What are the three 255's before the image in a monochrome .bmp? - bmp

Question

I'm trying to figure out bitmap formatting for a project I'm working on, but there's one thing I don't really get. In this .bmp:
00000000 42 4d aa 00 00 00 00 00 00 00 82 00 00 00 6c 00 |BM............l.|
00000010 00 00 0a 00 00 00 0a 00 00 00 01 00 01 00 00 00 |................|
00000020 00 00 28 00 00 00 13 0b 00 00 13 0b 00 00 02 00 |..(.............|
00000030 00 00 02 00 00 00 42 47 52 73 00 00 00 00 00 00 |......BGRs......|
00000040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000060 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 |................|
00000070 00 00 00 00 00 00 00 00 00 00 ff ff ff 00 00 00 |................|
00000080 00 00 ff c0 00 00 ff c0 00 00 ff c0 00 00 ff c0 |................|
*
000000a0 00 00 ff c0 00 00 aa 80 00 00 |..........|
000000aa
what are the three 256's at offset 122 (line 7). I assume they're some sort of color indicators, but I'm not sure.
This is the image I'm using.

Answer 1

The top part of your hex dump is a "BITMAPINFO structure" (https://msdn.microsoft.com/en-us/library/dd183375(v=vs.85).aspx). This is immediately followed by a color index array bmiColors (although its length may be 0, and you should check this in the BITMAPINFO data).
Although some say that
[t]hough seemingly a simple format, it is complicated by its many different versions, lack of an official specification, lack of any version control process, and ambiguities and contradictions in the documentation.
(http://fileformats.archiveteam.org/wiki/BMP)
you don't actually need to understand each single byte. The various structures either have a fixed size (the initial BITMAPFILEHEADER for example), or have their length as its first value.
A line by line annotation of most well-documented parts:
-------- BITMAPFILEHEADER
00000000 42 4d file type identifier
00000002 aa 00 00 00 size in bytes of the entire file
00000006 00 00 (reserved and must be 0)
00000008 00 00 (reserved and must be 0)
0000000A 82 00 00 00 offset from the beginning of the BITMAPFILEHEADER structure to the bitmap bits
-------- BITMAPINFOHEADER
0000000E 6c 00 00 00 BITMAPINFOHEADER structure size
00000012 0a 00 00 00 image width in pixels
00000016 0a 00 00 00 image height in pixels
0000001A 01 00 number of planes
0000001C 01 00 number of bits per pixel
0000001E 00 00 00 00 compression
00000022 28 00 00 00 size in bytes of image data
00000026 13 0b 00 00 horizontal resolution in pixels-per-meter
0000002A 13 0b 00 00 vertical resolution in pixels-per-meter
0000002E 02 00 00 00 number of colors in the color table that are actually used by the bitmap
00000032 02 00 00 00 number of color indexes that are required ("important")
........ badly documented stuff ........
00000036 42 47 52 73 00 00 00 00 00 00
00000040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00000060 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00
00000070 00 00 00 00 00 00 00 00 00 00
-------- end of BITMAPINFOHEADER
-------- bmiColors array
0000007A ff ff ff 00 color table entry #0
0000007E 00 00 00 00 color table entry #1
-------- Image data
00000082 ff c0 00 00 ff c0 00 00 ff c0 00 00 ff c0
000000a0 00 00 ff c0 00 00 aa 80 00 00
The "number of bits per pixel" at 0000001C is 1:
"1 = The bitmap is monochrome, and the bmiColors member of BITMAPINFO 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 bmiColors table; if the bit is set, the pixel has the color of the second entry in the table."
(https://msdn.microsoft.com/en-us/library/dd183376(v=vs.85).aspx)
and the number of colors in the array is reported to be 2. So the bmiColors array contains 2 elements in Microsoft's RGBQUAD format, with values in the odd order Blue, Green, Red, and Reserved.
In short: in your image, a pixel value of 0 (a 0 bit in a monochrome image) is FFFFFF: white, and a pixel value of 1 is 000000: black.