BMP

Introduction


BMP is a standard file format for computers running the Windows operating system. The format was developed by Microsoft for storing bitmap files in a device-independent bitmap (DIB) format that will allow Windows to display the bitmap on any type of display device. The term "device independent" means that the bitmap specifies pixel color in a form independent of the method used by a display to represent color.

Basic structure


A .bmp file contains of the following data structures:


1. BITMAPFILEHEADER - a bitmap-file header: this contains information about the type, size, and layout of a device-independent bitmap file.


2. BITMAPINFOHEADER - a bitmap-information header which specifies the dimensions, compression type, and color format for the bitmap.


3. RGBQUAD - a color table, defined as an array of RGBQUAD structures, contains as many elements as there are colors in the bitmap. The color table is not present for bitmaps with 24 color bits because each pixel is represented by 24-bit red-green-blue (RGB) values in the actual bitmap data area.


4. BYTE - an array of bytes that defines the bitmap bits. These are the actual image data, represented by consecutive rows, or "scan lines," of the bitmap. Each scan line consists of consecutive bytes representing the pixels in the scan line, in left-to-right order.

The BITMAPFILEHEADER

The header consists of the following fields. The header is 14 bytes in length.

Size
 Name
 Purpose
2 bytes bfType must always be set to 'BM' to declare that this is a .bmp-file.
4 bytes bfSize specifies the size of the file in bytes.
2 bytes bfReserved1 must always be set to zero.
2 bytes bfReserved2 must always be set to zero.
4 bytes bfOffBits specifies the offset from the beginning of the file to the bitmap data.

The BITMAPINFOHEADER

Size
 name
 purpose
4 bytes biSize specifies the size of the BITMAPINFOHEADER structure, in bytes.
4 bytes biWidth specifies the width of the image, in pixels.
4 bytes biHeight specifies the height of the image, in pixels.
2 bytes biPlanes specifies the number of planes of the target device, must be set to zero.
2 bytes biBitCount specifies the number of bits per pixel and the maximum number of colors in the bitmap.
4 bytes biCompression

Specifies the type of compression, usually set to zero (no compression).

0 - no compression
1 - 8 bit run length encoding.
2 - 4 bit run length encoding
3 - RGB bitmap with mask
4 bytes biSizeImage specifies the size of the image data, in bytes. If there is no compression, it is valid to set this member to zero.
4 bytes biXPelsPerMeter specifies the the horizontal pixels per meter on the designated targer device, usually set to zero.
4 bytes biYPelsPerMeter specifies the the vertical pixels per meter on the designated targer device, usually set to zero.
4 bytes BiClrUsed

specifies the number of colors used in the bitmap, if set to zero the number of colors is calculated using the biBitCount member.

The maximum number of colors = 2N. Where N = biBitCount. 2N is the same as 1 << N.
4 bytes BiClrImportant specifies the number of color that are 'important' for the bitmap, if set to zero, all colors are important.

 

The field 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 second.

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.

In 8-bit mode every byte represents a pixel. The value points to an entry in the color table which contains 256 entries.

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:

Size
 		Name
 		Purpose
1 rgbBlue specifies the blue part of the color
1 rgbGreen specifies the green part of the color
1 rgbRed specifies the red part of the color
1 rgbReserved must always be set to zero

The Pixel Data

It depends on the BITMAPINFOHEADER structure how the pixel data is to be interpreted. Its size is equal to the biSizeImage field of BITMAPINFOHEADER structure. 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

 

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