Die Implementierung ist dann straight forward :P
char bfType; // 19778 must always be set to 'BM' to declare that this is a .bmp-file.
uint32_t bfSize; // the size of the file in bytes.
uint16_t bfReserved1; // must always be set to zero.
uint16_t bfReserved2; // must always be set to zero.
uint32_t bfOffBits; // 1078 offset from the beginning of the file to the bitmap data.
} __attribute__ ((__packed__)) BMPFileHeader;
uint32_t biSize; // 40 size of the BITMAPINFOHEADER structure, in bytes.
uint32_t biWidth; // width of the image, in pixels.
uint32_t biHeight; // height of the image, in pixels.
uint16_t biPlanes; // 1 planes of the target device, must be set to zero.
uint16_t biBitCount; // 8 bits per pixel.
uint32_t biCompression; // 0 the type of compression, usually set to zero (no compression).
uint32_t biSizeImage; // 0 the size of the image data, in bytes. If there is no compression, it is valid to set this member to zero.
uint32_t biXPelsPerMeter; // 0 usually set to zero.
uint32_t biYPelsPerMeter; // 0 usually set to zero.
uint32_t biClrUsed; // 0 number of colors in the bitmap, if zero, the number of colors is calculated using biBitCount
uint32_t biClrImportant; // 0 number of color that are 'important' for the bitmap, if zero, all colors are important.
} __attribute__ ((__packed__)) BMPInfoHeader;
} __attribute__ ((__packed__)) RGBQUAD;
#endif /* _BMP_BASIC_H_ */
The .bmp file format
The .bmp file format (sometimes also saved as .dib) is the standard
for a Windows 3.0 or later DIB(device independent bitmap) file. It may
use compression (though I never came across a compressed .bmp-file)
and is (by itself) not capable of storing animation. However, you can
animate a bitmap using different methods but you have to write the
code which performs the animation. There are different ways to
compress a .bmp-file, but I won't explain them here because they are
so rarely used. The image data itself can either contain pointers to
entries in a color table or literal RGB values (this is explained
A .bmp file contains of the following data structures:
bmfh contains some information about the bitmap file (about the file,
not about the bitmap itself). bmih contains information about the
bitmap such as size, colors,... The aColors array contains a color
table. The rest is the image data, which format is specified by the
The following tables give exact information about the data structures
and also contain the settings for a bitmap with the following
dimensions: size 100x100, 256 colors, no compression. The start-value
is the position of the byte in the file at which the explained data
element of the structure starts, the size-value contains the nuber of
bytes used by this data element, the name-value is the name assigned
to this data element by the Microsoft API documentation. Stdvalue
stands for standard value. There actually is no such a thing as a
standard value but this is the value Paint assigns to the data element
if using the bitmap dimensions specified above (100x100x256). The
meaning-column gives a short explanation of the purpose of this data
start size name stdvalue purpose
1 2 bfType 19778 must always be set to 'BM' to declare that this is a .bmp-file.
3 4 bfSize ?? specifies the size of the file in bytes.
7 2 bfReserved1 0 must always be set to zero.
9 2 bfReserved2 0 must always be set to zero.
11 4 bfOffBits 1078 specifies the offset from the beginning of the file to the bitmap data.
start size name stdvalue purpose
15 4 biSize 40 specifies the size of the BITMAPINFOHEADER structure, in bytes.
19 4 biWidth 100 specifies the width of the image, in pixels.
23 4 biHeight 100 specifies the height of the image, in pixels.
27 2 biPlanes 1 specifies the number of planes of the target device, must be set to zero.
29 2 biBitCount 8 specifies the number of bits per pixel.
31 4 biCompression 0 Specifies the type of compression, usually set to zero (no compression).
35 4 biSizeImage 0 specifies the size of the image data, in bytes. If there is no compression, it is valid to set this member to zero.
39 4 biXPelsPerMeter 0 specifies the the horizontal pixels per meter on the designated targer device, usually set to zero.
43 4 biYPelsPerMeter 0 specifies the the vertical pixels per meter on the designated targer device, usually set to zero.
47 4 biClrUsed 0 specifies the number of colors used in the bitmap, if set to zero the number of colors is calculated using the biBitCount member.
51 4 biClrImportant 0 specifies the number of color that are 'important' for the bitmap, if set to zero, all colors are important.
Note that biBitCount actually specifies the color resolution of the
bitmap. The possible values are: 1 (black/white); 4 (16 colors); 8
(256 colors); 24 (16.7 million colors). The biBitCount data element
also decides if there is a color table in the file and how it looks
like. In 1-bit mode the color table has to contain 2 entries (usually
white and black). If a bit in the image data is clear, it points to
the first palette entry. If the bit is set, it points to the
In 4-bit mode the color table must contain 16 colors. Every
byte in the image data represents two pixels. The byte is split into
the higher 4 bits and the lower 4 bits and each value of them points
to a palette entry. There are also standard colors for 16 colors mode
(16 out of Windows 20 reserved colors (without the entries 8, 9, 246,
247)). Note that you do not need to use this standard colors if the
bitmap is to be displayed on a screen which support 256 colors or
more, however (nearly) every 4-bit image uses this standard colors.
In 8-bit mode every byte represents a pixel. The value points to an entry
in the color table which contains 256 entries (for details see
Palettes in Windows.
In 24-bit mode three bytes represent one
pixel. The first byte represents the red part, the second the green
and the third the blue part. There is no need for a palette because
every pixel contains a literal RGB-value, so the palette is omitted.
The RGBQUAD array:
The following table shows a single RGBQUAD structure:
start size name stdvalue purpose
1 1 rgbBlue - specifies the blue part of the color.
2 1 rgbGreen - specifies the green part of the color.
3 1 rgbRed - specifies the red part of the color.
4 1 rgbReserved - must always be set to zero.
Note that the term palette does not refer to a RGBQUAD array, which is
called color table instead. Also note that, in a color table
(RGBQUAD), the specification for a color starts with the blue byte. In
a palette a color always starts with the red byte. There is no simple
way to map the whole color table into a LOGPALETTE structure, which
you will need to display the bitmap. You will have to write a function
that copies byte after byte.
The pixel data:
It depens on the BITMAPINFOHEADER structure how the pixel data is to
be interpreted (see above).
It is important to know that the rows of a DIB are stored upside
down. That means that the uppest row which appears on the screen
actually is the lowest row stored in the bitmap, a short example:
pixels displayed on the screen pixels stored in .bmp-file
You do not need to turn around the rows manually. The API functions
which also display the bitmap will do that for you automatically.
Another important thing is that the number of bytes in one row must
always be adjusted to fit into the border of a multiple of four. You
simply append zero bytes until the number of bytes in a row reaches a
multiple of four, an example:
6 bytes that represent a row in the bitmap: A0 37 F2 8B 31 C4
must be saved as: A0 37 F2 8B 31 C4 00 00
to reach the multiple of four which is the next higher after six
(eight). If you keep these few rules in mind while working with .bmp
files it should be easy for you, to master it.