color planes */
WORD BitsPerPixel; /* Number of bits per pixel */
DWORD Compression; /* Compression methods used */
DWORD SizeOfBitmap; /* Size of bitmap in bytes */
LONG HorzResolution; /* Horizontal resolution in pixels per meter */
LONG VertResolution; /* Vertical resolution in pixels per meter */
DWORD ColorsUsed; /* Number of colors in the image */
DWORD ColorsImportant; /* Minimum number of important colors */
} WINNTBITMAPHEADER;
All fields are the same as in the v3.x BMP format, except for the Compression field.
Compression indicates the type of encoding method used to compress the bitmap data. 0 indicates that the data is uncompressed; 1 indicates that the 8-bit RLE algorithm was used; 2 indicates that the 4-bit RLE algorithm was used; and 3 indicates that bitfields encoding was used. If the bitmap contains 16 or 32 bits per pixel, then only a Compression value of 3 is supported and the RedMask, GreenMask, and BlueMask fields will be present following the header in place of a color palette. If Compression is a value other than 3, then the file is identical to a Windows 3.x BMP file.
typedef _WinNtBitfieldsMasks
{
DWORD RedMask; /* Mask identifying bits of red component */
DWORD GreenMask; /* Mask identifying bits of green component */
DWORD BlueMask; /* Mask identifying bits of blue component */
} WINNTBITFIELDSMASKS;
RedMask, GreenMask, and BlueMask specify which bits in a pixel value correspond to a specific color in 16- and 32-bit bitmaps. The bits in these mask values must be contiguous and must not contain overlapping fields. The bits in the pixel are ordered from most significant to least significant bits. For 16-bit bitmaps, the RGB565 format is often used to specify five bits each of red and blue values, and six bits of green:
RedMask = 0xF8000000; /* 1111 1000 0000 0000 0000 0000 0000 0000 */
GreenMask = 0x07E00000; /* 0000 0111 1110 0000 0000 0000 0000 0000 */
BlueMask = 0x001F0000; /* 0000 0000 0001 1111 0000 0000 0000 0000 */
For 32-bit bitmaps, the RGB101010 format can be used to specify 10 bits each of red, green, and blue:
RedMask = 0xFFC00000; /* 1111 1111 1100 0000 0000 0000 0000 0000 */
GreenMask = 0x003FF000; /* 0000 0000 0011 1111 1111 0000 0000 0000 */
BlueMask = 0x00000FFC; /* 0000 0000 0000 0000 0000 1111 1111 1100 */
BMP Version 4 (Microsoft Windows 95)
Version 4.x BMP files begin with the same 14-byte header as v2.x and v3.x BMP files. The file header is also followed by a bitmap header, which is an expanded version of the v3.x bitmap header, incorporating the mask fields of the NT BMP format. This v4.x bitmap header is 108-bytes in size and contains 17 additional fields:
typedef struct _Win4xBitmapHeader
{
DWORD Size; /* Size of this header in bytes */
LONG Width; /* Image width in pixels */
LONG Height; /* Image height in pixels */
WORD Planes; /* Number of color planes */
WORD BitsPerPixel; /* Number of bits per pixel */
DWORD Compression; /* Compression methods used */
DWORD SizeOfBitmap; /* Size of bitmap in bytes */
LONG HorzResolution; /* Horizontal resolution in pixels per meter */
LONG VertResolution; /* Vertical resolution in pixels per meter */
DWORD ColorsUsed; /* Number of colors in the image */
DWORD ColorsImportant; /* Minimum number of important colors */
/* Fields added for Windows 4.x follow this line */
DWORD RedMask; /* Mask identifying bits of red component */
DWORD GreenMask; /* Mask identifying bits of green component */
DWORD BlueMask; /* Mask identifying bits of blue component */
DWORD AlphaMask; /* Mask identifying bits of alpha component */
DWORD CSType; /* Color space type */
LONG RedX; /* X coordinate of red endpoint */
LONG RedY; /* Y coordinate of red endpoint */
LONG RedZ; /* Z coordinate of red endpoint */
LONG GreenX; /* X coordinate of green endpoint */
LONG GreenY; /* Y coordinate of green endpoint */
LONG GreenZ; /* Z coordinate of green endpoint */
LONG BlueX; /* X coordinate of blue endpoint */
LONG BlueY; /* Y coordinate of blue endpoint */
LONG BlueZ; /* Z coordinate of blue endpoint */
DWORD GammaRed; /* Gamma red coordinate scale value */
DWORD GammaGreen; /* Gamma green coordinate scale value */
DWORD GammaBlue; /* Gamma blue coordinate scale value */
} WIN4XBITMAPHEADER;
Size is the size of the header in bytes. For Windows 4.x BMP files, this value is always 108.
Width and Height are the width and height of the image in pixels, respectively. If Height is a positive number, then the image is a "bottom-up" bitmap with the origin in the lower-left corner. If Height is a negative number, then the image is a "top-down" bitmap with the origin in the upper-left corner. Width does not include any scan-line boundary padding.
Planes is the number of color planes used to represent the bitmap data. BMP files contain only one color plane, so this value is always 1.
BitsPerPixel is the number of bits in each pixel. This value is in the range 1 to 24; the values 1, 4, 8, 16, 24, and 32 are the only values considered legal by the Windows 4.xAPI.
Compression indicates the type of encoding method used to compress the bitmap data. 0 indicates that the data is uncompressed; 1 indicates that the 8-bit RLE algorithm was used; 2 indicates that the 4-bit RLE algorithm was used; and 3 indicates that bitfields encoding was used. If the bitmap contains a 16- or 32-bit bitmap, then only a compression value of 3 is supported.
SizeOfBitmap is the size of the stored bitmap in bytes. This value is typically zero when the bitmap data is uncompressed (including bitfields-encoded bitmaps); in this case, the decoder computes the size from the image dimensions.
HorzResolution and VertResolution are the horizontal and vertical resolutions of the bitmap in pixels per meter. These values are used to help a BMP reader choose a proper resolution when printing or displaying a BMP file.
ColorsUsed is the number of colors present in the palette. If this value is zero and the BMP file contains a color palette, then the number of entries is equal to the maximum size possible for the color palette. If the bitmap has a pixel depth of 16 or greater, there is never a color palette, and this value will be zero.
ColorsImportant is the number of significant colors in the palette, determined by their frequency of appearance in the bitmap data; the more frequent the occurrence of a color, the more important it is. See the explanation of this field for the Windows 3.x bitmap header for more information.
RedMask, GreenMask, BlueMask, and AlphaMask specify which bits in a pixel value correspond to a specific color or alpha channel in 16- and 32-bit bitmaps. The bits in these mask values must be contiguous and must not contain overlapping fields. The bits in the pixel are ordered from most significant to least significant bits. For example, a 16-bit bitmap using the RGB555 format would specify five bits each of red, green, blue, and alpha as follows:
AlphaMask = 0xF8000000; /* 1111 1000 0000 0000 0000 0000 0000 0000 */
RedMask = 0x07C00000; /* 0000 0111 1100 0000 0000 0000 0000 0000 */
GreenMask = 0x003E0000; /* 0000 0000 0011 1110 0000 0000 0000 0000 */
BlueMask = 0x0001F000; /* 0000 0000 0000 0001 1111 0000 0000 0000 */
A 32-bit bitmap using the RGB888 format would specify eight bits each of red, green, and blue using the mask values as follows:
AlphaMask = 0xFF000000; /* 1111 1111 0000 0000 0000 0000 0000 0000 */
RedMask = 0x00FF0000; /* 0000 0000 1111 1111 0000 0000 0000 0000 */
GreenMask = 0x0000FF00; /* 0000 0000 0000 0000 1111 1111 0000 0000 */
BlueMask = 0x000000FF; /* 0000 0000 0000 0000 0000 0000 1111 1111 */
Note that Windows 95 only supports the RGB555 and RGB565 masks for 16-bit BMP bitmaps and RGB888 for 32-bit BMP bitmaps.
CSType is the color space type used by the bitmap data. Values for this field include 00h (calibrated RGB), 01h (device-dependent RGB), and 02h (device-dependent CMYK). Device-dependent RGB is the default color space. Calibrated RGB is defined by the 1931 CIE XYZ standard.
RedX, RedY, and RedZ specify the CIE X, Y, and Z coordinates, respectively, for the endpoint of the red component of a specified logical color space. These fields are used only when CSType is 00h (calibrated RGB).
GreenX, GreenY, and GreenZ specify the CIE X, Y, and Z coordinates, respectively, for the endpoint of the green component of a specified logical color space. These fields are used only when CSType is 00h (calibrated RGB).
BlueX, BlueY, and BlueZ specify the CIE X, Y, and Z coordinates, respectively, for the endpoint of the blue component of a specified logical color space. These fields are used only when CSType is 00h (calibrated RGB).
GammaRed, GammaGreen, and GammaBlue are the red, green, and blue gamma coordinate scale values, respectively, for this bitmap.
All of the additional fields added to the Windows 4.x bitmap header are used to support 16- and 32-bit bitmaps and color matching and color characterization of the bitmap data. Color processing may be performed on an image and the ICM (Image Color Matching) information stored in the BMP file. This data is used to provide color matching processing when the bitmap is printed or displayed.
Color Palette
颜色板
As we have seen, a BMP color palette is an array of structures that specify the red, green, and blue intensity values of each color in a display device's color palette. Each pixel in the bitmap data stores a single value used as an index into the color palette. The color information stored in the element at that index specifies the color of that pixel.
如我们所看到的，BMP颜色板
One-, 4-, and 8-bit BMP files are expected to always contain a color palette. Sixteen-, 24-, and 32-bit BMP files never contain color palettes. Sixteen- and 32-bit BMP files contain bitfields mask values in place of the color palette.
You must be sure to check the Size field of the bitmap header to know if you are reading 3-byte or 4-byte color palette elements. A Size value of 12 indicates a Windows 2.x(or possibly an OS/2 1.x) BMP file with 3-byte elements. Larger numbers (such as 40 and 108) indicate later versions of BMP, which all use 4-byte color palette elements.
Windows BMP File Types
Each new version of BMP has added new information to the bitmap header. In some cases, the newer versions have changed the size of the color palette and added features to the format itself. Unfortunately, a field wasn't included in the header to easily indicate the version of the file's format or the type of operating system that created the BMP file. If we add Windows' four versions of BMP to OS/2's two versions--each with four possible variations--we find that as many as twelve different related file formats all have the file extension ".BMP".
It is clear that you cannot know the internal format of a BMP file based on the file extension alone. But, fortunately, you can use a short algorithm to determine the internal format of BMP files.
The FileType field of the file header is where we start. If these two byte values are 424Dh ("BM"), then you have a single-image BMP file that may have been created under Windows or OS/2. If FileType is the value 4142h ("BA"), then you have an OS/2 bitmap array file. Other OS/2 BMP variations have the file extensions .ICO and .PTR.
If your file type is "BM", then you must now read the Size field of the bitmap header to determine the version of the file. Size will be 12 for Windows 2.x BMP and OS/2 1.xBMP, 40 for Windows 3.x and Windows NT BMP, 12 to 64 for OS/2 2.x BMP, and 108 for Windows 4.x BMP. A Windows NT BMP file will always have a Compression value of 3; otherwise, it is read as a Windows 3.x BMP file.
Note that the only difference between Windows 2.x BMP and OS/2 1.x BMP is the data type of the Width and Height fields. For Windows 2.x, they are signed shorts and for OS/2 1.x, they are unsigned shorts. Windows 3.x, Windows NT, and OS/2 2.x BMP files only vary in the number of fields in the bitmap header and in the interpretation of the Compression field.
Image Data and Compression
Uncompressed data is a series of values representing either color palette indices or actual RGB color values. Pixels are packed into bytes and arranged as scan lines. Each scan line must end on a 4-byte boundary, so one, two, or three bytes of padding may follow each scan line.
Scan lines are stored from the bottom up if the value of the Height field in the bitmap header is a positive value; they are stored from the top down if the Height field value is negative. The bottom-up configuration is the most common, because scan lines stored from the top down cannot be compressed.
Monochrome bitmaps contain one bit per pixel, eight pixels per byte (with the most significant bit being the leftmost pixel), and have a 2-element color palette. If a BMP reader chooses to ignore the color palette, all "one" bits are set to the display's foreground color and all "zero" bits are set to the background color.
Four-bit pixels are packed two per byte with the most significant nibble being the leftmost pixel. Eight-bit pixels are stored one per byte. Both 4- and 8-bit pixel values are indices into color palettes 16 and 256 elements in size respectively.
Sixteen-bit pixels in the Windows NT format are two bytes in size and are stored in big-endian order. In other words, on little-endian machines these bytes must be read and flipped into little-endian order before they are used. The organization of the bit fields in the 16-bit pixels is defined by the values of the RedMask, GreenMask, and BlueMask fields in the header. The most common masks are RGB555 and RGB565. The Compression field must always be a value of 3 (bitfields encoding) when a file stores 16-bit data.
In the v4.x BMP format, 16- and 32-bit pixels are stored as little-endian 4-byte RGB values. Common masks for 32-bit data include RGB888 and RGB101010. These bit depths also require bitfields encoding and the mask fields in the bitmap header to define their pixel format. 24-bit bitmap data is always stored as 3-byte RGB values.
The Windows BMP format supports a simple run-length encoded (RLE) compression scheme for compressing 4-bit and 8-bit bitmap data. Since this is a byte-wise RLE scheme, 1-, 16-, 24-, and 32-bit bitmaps cannot be compressed using it, due to the typical lack of long runs of bytes with identical values in these types of data.
BMP uses a two-value RLE scheme. The first value contains a count of the number of pixels in the run, and the second value contains the value of the pixel. Runs of up to 255 identical pixel values may be encoded as only two bytes of data. Actually, it's a bit more complex than this. In addition to encoded runs, there are unencoded runs, delta markers, end-of-scan-line markers, and an end-of-RLE-data marker.
The 8-bit RLE algorithm (RLE8) stores repeating pixel values as encoded runs. The first byte of an encoded run will be in the range of 1 to 255. The second byte is the value of the 8-bit pixels in the run. For example, an encoded run of 05 18 would decode into five pixels each with the value 18, or 18 18 18 18 18.
When a scan line does not contain enough pixel runs to achieve a significant amount of compression, contiguous pixel values may be stored as literal or unencoded runs. An unencoded run may contain from 3 to 255 pixel values. The first byte of an unencoded run is always zero. This makes it possible to tell the difference between the start of an encoded and the start of an unencoded run. The second byte value is the number of unencoded pixel values that follow. If the number of pixels is odd, then a 00 padding value also follows. This padding value is not part of the original pixel data and should not be written to the decoded data stream. Here are some examples of encoded and unencoded data streams:
|
Encoded Bytes |
Decoded Bytes |
|
05 10 |
10 10 10 10 10 |
|
00 05 23 65 34 56 45 00 |
23 65 34 56 45 |
|
0A 0A |
0A 0A 0A 0A 0A 0A 0A 0A 0A 0A |
|
00 04 46 57 68 79 |
46 57 68 79 |
Three marker values may also be found in the RLE data. Each of these markers also begins with a zero-byte value. The second byte value indicates the type of marker. These markers specify positional information relating to the decoded bitmap data and do not generate any data themselves.
The first marker is the end-of-scan-line marker and is identified by two byte values 00 and 00. This marker is an indication that the end of data for the current scan line has been reached. Encoded data occurring after this marker is decoded starting at the beginning of the next scan line. If an end-of-scan-line marker is not present in the encoded data, then the pixels will automatically wrap from the end of one scan line to the start of the next.
This marker is only used when you want to force the decoding of a scan line to end at a particular place. If the end-of-scan-line marker occurs in the middle of a scan line, all remaining pixels in the decoded bitmap for the line are ignored. This "short scan line" technique is used to omit unneeded portions of scan lines. Most often, it is found in icon and pointer BMP files.
The next marker is the end of RLE data marker. It is identified by the two byte values 00 and 01. This marker occurs only as the last two bytes of the RLE data. This marker is an indication that the reader should stop decoding data.
The last marker is the run offset marker, also called a delta or vector code. This marker is four bytes in size, with the first two bytes being the values 00 and 02, and the last two values specifying a pixel address using unsigned X and Y values as an offset from the current bitmap cursor position. The X value is the number of pixels across the scan line, and the Y value is the number of rows forward in the bitmap.
This run offset marker indicates the location in the bitmap where the next decoded run of pixels should be written. For example, a run offset marker value of 00 02 05 03 would indicate that the offset of the bitmap cursor should move five pixels down the scan line, three rows forward, and write out the next run. The cursor then continues writing decoded data from its new position moving forward.
Run offset markers are used when a bitmap may contain a large amount of "don't care" pixels. For example, if the BMP file holds a bitmap used as a mask (such as those used with icons and pointers), many of the pixels in the rectangular bitmap may not be used. Rather than store these unused pixels in the BMP file, only the significant pixels are stored, and the delta markers are used as "jumps" to skip over the parts of the bitmap not actually used in the mask.
The following are the BMP RLE markers:
00 00 End of scan line
00 01 End of bitmap data
00 02 XX YY Run offset marker
Here is an example of decoding an 8-bit data stream. Each of the values is an 8-bit index value into the color palette and not an actual color value.
|
Encoded Bytes |
Decoding Description |
Decoded Bytes |
|
04 16 |
Four bytes of value 16 |
16 16 16 16 |
|
08 45 |
Eight bytes of value 45 |
45 45 45 45 45 45 45 45 |
|
00 00 |
End of scan line |
None |
|
00 02 04 02 |
Move to offset four pixels forward and two rows up |
None |
|
03 E4 |
Three bytes of value E4 |
E4 E4 E4 |
|
00 03 12 A4 46 00 |
Three bytes of unencoded data |
12 A4 46 |
|
00 00 |
End of scan line |
None |
|
00 01 |
End of RLE data |
None |
The 4-bit RLE algorithm (RLE4) stores repeating pixel values in a very similar manner to RLE8. All of the markers are the same. The only real difference is that two pixel values are packed per byte, and these pixel values alternate when decoded. For example, an RLE4-encoded data stream of 07 48 would decode to seven pixels, alternating in value as 04 08 04 08 04 08 04.
If this looks bizarre, it's because you rarely see alternating runs of pixel values in bitmaps eight bits or greater in depth. Four-bit (16-color) bitmaps, however, usually contains a lot of dithering. Most dithering algorithms will yield relatively large runs of alternating pixels. Runs of repeating sequences of three and four pixels are also fairly common output from many dithering algorithms. But the ability to efficiently encode these types of pixel runs is not currently supported in the BMP RLE scheme.
In case you are thinking that runs of identical pixel values cannot be encoded by RLE4, you are incorrect. For example, a run of twelve pixels all of the value 9 would be RLE4-encoded as 0C 99 and would decode to the run 09 09 09 09 09 09 09 09 09 09 09 09.
Padding is added to unencoded pixel runs that are an odd number of bytes, rather than pixels, in length. And an unused final nibble in odd-length runs is set to zero. For example, the six pixel values 1 3 5 7 9 0 would be stored as the unencoded run 00 06 13 57 90 00, while the five pixel values 1 3 5 7 9 would be stored as the unencoded run 00 05 13 57 90 00.
Following is an example of decoding a 4-bit data stream. Each of the values is a 4-bit index value into the color palette and not an actual color value.
|
Encoded Bytes |
Decoding Description |
Decoded Bytes |
|
04 16 |
Four values alternating 1 and 6 |
1 6 1 6 |
|
08 44 |
Eight values alternating 4 and 4 |
4 4 4 4 4 4 4 4 |
|
00 00 |
End of scan line |
None |
|
00 02 04 02 |
Move to offset four pixels forward and two rows up |
None |
|
03 E4 |
Three values alternating E and 4 |
E 4 E |
|
00 06 12 A4 46 00 |
Six values of unencoded data |
1 2 A 4 4 6 |
|
00 00 |
End of scan line |
None |
|
00 01 |
End of RLE data |
None |
Here is a summary of Windows BMP data characteristics:
|
Pixel Depth |
Pixel Size |
Compression |
Color Palette |
Color Masks |
|
1 bit |
1 bit |
0 |
Yes |
No |
|
4 bits |
4 bits |
0,2 |
Yes |
No |
|
8 bits |
1 byte |
0,1 |
Yes |
No |
|
16 bits |
4 bytes |
3 |
No |
Yes |
|
24 bits |
3 bytes |
0 |
No |
No |
|
32 bits |
4 bytes |
3 |
No |
Yes |
For Further Information
获取更多信息
For further information about the Microsoft Windows Bitmap format, see the documentation included on the CD-ROM. Although you probably will not be able to get any information directly from them, here is Microsoft's address:
想要获取微软windows位图格式更多信息，查看CD-ROM中的文件。尽管你可能不能直接得到你想要的信息，这里是微软的地址：
Microsoft
Corporation
One Microsoft Way
Redmond, WA 98052-6399
Voice: 206-882-8080
FAX: 206-936-7329
BBS: 206-637-9009
FTP: ftp://ftp.microsoft.com
WWW: http://www.microsoft.com
The closest thing there is to an archive of Microsoft file format specifications is the Microsoft Developers Network Knowledge Base available on the MSDN CD-ROM and at Microsoft's Web site. The Win16 and Win32 Software Development Kits (SDKs) also have information on BMP.
关于微软文件格式的说明书可以在MSDN的微软开发者网络知识库和微软的网站中找到。Win16和win32软件开发工具箱（SDKs）也有关于BMP的信息。
Information about the Windows BMP format can also be found in the following references:
关于windows BMP格式的信息也可以从下面的引用中得到：
Charlap, David, "The BMP File Format: Part I," Dr. Dobb's Journal, vol. 20, no. 228, March 1995.
Charlap, David, "The BMP File Format: Part II," Dr. Dobb's Journal, vol. 20, no. 229, April 1995.
Luse, Marv, "The BMP File Format," Dr. Dobb's Journal, vol. 9, no. 219, September 1994, pp. 18-22.
Microsoft Corporation, Microsoft Win32 Programmer's Reference, vol. 5, Microsoft Press, Redmond, WA, 1993.
Microsoft Corporation, Microsoft Windows Programmer's Reference, vol. 2, v3, Microsoft Press, Redmond, WA, 1990.
Petzold, Charles, "Preserving a Device-Independent Bitmap: The Packed-DIB Format," PC Magazine, July 1991, pp. 433-439.
Petzold, Charles, "What's New in Bitmap Formats: A Look at Windows and OS/2," PC Magazine, 11 September 1990, pp. 403-410.
Swan, Tom, Inside Windows File Formats, Sams Publishing, 1993.
The code for the above issues of Dr. Dobb's Journal are available at:
ftp://ftp.mv.com/pub/ddj/1994/1194.09/bmp.zip
ftp://ftp.mv.com/pub/ddj/1995/1195.03/bmp.zip
The two Dr. Dobb's Journal articles by David Charlap contain a complete collection of source code for working with Windows 2.x, 3.x, NT, and OS/2 BMP file formats. It is available at the above FTP site.
This page is taken from the Encyclopedia of Graphics File Formats and is licensed by O'Reilly under the Creative Common/Attribution license.
参考文章：http://www.fileformat.info/format/bmp/egff.htm
