This question was asked in the comments of another question, and I think it's a good question, but I wanted to make sure this was addressed in the proper place:
How do I know RGB triple is 3 bytes? Is it a fixed value or variable according to the bmp's type?
If you take a look in bmp.h, which is included as a header file to copy.c, you'll see a definition for structures named RGBTRIPLE:
typedef struct { BYTE rgbtBlue; BYTE rgbtGreen; BYTE rgbtRed; } __attribute__((__packed__)) RGBTRIPLE; So a triple has 3 variables in it of type BYTE. BYTE is defined in the header of the same bmp.h:
#include <stdint.h> /** * Common Data Types * * The data types in this section are essentially aliases for C/C++ * primitive data types. * * Adapted from http://msdn.microsoft.com/en-us/library/cc230309.aspx. * See http://en.wikipedia.org/wiki/Stdint.h for more on stdint.h. */ typedef uint8_t BYTE; SO: BYTE was explicitly defined as 8 bits of data via stdint.h, which is our standard idea of what a byte is anyway.
The structure called RGBTRIPLE therefore has 3 variables, each using 1 byte of space, for a total of 3 bytes.
We could change the size of an RGBTRIPLE by changing its definition in the bmp.h file, although I suspect we would also need to make some adjustments to our file/info headers.
The missing part of Dr.Queso's response is this. A pixel is made of 3 parts - one red, one green and one blue. Each of these is represented by a number that determines it's intensity. The range for each color part of a pixel runs from 0 to 255, with 0 being the absence of that color and 255 being the maximum. Of course, that's in base 10. In hexadecimal, that would be from 0x00 to 0xFF. And since a byte can hold two hexadecimal digits (4 bits each ), each color is represented by one byte and a pixel is represented by 3 bytes. Thus the definition of an RGBTRIPLE as a 3 byte unsigned hexadecimal number - one byte for each of the primary colors.
And yes, it's also an industry standard.
