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Saturday, October 9, 2010

Bmp Image Format



BITMAP Image Format 

                 The BMP image file also called DIB image file. DIB means (Device Independent Bitmap). It is an image file used to store image data with different bit levels. This article describes the BITMAP file formats used in Microsoft windows, and IBM os/2 machines in byte by byte level. Its file extension is either BMP or DIB. The sample 24 bit BMP and DIP image are shown below.
BMP image hex data
Sample Bitmap.dip
 
Sample Bitmap.bmp
Contents :
DIM or BMP Image file format:

  A typical BMP has the following structure. It consist if BMP File Header, Bitmap Information, Color palette and Bitmap Data.


BMP File HeaderBMP file general information.
Bitmap Information (DIB header)Detailed information about the BMP image.
Color PaletteColor definitions used for indexed color bitmaps.
Bitmap Datapixel by pixel image data
To explain the information in details, the above bmp image is taken as a sample image and its hex values are displayed.

Bitmap File Header:
                For any file types first few bytes are identify of that files. It identifies the file types. Like that here also first two bytes are the identity bytes. Generally first two bytes has the value of 'BM'.  All the values are stores in little-endian encoding format. Generally the first 14 bytes contains the BMP file header information. 


OffsetSizePurpose
0000h2 bytes
  • BM – Windows 3.1x, 95, NT, ... etc.
  • BA – OS/2 Bitmap Array
  • CI – OS/2 Color Icon
  • CP – OS/2 Color Pointer
  • IC – OS/2 Icon
  • PT – OS/2 Pointer
0002h4 bytesBMP file size in bytes
0006h2 bytesreserved
0008h2 bytesreserved
000Ah4 bytesthe offset, i.e. starting address, of the byte where the bitmap data can be found.

Bitmap File Header Field details:


Identifier:
  From the sample image. The first two bytes are the identity bytes so it's 'B' and 'M'.

File Size:
                       The Image size is 47174 bytes (refer size.png). 47174 equvalent of  hex value is B846. The image representation is little-endian so the value becomes 05 th byte = 00, 04 th byte = 00, 03rd byte = B8, 02 nd byte = 46.
Reserved:  Bytes 6,7,8 and 9 are reserved so its value will be zero

Offset : Starting address of bitmap data. I.e the starting address of the bitmap data. In the sample image it is 36 (Represented in byte 0Ah) because it does have any extra color palate data or header information so it would be 36.

Bitmap Information (DIB header):
                The header bytes show the detailed information of BMP image. Here I am giving five versions of header two from IBM OS 2 versions and three from Microsoft versions. Because Microsoft extended the DIB format several times. So that the new header format is used  instead of the older ones for providing more functionality. Since windows GDI loads the bitmap file into the system.

SizeHeaderIdentified bySupported by the GDI of
12OS/2 V1BITMAPCOREHEADEROS/2 and Windows 3.0 and above
64OS/2 V2BITMAPCOREHEADER2
40Windows V3BITMAPINFOHEADERWindows 3.0 and above
108Windows V4BITMAPV4HEADERWindows 95/NT4 and above
124Windows V5BITMAPV5HEADERWindows 98/2000 and above

The table has all the five formats of header information’s. From the size of the header system identifies which version of header is used. The header size is in offset 0Eh.

BITMAPCOREHEADER  or OS/2 V1 Header Structure
  This structure is used in all OS/2 and also all Windows versions since Windows 3.0. This is a simple structure its size is 12 bytes. It has size, width, height, planes and bitcount. The offset and byte values are shown in the following table. This is also called as OS/2 V1 bitmaps. This format does not use any compression algorithms and it does not have 16 or 32 bit depths. The values in the header are unsigned integers. 32 bit version with alpha channel is introduced in Windows XP and it is used for its login screen. It is supported by Adobe Photoshop version 7 and above, Adobe flash and GIMP

OffsetSizePurpose
Eh4size of the header (12 bytes)
12h2bitmap width in pixels.
14h2bitmap height in pixels.
16h2the number of color planes; 1 is the only legal value
18h2the number of bits per pixel. Typical values are 1, 4, 8 and 24.
The typical structure used here is

typedef struct tagBITMAPCOREHEADER {
  DWORD bcSize;
  WORD  bcWidth;
  WORD  bcHeight;
  WORD  bcPlanes;
  WORD  bcBitCount;
} BITMAPCOREHEADER, *PBITMAPCOREHEADER;

BITMAPCOREHEADER2  or OS/2 V2 Header Structure

  This version has few additional fields over BITMAPINFOHEADER. The BITMAPINFOHEADER header info is given below. This is also called OS/2 V2 header used in IBM OS/2 Versions. Its header size would be 64 bytes. The typical C structure is given below.

typedef struct {
     DWORD bc2Size;
     DWORD bc2Width;
     DWORD bc2Height;
     WORD  bc2Planes;
     WORD  bc2BitCount;
     DWORD bc2Compression;
     DWORD bc2SizeImage;
     DWORD bc2XRes;
     DWORD bc2YRes;
     DWORD bc2ClrUsed;
     DWORD bc2ClrImportant;
    /* same as BITMAPINFOHEADER until this point */
     WORD  bc2ResUnit;
     WORD  bc2Reserved;
     WORD  bc2Orientation;
     WORD  bc2Halftoning;
     DWORD bc2HalftoneSize1;
     DWORD bc2HalftoneSize2;
     DWORD bc2ColorSpace;
     DWORD bc2AppData;
 } BITMAPCOREHEADER2;

BITMAPINFOHEADER Header Structure:
  All Windows versions since Windows 3.0 uses this header format. Its size is 40 bytes. For compatibility reason almost all versions of OS uses this header format. It has all the information generally used. All values are unsigned integers.

Offset #SizePurpose
0Eh4size of the header (40 bytes)
12h4bitmap width in pixels (signed integer).
16h4bitmap height in pixels (signed integer).
1Ah2number of color planes being used. Must be set to 1.
1Ch2the number of bits per pixel, I.e color depth of the image. Values are 1, 4, 8, 16, 24 and 32.
1Eh4compression method being used.
22h4image size. This is the size of the raw bitmap data (0 for BI_RGB bitmaps), and should not be confused with the file size.
26h4horizontal resolution of the image. (pixel per meter, signed integer)
2Ah4vertical resolution of the image. (pixel per meter, signed integer)
2Eh4number of colors in the color palette, or 0 to default to 2n.
32h4number of important colors used, or 0 when every color is important; generally ignored.
The sample image has BITMAPINFOHEADER header which is shown in the following image. Verify the offset 0Eh to 11h value would be 26h (=40 in decimal).

typedef struct {
  uint32_t header_sz;
  int32_t width;
  int32_t height;
  uint16_t nplanes;
  uint16_t bitspp;
  uint32_t compress_type;
  uint32_t bmp_bytesz;
  int32_t hres;
  int32_t vres;
  uint32_t ncolors;
  uint32_t nimpcolors;
} bmp_dib_v3_header_t;

Bitmap Compression Methods.

  The following compression methods are used in BMP Image which are shown in the following table.


ValueMeaning
BI_RGBAn uncompressed format.
BI_RLE8RLE compression format for bitmaps with 8 bpp. The compression format is a two-byte format consisting of a count byte followed by a byte containing a color index. If bV5Compression is BI_RGB and the bV5BitCountmember is 16, 24, or 32, the bitmap array specifies the actual intensities of blue, green, and red rather than using color table indexes. 
BI_RLE4RLE compression format for bitmaps with 4 bpp. The compression format is a two-byte format consisting of a count byte followed by two word-length color indexes. 
BI_BITFIELDSSpecifies that the bitmap is not compressed and that the color table consists of three DWORD color masks that specify the red, green, and blue components of each pixel. Valid when used with 16- and 32-bpp bitmaps.
BI_JPEGJPEG Compression is used. It can achieve a compression ratio of 20:1 with little noticeable loss.
BI_PNGPNG file Interchange Format compression.



typedef enum {
  BI_RGB = 0,
  BI_RLE8,
  BI_RLE4,
  BI_BITFIELDS,
  BI_JPEG,
  BI_PNG,
} bmp_compression_method_t;

BITMAPV4HEADER Structure:

  The BITMAPV4HEADER structure is an extended version of the BITMAPINFOHEADER structure. This header size is 108 bytes. It is supported by Windows 95/NT4 and above. The C structure for this header is

typedef struct {
  DWORD        bV4Size;
  LONG         bV4Width;
  LONG         bV4Height;
  WORD         bV4Planes;
  WORD         bV4BitCount;
  DWORD        bV4V4Compression;
  DWORD        bV4SizeImage;
  LONG         bV4XPelsPerMeter;
  LONG         bV4YPelsPerMeter;
  DWORD        bV4ClrUsed;
  DWORD        bV4ClrImportant;
  DWORD        bV4RedMask;
  DWORD        bV4GreenMask;
  DWORD        bV4BlueMask;
  DWORD        bV4AlphaMask;
  DWORD        bV4CSType;
  CIEXYZTRIPLE bV4Endpoints;
  DWORD        bV4GammaRed;
  DWORD        bV4GammaGreen;
  DWORD        bV4GammaBlue;
} BITMAPV4HEADER, *PBITMAPV4HEADER;

refer : http://msdn.microsoft.com/en-us/library/dd183380(VS.85).aspx for more about BITMAPV4HEADER Structure.

BITMAPV5HEADER Structure:

              The BITMAPV4HEADER structure is also an extended version of the BITMAPINFOHEADER structure. This header size is 124 bytes. It is supported by Windows 98/2000 and above. The C structure for this header is

typedef struct {
  DWORD        bV5Size;
  LONG         bV5Width;
  LONG         bV5Height;
  WORD         bV5Planes;
  WORD         bV5BitCount;
  DWORD        bV5Compression;
  DWORD        bV5SizeImage;
  LONG         bV5XPelsPerMeter;
  LONG         bV5YPelsPerMeter;
  DWORD        bV5ClrUsed;
  DWORD        bV5ClrImportant;
  DWORD        bV5RedMask;
  DWORD        bV5GreenMask;
  DWORD        bV5BlueMask;
  DWORD        bV5AlphaMask;
  DWORD        bV5CSType;
  CIEXYZTRIPLE bV5Endpoints;
  DWORD        bV5GammaRed;
  DWORD        bV5GammaGreen;
  DWORD        bV5GammaBlue;
  DWORD        bV5Intent;
  DWORD        bV5ProfileData;
  DWORD        bV5ProfileSize;
  DWORD        bV5Reserved;
} BITMAPV5HEADER, *PBITMAPV5HEADER;

refer : http://msdn.microsoft.com/en-us/library/dd183381(VS.85).aspx for more about BITMAPV5HEADER Structure.


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