.bmp image data format

Description of Header structure

1. College of Engineering Pune (COEP)
Forerunners in Technical Education

A seminar on
.bmp Image Data format
Presented By
Mahesh Pawar (121697010)

2. College of Engineering Pune (COEP)
Forerunners in Technical Education

Some Common Image Formats
-JPG : Used for most digital photos but “lossy”
-GIF : Best for line drawings. 256 colors maximum. No loss
-BMP : Bitmap. Every pixel stored individually
-TIF : Standard in many technical applications. No loss
-PNG : Lossless image format
-RAW : Data as captured by digital camera

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Forerunners in Technical Education

Test Image in Various Formats
-JPG 67 kb
-GIF 185 kb (256 colors, optimized)
-BMP 899 kb (640*479*3/1024 =
898.125)
-TIF 902 kb
-PNG 418 kb
-RAW 900 kb
Test Image (640 x 479 pixels)

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Forerunners in Technical Education

GIF Versus JPG
GIF: 11 kb JPG: 78 kb

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Forerunners in Technical Education

.bmp format
-BMP file format called as bitmap or DIB file format
-DIB in memory
-Pixel storage :
- Pixels are stored with colour depth of 1,4,8,16,24,32 bits/pixel
Uncompressed bitmap files are typically much larger than compressed
-Colour palette

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Forerunners in Technical Education

Bitmap File structure
-Bitmap-file Header
-Bitmap information Header
-Colour table
Fig. Bitmap File Format

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Forerunners in Technical Education

Bitmap file Header
uint 2 bytes Filetype
Dword 4 bytes File size in bytes
Uint 2 bytes Reserved
Uint 2 bytes Reserved
dword 4 bytes Offset to data in
bytes
-Contains information about type, size and layout of a bitmap file
-First two characters – BM (0x42 0x4D)
-Next four bytes – file size
-Next four bytes are unused
-Final four bytes are an offset

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Forerunners in Technical Education

Bitmap information header
-Specifies the dimensions, compression type, and colour format for the bitmap
-First four bytes are header size – 40 bytes followed by width and height of the image
-Next two bytes – 1 i.e. number of planes
-Next two bytes – number of bits used to represent colour intensities of a pixel
-Next four bytes store the compression followed by the image size
-Next eight bytes store the X and Y resolution
-Final entries – number of colour map entries and the number of significant colours

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dword 4 Bitmap info header structure
size
Long 4 Bitmap width in pixels
Long 4 Bitmap height in pixels
Word 2 1
Word 2 Bits/pixel( 1 = monochrome )
Dword 4 Compression 0,8, 4
Dword 4 Image size in bytes
Long 4 Pixels/metre
Long 4 Pixels/metre
Dword 4 Number of colour indexes
Continued…….

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Forerunners in Technical Education

Colour Table & Image data
-Colour table is not present for bitmaps with 24 bit files.
-The format of the file depends on the number of bits used
to represent each pixel
-Bitmap data immediately following the colour table.
-Bitmap pixel data is stored upside down from its actual image
-A scan line must be zero padded to end
-The scan lines in the bitmap are stored from bottom to the top
-One important issue : managing junk bytes

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Forerunners in Technical Education

Usage of BMP format
-Simplicity
-Well documented and free of patents
-Most BMP files have a relatively large file size & can be compressed
with lossless data compression algorithm

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Forerunners in Technical Education

References
[1] https://en.Wikipedia.org/wiki/Image_file_formats
[2] John Miano, "Compressed Image File Formats“, ISBN - 0-201-60443-4,1999
[3] R. Witrow, “ OpenGL Graphics through Applications”, Springer-verlag London
Limited, 2008

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Forerunners in Technical Education

Thank you