BMP format

REF1: wikipedia

REF2

REF3: martinreddy

REF4: microsoft

Bitmap file header

Header 14 bytes  Windows Structure: BITMAPFILEHEADER

Field	Length	Offset	Meaning
Signature	2 bytes	0000h	'BM'
FileSize	4 bytes	0002h	File size in bytes
reserved	4 bytes	0006h	unused (=0)
DataOffset	4 bytes	000Ah	Offset from beginning of file to the beginning of the bitmap data

InfoHeader 40 bytes  Windows Structure: BITMAPINFOHEADER

Field	Length	Offset	Meaning
Size	4 bytes	000Eh	Size of InfoHeader =40
Width	4 bytes	0012h	Horizontal width of bitmap in pixels
Height	4 bytes	0016h	Vertical height of bitmap in pixels
Planes	2 bytes	001Ah	Number of Planes (=1)
Bits Per Pixel	2 bytes	001Ch	Bits per Pixel used to store palette entry information. This also identifies in an indirect way the number of possible colors. Possible values are:1 = monochrome palette. NumColors = 1  4 = 4bit palletized. NumColors = 16  8 = 8bit palletized. NumColors = 256 16 = 16bit RGB. NumColors = 6553624 = 24bit RGB. NumColors = 16M
Compression	4 bytes	001Eh	Type of Compression  0 = BI_RGB   no compression  1 = BI_RLE8 8bit RLE encoding  2 = BI_RLE4 4bit RLE encoding
ImageSize	4 bytes	0022h	(compressed) Size of Image It is valid to set this =0 if Compression = 0
XpixelsPerM	4 bytes	0026h	horizontal resolution: Pixels/meter
YpixelsPerM	4 bytes	002Ah	vertical resolution: Pixels/meter
Colors Used	4 bytes	002Eh	Number of actually used colors. For a 8-bit / pixel bitmap this will be 100h or 256.
Important Colors	4 bytes	0032h	Number of important colors

If color_used greater than 0, we will have color table

Field	Length	Offset	Meaning
ColorTable	4 * NumColors bytes	0036h	present only if Info.BitsPerPixel less than 8  colors should be ordered by importance

If Color table present， it contains color entry like below

Field	Length		Meaning
Blue	1 byte		Red intensity
Green	1 byte		Green intensity
Red	1 byte		Blue intensity
reserved	1 byte		unused (=0)

Bits Per Pixel Field

Value	Description
1	The bitmap is monochrome, and the palette contains two entries. Each bit in the bitmap array represents a pixel. If the bit is clear, the pixel is displayed with the color of the first entry in the palette; if the bit is set, the pixel has the color of the second entry in the table.
4	The bitmap has a maximum of 16 colors, and the palette contains up to 16 entries. Each pixel in the bitmap is represented by a 4-bit index into the palette. For example, if the first byte in the bitmap is 1Fh, the byte represents two pixels. The first pixel contains the color in the second palette entry, and the second pixel contains the color in the sixteenth palette entry.
8	The bitmap has a maximum of 256 colors, and the palette contains up to 256 entries. In this case, each byte in the array represents a single pixel.
16	The bitmap has a maximum of 2^16 colors. If the Compression field of the bitmap file is set to BI_RGB, the Palette field does not contain any entries. Each word in the bitmap array represents a single pixel. The relative intensities of red, green, and blue are represented with 5 bits for each color component. The value for blue is in the least significant 5 bits, followed by 5 bits each for green and red, respectively. The most significant bit is not used.If the Compression field of the bitmap file is set to BI_BITFIELDS, the Palette field contains three 4 byte color masks that specify the red, green, and blue components, respectively, of each pixel.  Each 2 bytes in the bitmap array represents a single pixel.
24	The bitmap has a maximum of 2^24 colors, and the Palette field does not contain any entries. Each 3-byte triplet in the bitmap array represents the relative intensities of blue, green, and red, respectively, for a pixel.

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Note: Additional Info

Each scan line is zero padded to the nearest 4-byte boundary. If the image has a width that is not divisible by four, say, 21 bytes, there would be 3 bytes of padding at the end of every scan line.

Scan lines are stored bottom to top instead of top to bottom.

RGB values are stored backwards i.e. BGR.

4 bit & 8 bit BMPs can be compressed. BMPs use a very simple form of compression called Run Length Encoded (RLE). Instead of storing a value for each pixel RLE stores a number, N, followed by an index. This means that the next N pixels are of the color for this index.

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run-length encoded format uses two modes: encoded mode and absolute mode.

Encoded mode involves two bytes. If the first byte of a pair is greater than zero, it specifies the number of consecutive pixels to be drawn using the color index that is contained in the second byte.

If the first byte of a pair is zero and the second byte is 0x02 or less, the second byte is an escape value that can denote the end of a line, the end of the bitmap, or a relative pixel position, as follows.

Second byte value	Meaning
0x00	End of line
0x01	End of bitmap
0x02	Delta

When a delta is specified, the 2 bytes following the escape value contain unsigned values indicating the horizontal and vertical offsets of the next pixel relative to the current position.

In absolute mode, the first byte is zero, and the second byte is a value in the range 0x03 through 0xFF. The second byte represents the number of bytes that follow, each of which contains the color index of a single pixel. In absolute mode, each run is aligned on a word boundary.

The following example shows the hexadecimal contents of an 8-bit compressed bitmap.

03 04 05 06 00 03 45 56 67 00 02 78 00 02 05 01 02 78 00 00 09 1E 00 01

This bitmap is interpreted as follows:

• 03 04: Encoded mode, specifying 3 pixels with the value 0x04.
• 05 06: Encoded mode, specifying 5 pixels with the value 0x06.
• 00 03 45 56 67 00: Absolute mode, specifying 3 pixels with the values 0x45, 0x56, and 0x67, padded to a word boundary.
• 02 78: Encoded mode, specifying 2 pixels with the value 0x78.
• 00 02 05 01: Encoded mode, specifying a new relative position 5 pixels to the right and 1 line down.
• 02 78: Encoded mode, specifying 2 pixels with the value 0x78.
• 00 00: Encoded mode, specifying the end of a line.
• 09 1E: Encoded mode, specifying 9 pixels with the value 1E.
• 00 01: Encoded mode, specifying the end of the bitmap.